[dev.typeparams] cmd/compile/internal/importer, types2: initial check-in of types2 and importer

This is a copy of the importer and types2 (unreviewed) prototype version
excluding the testdata directory containing tests (see below). Each file
is marked with the comment

// UNREVIEWED

on the first line. The plan is to check in this code wholesale (it runs and
passes all tests) and then review the code file-by-file via subsequent CLs
and remove the "// UNREVIEWED" comments as we review the files.

Since most tests are unchanged from the original go/types, the next CL will
commit those tests as they don't need to be reviewed again. (Eventually we
may want to factor them out and share them from a single place, e.g. the
test directory.)

The existing file fmtmap_test.go was updated.

Change-Id: I9bd0ad1a7e7188b501423483a44d18e623c0fe71
Reviewed-on: https://go-review.googlesource.com/c/go/+/263624
Trust: Robert Griesemer <gri@golang.org>
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/fmtmap_test.go

@@ -42,6 +42,10 @@ var knownFormats = map[string]string{
 	"*cmd/compile/internal/ssa.Value %s":              "",
 	"*cmd/compile/internal/ssa.Value %v":              "",
 	"*cmd/compile/internal/ssa.sparseTreeMapEntry %v": "",
+	"*cmd/compile/internal/syntax.CallExpr %s":        "",
+	"*cmd/compile/internal/syntax.CallExpr %v":        "",
+	"*cmd/compile/internal/syntax.FuncLit %s":         "",
+	"*cmd/compile/internal/syntax.IndexExpr %s":       "",
 	"*cmd/compile/internal/types.Field %p":            "",
 	"*cmd/compile/internal/types.Field %v":            "",
 	"*cmd/compile/internal/types.Sym %0S":             "",
@@ -58,6 +62,25 @@ var knownFormats = map[string]string{
 	"*cmd/compile/internal/types.Type %p":             "",
 	"*cmd/compile/internal/types.Type %s":             "",
 	"*cmd/compile/internal/types.Type %v":             "",
+	"*cmd/compile/internal/types2.Basic %s":           "",
+	"*cmd/compile/internal/types2.Chan %s":            "",
+	"*cmd/compile/internal/types2.Func %s":            "",
+	"*cmd/compile/internal/types2.Initializer %s":     "",
+	"*cmd/compile/internal/types2.Interface %s":       "",
+	"*cmd/compile/internal/types2.MethodSet %s":       "",
+	"*cmd/compile/internal/types2.Named %s":           "",
+	"*cmd/compile/internal/types2.Named %v":           "",
+	"*cmd/compile/internal/types2.Package %s":         "",
+	"*cmd/compile/internal/types2.Package %v":         "",
+	"*cmd/compile/internal/types2.Scope %p":           "",
+	"*cmd/compile/internal/types2.Selection %s":       "",
+	"*cmd/compile/internal/types2.Signature %s":       "",
+	"*cmd/compile/internal/types2.TypeName %s":        "",
+	"*cmd/compile/internal/types2.TypeName %v":        "",
+	"*cmd/compile/internal/types2.TypeParam %s":       "",
+	"*cmd/compile/internal/types2.Var %s":             "",
+	"*cmd/compile/internal/types2.operand %s":         "",
+	"*cmd/compile/internal/types2.substMap %s":        "",
 	"*cmd/internal/obj.Addr %v":                       "",
 	"*cmd/internal/obj.LSym %v":                       "",
 	"*math/big.Float %f":                              "",
@@ -67,6 +90,8 @@ var knownFormats = map[string]string{
 	"[16]byte %x":                                     "",
 	"[]*cmd/compile/internal/ssa.Block %v":            "",
 	"[]*cmd/compile/internal/ssa.Value %v":            "",
+	"[]*cmd/compile/internal/types2.Func %v":          "",
+	"[]*cmd/compile/internal/types2.TypeName %s":      "",
 	"[][]string %q":                                   "",
 	"[]byte %s":                                       "",
 	"[]byte %x":                                       "",
@@ -75,6 +100,8 @@ var knownFormats = map[string]string{
 	"[]cmd/compile/internal/ssa.posetNode %v":         "",
 	"[]cmd/compile/internal/ssa.posetUndo %v":         "",
 	"[]cmd/compile/internal/syntax.token %s":          "",
+	"[]cmd/compile/internal/types2.Type %s":           "",
+	"[]int %v":                                        "",
 	"[]string %v":                                     "",
 	"[]uint32 %v":                                     "",
 	"bool %v":                                         "",
@@ -100,6 +127,7 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/gc.fmtMode %d":              "",
 	"cmd/compile/internal/gc.initKind %d":             "",
 	"cmd/compile/internal/gc.itag %v":                 "",
+	"cmd/compile/internal/importer.itag %v":           "",
 	"cmd/compile/internal/ssa.BranchPrediction %d":    "",
 	"cmd/compile/internal/ssa.Edge %v":                "",
 	"cmd/compile/internal/ssa.GCNode %v":              "",
@@ -122,9 +150,13 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/ssa.regMask %d":             "",
 	"cmd/compile/internal/ssa.register %d":            "",
 	"cmd/compile/internal/ssa.relation %s":            "",
+	"cmd/compile/internal/syntax.ChanDir %d":          "",
+	"cmd/compile/internal/syntax.Decl %T":             "",
 	"cmd/compile/internal/syntax.Error %q":            "",
 	"cmd/compile/internal/syntax.Error %v":            "",
 	"cmd/compile/internal/syntax.Expr %#v":            "",
+	"cmd/compile/internal/syntax.Expr %T":             "",
+	"cmd/compile/internal/syntax.Expr %s":             "",
 	"cmd/compile/internal/syntax.LitKind %d":          "",
 	"cmd/compile/internal/syntax.Node %T":             "",
 	"cmd/compile/internal/syntax.Operator %s":         "",
@@ -136,12 +168,22 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/types.EType %d":             "",
 	"cmd/compile/internal/types.EType %s":             "",
 	"cmd/compile/internal/types.EType %v":             "",
+	"cmd/compile/internal/types2.Object %T":           "",
+	"cmd/compile/internal/types2.Object %p":           "",
+	"cmd/compile/internal/types2.Object %s":           "",
+	"cmd/compile/internal/types2.Object %v":           "",
+	"cmd/compile/internal/types2.Type %T":             "",
+	"cmd/compile/internal/types2.Type %s":             "",
+	"cmd/compile/internal/types2.Type %v":             "",
+	"cmd/compile/internal/types2.color %s":            "",
 	"cmd/internal/obj.ABI %v":                         "",
+	"error %s":                                        "",
 	"error %v":                                        "",
 	"float64 %.2f":                                    "",
 	"float64 %.3f":                                    "",
 	"float64 %.6g":                                    "",
 	"float64 %g":                                      "",
+	"go/constant.Value %s":                            "",
 	"int %#x":                                         "",
 	"int %-12d":                                       "",
 	"int %-6d":                                        "",
@@ -176,6 +218,7 @@ var knownFormats = map[string]string{
 	"interface{} %v":                                  "",
 	"map[*cmd/compile/internal/gc.Node]*cmd/compile/internal/ssa.Value %v":           "",
 	"map[*cmd/compile/internal/gc.Node][]*cmd/compile/internal/gc.Node %v":           "",
+	"map[*cmd/compile/internal/types2.TypeParam]cmd/compile/internal/types2.Type %s": "",
 	"map[cmd/compile/internal/ssa.ID]uint32 %v":                                      "",
 	"map[int64]uint32 %v":  "",
 	"math/big.Accuracy %s": "",
@@ -186,6 +229,7 @@ var knownFormats = map[string]string{
 	"string %-*s":          "",
 	"string %-16s":         "",
 	"string %-6s":          "",
+	"string %T":            "",
 	"string %q":            "",
 	"string %s":            "",
 	"string %v":            "",
@@ -205,6 +249,7 @@ var knownFormats = map[string]string{
 	"uint64 %08x":          "",
 	"uint64 %b":            "",
 	"uint64 %d":            "",
+	"uint64 %v":            "",
 	"uint64 %x":            "",
 	"uint8 %#x":            "",
 	"uint8 %d":             "",

src/cmd/compile/internal/importer/exportdata.go

@@ -0,0 +1,92 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements FindExportData.
+
+package importer
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+func readGopackHeader(r *bufio.Reader) (name string, size int, err error) {
+	// See $GOROOT/include/ar.h.
+	hdr := make([]byte, 16+12+6+6+8+10+2)
+	_, err = io.ReadFull(r, hdr)
+	if err != nil {
+		return
+	}
+	// leave for debugging
+	if false {
+		fmt.Printf("header: %s", hdr)
+	}
+	s := strings.TrimSpace(string(hdr[16+12+6+6+8:][:10]))
+	size, err = strconv.Atoi(s)
+	if err != nil || hdr[len(hdr)-2] != '`' || hdr[len(hdr)-1] != '\n' {
+		err = fmt.Errorf("invalid archive header")
+		return
+	}
+	name = strings.TrimSpace(string(hdr[:16]))
+	return
+}
+
+// FindExportData positions the reader r at the beginning of the
+// export data section of an underlying GC-created object/archive
+// file by reading from it. The reader must be positioned at the
+// start of the file before calling this function. The hdr result
+// is the string before the export data, either "$$" or "$$B".
+//
+func FindExportData(r *bufio.Reader) (hdr string, err error) {
+	// Read first line to make sure this is an object file.
+	line, err := r.ReadSlice('\n')
+	if err != nil {
+		err = fmt.Errorf("can't find export data (%v)", err)
+		return
+	}
+
+	if string(line) == "!<arch>\n" {
+		// Archive file. Scan to __.PKGDEF.
+		var name string
+		if name, _, err = readGopackHeader(r); err != nil {
+			return
+		}
+
+		// First entry should be __.PKGDEF.
+		if name != "__.PKGDEF" {
+			err = fmt.Errorf("go archive is missing __.PKGDEF")
+			return
+		}
+
+		// Read first line of __.PKGDEF data, so that line
+		// is once again the first line of the input.
+		if line, err = r.ReadSlice('\n'); err != nil {
+			err = fmt.Errorf("can't find export data (%v)", err)
+			return
+		}
+	}
+
+	// Now at __.PKGDEF in archive or still at beginning of file.
+	// Either way, line should begin with "go object ".
+	if !strings.HasPrefix(string(line), "go object ") {
+		err = fmt.Errorf("not a Go object file")
+		return
+	}
+
+	// Skip over object header to export data.
+	// Begins after first line starting with $$.
+	for line[0] != '$' {
+		if line, err = r.ReadSlice('\n'); err != nil {
+			err = fmt.Errorf("can't find export data (%v)", err)
+			return
+		}
+	}
+	hdr = string(line)
+
+	return
+}

src/cmd/compile/internal/importer/gcimporter.go

@@ -0,0 +1,175 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// package importer implements Import for gc-generated object files.
+package importer
+
+import (
+	"bufio"
+	"cmd/compile/internal/types2"
+	"fmt"
+	"go/build"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// debugging/development support
+const debug = false
+
+var pkgExts = [...]string{".a", ".o"}
+
+// FindPkg returns the filename and unique package id for an import
+// path based on package information provided by build.Import (using
+// the build.Default build.Context). A relative srcDir is interpreted
+// relative to the current working directory.
+// If no file was found, an empty filename is returned.
+//
+func FindPkg(path, srcDir string) (filename, id string) {
+	if path == "" {
+		return
+	}
+
+	var noext string
+	switch {
+	default:
+		// "x" -> "$GOPATH/pkg/$GOOS_$GOARCH/x.ext", "x"
+		// Don't require the source files to be present.
+		if abs, err := filepath.Abs(srcDir); err == nil { // see issue 14282
+			srcDir = abs
+		}
+		bp, _ := build.Import(path, srcDir, build.FindOnly|build.AllowBinary)
+		if bp.PkgObj == "" {
+			id = path // make sure we have an id to print in error message
+			return
+		}
+		noext = strings.TrimSuffix(bp.PkgObj, ".a")
+		id = bp.ImportPath
+
+	case build.IsLocalImport(path):
+		// "./x" -> "/this/directory/x.ext", "/this/directory/x"
+		noext = filepath.Join(srcDir, path)
+		id = noext
+
+	case filepath.IsAbs(path):
+		// for completeness only - go/build.Import
+		// does not support absolute imports
+		// "/x" -> "/x.ext", "/x"
+		noext = path
+		id = path
+	}
+
+	if false { // for debugging
+		if path != id {
+			fmt.Printf("%s -> %s\n", path, id)
+		}
+	}
+
+	// try extensions
+	for _, ext := range pkgExts {
+		filename = noext + ext
+		if f, err := os.Stat(filename); err == nil && !f.IsDir() {
+			return
+		}
+	}
+
+	filename = "" // not found
+	return
+}
+
+// Import imports a gc-generated package given its import path and srcDir, adds
+// the corresponding package object to the packages map, and returns the object.
+// The packages map must contain all packages already imported.
+//
+func Import(packages map[string]*types2.Package, path, srcDir string, lookup func(path string) (io.ReadCloser, error)) (pkg *types2.Package, err error) {
+	var rc io.ReadCloser
+	var id string
+	if lookup != nil {
+		// With custom lookup specified, assume that caller has
+		// converted path to a canonical import path for use in the map.
+		if path == "unsafe" {
+			return types2.Unsafe, nil
+		}
+		id = path
+
+		// No need to re-import if the package was imported completely before.
+		if pkg = packages[id]; pkg != nil && pkg.Complete() {
+			return
+		}
+		f, err := lookup(path)
+		if err != nil {
+			return nil, err
+		}
+		rc = f
+	} else {
+		var filename string
+		filename, id = FindPkg(path, srcDir)
+		if filename == "" {
+			if path == "unsafe" {
+				return types2.Unsafe, nil
+			}
+			return nil, fmt.Errorf("can't find import: %q", id)
+		}
+
+		// no need to re-import if the package was imported completely before
+		if pkg = packages[id]; pkg != nil && pkg.Complete() {
+			return
+		}
+
+		// open file
+		f, err := os.Open(filename)
+		if err != nil {
+			return nil, err
+		}
+		defer func() {
+			if err != nil {
+				// add file name to error
+				err = fmt.Errorf("%s: %v", filename, err)
+			}
+		}()
+		rc = f
+	}
+	defer rc.Close()
+
+	var hdr string
+	buf := bufio.NewReader(rc)
+	if hdr, err = FindExportData(buf); err != nil {
+		return
+	}
+
+	switch hdr {
+	case "$$\n":
+		err = fmt.Errorf("import %q: old textual export format no longer supported (recompile library)", path)
+
+	case "$$B\n":
+		var data []byte
+		data, err = ioutil.ReadAll(buf)
+		if err != nil {
+			break
+		}
+
+		// The indexed export format starts with an 'i'; the older
+		// binary export format starts with a 'c', 'd', or 'v'
+		// (from "version"). Select appropriate importer.
+		if len(data) > 0 && data[0] == 'i' {
+			_, pkg, err = iImportData(packages, data[1:], id)
+		} else {
+			err = fmt.Errorf("import %q: old binary export format no longer supported (recompile library)", path)
+		}
+
+	default:
+		err = fmt.Errorf("import %q: unknown export data header: %q", path, hdr)
+	}
+
+	return
+}
+
+type byPath []*types2.Package
+
+func (a byPath) Len() int           { return len(a) }
+func (a byPath) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a byPath) Less(i, j int) bool { return a[i].Path() < a[j].Path() }

src/cmd/compile/internal/importer/gcimporter_test.go

@@ -0,0 +1,612 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importer
+
+import (
+	"bytes"
+	"cmd/compile/internal/types2"
+	"fmt"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+// skipSpecialPlatforms causes the test to be skipped for platforms where
+// builders (build.golang.org) don't have access to compiled packages for
+// import.
+func skipSpecialPlatforms(t *testing.T) {
+	switch platform := runtime.GOOS + "-" + runtime.GOARCH; platform {
+	case "darwin-arm64":
+		t.Skipf("no compiled packages available for import on %s", platform)
+	}
+}
+
+// compile runs the compiler on filename, with dirname as the working directory,
+// and writes the output file to outdirname.
+func compile(t *testing.T, dirname, filename, outdirname string) string {
+	// filename must end with ".go"
+	if !strings.HasSuffix(filename, ".go") {
+		t.Fatalf("filename doesn't end in .go: %s", filename)
+	}
+	basename := filepath.Base(filename)
+	outname := filepath.Join(outdirname, basename[:len(basename)-2]+"o")
+	cmd := exec.Command(testenv.GoToolPath(t), "tool", "compile", "-o", outname, filename)
+	cmd.Dir = dirname
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Fatalf("go tool compile %s failed: %s", filename, err)
+	}
+	return outname
+}
+
+func testPath(t *testing.T, path, srcDir string) *types2.Package {
+	t0 := time.Now()
+	pkg, err := Import(make(map[string]*types2.Package), path, srcDir, nil)
+	if err != nil {
+		t.Errorf("testPath(%s): %s", path, err)
+		return nil
+	}
+	t.Logf("testPath(%s): %v", path, time.Since(t0))
+	return pkg
+}
+
+const maxTime = 30 * time.Second
+
+func testDir(t *testing.T, dir string, endTime time.Time) (nimports int) {
+	dirname := filepath.Join(runtime.GOROOT(), "pkg", runtime.GOOS+"_"+runtime.GOARCH, dir)
+	list, err := ioutil.ReadDir(dirname)
+	if err != nil {
+		t.Fatalf("testDir(%s): %s", dirname, err)
+	}
+	for _, f := range list {
+		if time.Now().After(endTime) {
+			t.Log("testing time used up")
+			return
+		}
+		switch {
+		case !f.IsDir():
+			// try extensions
+			for _, ext := range pkgExts {
+				if strings.HasSuffix(f.Name(), ext) {
+					name := f.Name()[0 : len(f.Name())-len(ext)] // remove extension
+					if testPath(t, filepath.Join(dir, name), dir) != nil {
+						nimports++
+					}
+				}
+			}
+		case f.IsDir():
+			nimports += testDir(t, filepath.Join(dir, f.Name()), endTime)
+		}
+	}
+	return
+}
+
+func mktmpdir(t *testing.T) string {
+	tmpdir, err := ioutil.TempDir("", "gcimporter_test")
+	if err != nil {
+		t.Fatal("mktmpdir:", err)
+	}
+	if err := os.Mkdir(filepath.Join(tmpdir, "testdata"), 0700); err != nil {
+		os.RemoveAll(tmpdir)
+		t.Fatal("mktmpdir:", err)
+	}
+	return tmpdir
+}
+
+func TestImportTestdata(t *testing.T) {
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+
+	compile(t, "testdata", "exports.go", filepath.Join(tmpdir, "testdata"))
+
+	if pkg := testPath(t, "./testdata/exports", tmpdir); pkg != nil {
+		// The package's Imports list must include all packages
+		// explicitly imported by exports.go, plus all packages
+		// referenced indirectly via exported objects in exports.go.
+		// With the textual export format, the list may also include
+		// additional packages that are not strictly required for
+		// import processing alone (they are exported to err "on
+		// the safe side").
+		// TODO(gri) update the want list to be precise, now that
+		// the textual export data is gone.
+		got := fmt.Sprint(pkg.Imports())
+		for _, want := range []string{"go/ast", "go/token"} {
+			if !strings.Contains(got, want) {
+				t.Errorf(`Package("exports").Imports() = %s, does not contain %s`, got, want)
+			}
+		}
+	}
+}
+
+func TestVersionHandling(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	const dir = "./testdata/versions"
+	list, err := ioutil.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	corruptdir := filepath.Join(tmpdir, "testdata", "versions")
+	if err := os.Mkdir(corruptdir, 0700); err != nil {
+		t.Fatal(err)
+	}
+
+	for _, f := range list {
+		name := f.Name()
+		if !strings.HasSuffix(name, ".a") {
+			continue // not a package file
+		}
+		if strings.Contains(name, "corrupted") {
+			continue // don't process a leftover corrupted file
+		}
+		pkgpath := "./" + name[:len(name)-2]
+
+		if testing.Verbose() {
+			t.Logf("importing %s", name)
+		}
+
+		// test that export data can be imported
+		_, err := Import(make(map[string]*types2.Package), pkgpath, dir, nil)
+		if err != nil {
+			// ok to fail if it fails with a no longer supported error for select files
+			if strings.Contains(err.Error(), "no longer supported") {
+				switch name {
+				case "test_go1.7_0.a", "test_go1.7_1.a",
+					"test_go1.8_4.a", "test_go1.8_5.a",
+					"test_go1.11_6b.a", "test_go1.11_999b.a":
+					continue
+				}
+				// fall through
+			}
+			// ok to fail if it fails with a newer version error for select files
+			if strings.Contains(err.Error(), "newer version") {
+				switch name {
+				case "test_go1.11_999i.a":
+					continue
+				}
+				// fall through
+			}
+			t.Errorf("import %q failed: %v", pkgpath, err)
+			continue
+		}
+
+		// create file with corrupted export data
+		// 1) read file
+		data, err := ioutil.ReadFile(filepath.Join(dir, name))
+		if err != nil {
+			t.Fatal(err)
+		}
+		// 2) find export data
+		i := bytes.Index(data, []byte("\n$$B\n")) + 5
+		j := bytes.Index(data[i:], []byte("\n$$\n")) + i
+		if i < 0 || j < 0 || i > j {
+			t.Fatalf("export data section not found (i = %d, j = %d)", i, j)
+		}
+		// 3) corrupt the data (increment every 7th byte)
+		for k := j - 13; k >= i; k -= 7 {
+			data[k]++
+		}
+		// 4) write the file
+		pkgpath += "_corrupted"
+		filename := filepath.Join(corruptdir, pkgpath) + ".a"
+		ioutil.WriteFile(filename, data, 0666)
+
+		// test that importing the corrupted file results in an error
+		_, err = Import(make(map[string]*types2.Package), pkgpath, corruptdir, nil)
+		if err == nil {
+			t.Errorf("import corrupted %q succeeded", pkgpath)
+		} else if msg := err.Error(); !strings.Contains(msg, "version skew") {
+			t.Errorf("import %q error incorrect (%s)", pkgpath, msg)
+		}
+	}
+}
+
+func TestImportStdLib(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	dt := maxTime
+	if testing.Short() && testenv.Builder() == "" {
+		dt = 10 * time.Millisecond
+	}
+	nimports := testDir(t, "", time.Now().Add(dt)) // installed packages
+	t.Logf("tested %d imports", nimports)
+}
+
+var importedObjectTests = []struct {
+	name string
+	want string
+}{
+	// non-interfaces
+	{"crypto.Hash", "type Hash uint"},
+	{"go/ast.ObjKind", "type ObjKind int"},
+	{"go/types.Qualifier", "type Qualifier func(*Package) string"},
+	{"go/types.Comparable", "func Comparable(T Type) bool"},
+	{"math.Pi", "const Pi untyped float"},
+	{"math.Sin", "func Sin(x float64) float64"},
+	{"go/ast.NotNilFilter", "func NotNilFilter(_ string, v reflect.Value) bool"},
+	{"go/internal/gcimporter.FindPkg", "func FindPkg(path string, srcDir string) (filename string, id string)"},
+
+	// interfaces
+	{"context.Context", "type Context interface{Deadline() (deadline time.Time, ok bool); Done() <-chan struct{}; Err() error; Value(key interface{}) interface{}}"},
+	{"crypto.Decrypter", "type Decrypter interface{Decrypt(rand io.Reader, msg []byte, opts DecrypterOpts) (plaintext []byte, err error); Public() PublicKey}"},
+	{"encoding.BinaryMarshaler", "type BinaryMarshaler interface{MarshalBinary() (data []byte, err error)}"},
+	{"io.Reader", "type Reader interface{Read(p []byte) (n int, err error)}"},
+	{"io.ReadWriter", "type ReadWriter interface{Reader; Writer}"},
+	{"go/ast.Node", "type Node interface{End() go/token.Pos; Pos() go/token.Pos}"},
+	// go/types.Type has grown much larger - excluded for now
+	// {"go/types.Type", "type Type interface{String() string; Underlying() Type}"},
+}
+
+func TestImportedTypes(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	for _, test := range importedObjectTests {
+		s := strings.Split(test.name, ".")
+		if len(s) != 2 {
+			t.Fatal("inconsistent test data")
+		}
+		importPath := s[0]
+		objName := s[1]
+
+		pkg, err := Import(make(map[string]*types2.Package), importPath, ".", nil)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		obj := pkg.Scope().Lookup(objName)
+		if obj == nil {
+			t.Errorf("%s: object not found", test.name)
+			continue
+		}
+
+		got := types2.ObjectString(obj, types2.RelativeTo(pkg))
+		if got != test.want {
+			t.Errorf("%s: got %q; want %q", test.name, got, test.want)
+		}
+
+		if named, _ := obj.Type().(*types2.Named); named != nil {
+			verifyInterfaceMethodRecvs(t, named, 0)
+		}
+	}
+}
+
+// verifyInterfaceMethodRecvs verifies that method receiver types
+// are named if the methods belong to a named interface type.
+func verifyInterfaceMethodRecvs(t *testing.T, named *types2.Named, level int) {
+	// avoid endless recursion in case of an embedding bug that lead to a cycle
+	if level > 10 {
+		t.Errorf("%s: embeds itself", named)
+		return
+	}
+
+	iface, _ := named.Underlying().(*types2.Interface)
+	if iface == nil {
+		return // not an interface
+	}
+
+	// check explicitly declared methods
+	for i := 0; i < iface.NumExplicitMethods(); i++ {
+		m := iface.ExplicitMethod(i)
+		recv := m.Type().(*types2.Signature).Recv()
+		if recv == nil {
+			t.Errorf("%s: missing receiver type", m)
+			continue
+		}
+		if recv.Type() != named {
+			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), named)
+		}
+	}
+
+	// check embedded interfaces (if they are named, too)
+	for i := 0; i < iface.NumEmbeddeds(); i++ {
+		// embedding of interfaces cannot have cycles; recursion will terminate
+		if etype, _ := iface.EmbeddedType(i).(*types2.Named); etype != nil {
+			verifyInterfaceMethodRecvs(t, etype, level+1)
+		}
+	}
+}
+
+func TestIssue5815(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	pkg := importPkg(t, "strings", ".")
+
+	scope := pkg.Scope()
+	for _, name := range scope.Names() {
+		obj := scope.Lookup(name)
+		if obj.Pkg() == nil {
+			t.Errorf("no pkg for %s", obj)
+		}
+		if tname, _ := obj.(*types2.TypeName); tname != nil {
+			named := tname.Type().(*types2.Named)
+			for i := 0; i < named.NumMethods(); i++ {
+				m := named.Method(i)
+				if m.Pkg() == nil {
+					t.Errorf("no pkg for %s", m)
+				}
+			}
+		}
+	}
+}
+
+// Smoke test to ensure that imported methods get the correct package.
+func TestCorrectMethodPackage(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	imports := make(map[string]*types2.Package)
+	_, err := Import(imports, "net/http", ".", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mutex := imports["sync"].Scope().Lookup("Mutex").(*types2.TypeName).Type()
+	mset := types2.NewMethodSet(types2.NewPointer(mutex)) // methods of *sync.Mutex
+	sel := mset.Lookup(nil, "Lock")
+	lock := sel.Obj().(*types2.Func)
+	if got, want := lock.Pkg().Path(), "sync"; got != want {
+		t.Errorf("got package path %q; want %q", got, want)
+	}
+}
+
+func TestIssue13566(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	testoutdir := filepath.Join(tmpdir, "testdata")
+
+	// b.go needs to be compiled from the output directory so that the compiler can
+	// find the compiled package a. We pass the full path to compile() so that we
+	// don't have to copy the file to that directory.
+	bpath, err := filepath.Abs(filepath.Join("testdata", "b.go"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	compile(t, "testdata", "a.go", testoutdir)
+	compile(t, testoutdir, bpath, testoutdir)
+
+	// import must succeed (test for issue at hand)
+	pkg := importPkg(t, "./testdata/b", tmpdir)
+
+	// make sure all indirectly imported packages have names
+	for _, imp := range pkg.Imports() {
+		if imp.Name() == "" {
+			t.Errorf("no name for %s package", imp.Path())
+		}
+	}
+}
+
+func TestIssue13898(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// import go/internal/gcimporter which imports go/types partially
+	imports := make(map[string]*types2.Package)
+	_, err := Import(imports, "go/internal/gcimporter", ".", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// look for go/types package
+	var goTypesPkg *types2.Package
+	for path, pkg := range imports {
+		if path == "go/types" {
+			goTypesPkg = pkg
+			break
+		}
+	}
+	if goTypesPkg == nil {
+		t.Fatal("go/types not found")
+	}
+
+	// look for go/types2.Object type
+	obj := lookupObj(t, goTypesPkg.Scope(), "Object")
+	typ, ok := obj.Type().(*types2.Named)
+	if !ok {
+		t.Fatalf("go/types2.Object type is %v; wanted named type", typ)
+	}
+
+	// lookup go/types2.Object.Pkg method
+	m, index, indirect := types2.LookupFieldOrMethod(typ, false, nil, "Pkg")
+	if m == nil {
+		t.Fatalf("go/types2.Object.Pkg not found (index = %v, indirect = %v)", index, indirect)
+	}
+
+	// the method must belong to go/types
+	if m.Pkg().Path() != "go/types" {
+		t.Fatalf("found %v; want go/types", m.Pkg())
+	}
+}
+
+func TestIssue15517(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+
+	compile(t, "testdata", "p.go", filepath.Join(tmpdir, "testdata"))
+
+	// Multiple imports of p must succeed without redeclaration errors.
+	// We use an import path that's not cleaned up so that the eventual
+	// file path for the package is different from the package path; this
+	// will expose the error if it is present.
+	//
+	// (Issue: Both the textual and the binary importer used the file path
+	// of the package to be imported as key into the shared packages map.
+	// However, the binary importer then used the package path to identify
+	// the imported package to mark it as complete; effectively marking the
+	// wrong package as complete. By using an "unclean" package path, the
+	// file and package path are different, exposing the problem if present.
+	// The same issue occurs with vendoring.)
+	imports := make(map[string]*types2.Package)
+	for i := 0; i < 3; i++ {
+		if _, err := Import(imports, "./././testdata/p", tmpdir, nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestIssue15920(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue15920")
+}
+
+func TestIssue20046(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	// "./issue20046".V.M must exist
+	pkg := compileAndImportPkg(t, "issue20046")
+	obj := lookupObj(t, pkg.Scope(), "V")
+	if m, index, indirect := types2.LookupFieldOrMethod(obj.Type(), false, nil, "M"); m == nil {
+		t.Fatalf("V.M not found (index = %v, indirect = %v)", index, indirect)
+	}
+}
+func TestIssue25301(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue25301")
+}
+
+func TestIssue25596(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue25596")
+}
+
+func importPkg(t *testing.T, path, srcDir string) *types2.Package {
+	pkg, err := Import(make(map[string]*types2.Package), path, srcDir, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return pkg
+}
+
+func compileAndImportPkg(t *testing.T, name string) *types2.Package {
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	compile(t, "testdata", name+".go", filepath.Join(tmpdir, "testdata"))
+	return importPkg(t, "./testdata/"+name, tmpdir)
+}
+
+func lookupObj(t *testing.T, scope *types2.Scope, name string) types2.Object {
+	if obj := scope.Lookup(name); obj != nil {
+		return obj
+	}
+	t.Fatalf("%s not found", name)
+	return nil
+}

src/cmd/compile/internal/importer/iimport.go

@@ -0,0 +1,616 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Indexed package import.
+// See cmd/compile/internal/gc/iexport.go for the export data format.
+
+package importer
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/types2"
+	"encoding/binary"
+	"fmt"
+	"go/constant"
+	"go/token"
+	"io"
+	"sort"
+)
+
+type intReader struct {
+	*bytes.Reader
+	path string
+}
+
+func (r *intReader) int64() int64 {
+	i, err := binary.ReadVarint(r.Reader)
+	if err != nil {
+		errorf("import %q: read varint error: %v", r.path, err)
+	}
+	return i
+}
+
+func (r *intReader) uint64() uint64 {
+	i, err := binary.ReadUvarint(r.Reader)
+	if err != nil {
+		errorf("import %q: read varint error: %v", r.path, err)
+	}
+	return i
+}
+
+const predeclReserved = 32
+
+type itag uint64
+
+const (
+	// Types
+	definedType itag = iota
+	pointerType
+	sliceType
+	arrayType
+	chanType
+	mapType
+	signatureType
+	structType
+	interfaceType
+)
+
+// iImportData imports a package from the serialized package data
+// and returns the number of bytes consumed and a reference to the package.
+// If the export data version is not recognized or the format is otherwise
+// compromised, an error is returned.
+func iImportData(imports map[string]*types2.Package, data []byte, path string) (_ int, pkg *types2.Package, err error) {
+	const currentVersion = 1
+	version := int64(-1)
+	defer func() {
+		if e := recover(); e != nil {
+			if version > currentVersion {
+				err = fmt.Errorf("cannot import %q (%v), export data is newer version - update tool", path, e)
+			} else {
+				err = fmt.Errorf("cannot import %q (%v), possibly version skew - reinstall package", path, e)
+			}
+		}
+	}()
+
+	r := &intReader{bytes.NewReader(data), path}
+
+	version = int64(r.uint64())
+	switch version {
+	case currentVersion, 0:
+	default:
+		errorf("unknown iexport format version %d", version)
+	}
+
+	sLen := int64(r.uint64())
+	dLen := int64(r.uint64())
+
+	whence, _ := r.Seek(0, io.SeekCurrent)
+	stringData := data[whence : whence+sLen]
+	declData := data[whence+sLen : whence+sLen+dLen]
+	r.Seek(sLen+dLen, io.SeekCurrent)
+
+	p := iimporter{
+		ipath:   path,
+		version: int(version),
+
+		stringData:  stringData,
+		stringCache: make(map[uint64]string),
+		pkgCache:    make(map[uint64]*types2.Package),
+
+		declData: declData,
+		pkgIndex: make(map[*types2.Package]map[string]uint64),
+		typCache: make(map[uint64]types2.Type),
+	}
+
+	for i, pt := range predeclared {
+		p.typCache[uint64(i)] = pt
+	}
+
+	pkgList := make([]*types2.Package, r.uint64())
+	for i := range pkgList {
+		pkgPathOff := r.uint64()
+		pkgPath := p.stringAt(pkgPathOff)
+		pkgName := p.stringAt(r.uint64())
+		_ = r.uint64() // package height; unused by go/types
+
+		if pkgPath == "" {
+			pkgPath = path
+		}
+		pkg := imports[pkgPath]
+		if pkg == nil {
+			pkg = types2.NewPackage(pkgPath, pkgName)
+			imports[pkgPath] = pkg
+		} else if pkg.Name() != pkgName {
+			errorf("conflicting names %s and %s for package %q", pkg.Name(), pkgName, path)
+		}
+
+		p.pkgCache[pkgPathOff] = pkg
+
+		nameIndex := make(map[string]uint64)
+		for nSyms := r.uint64(); nSyms > 0; nSyms-- {
+			name := p.stringAt(r.uint64())
+			nameIndex[name] = r.uint64()
+		}
+
+		p.pkgIndex[pkg] = nameIndex
+		pkgList[i] = pkg
+	}
+
+	localpkg := pkgList[0]
+
+	names := make([]string, 0, len(p.pkgIndex[localpkg]))
+	for name := range p.pkgIndex[localpkg] {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		p.doDecl(localpkg, name)
+	}
+
+	for _, typ := range p.interfaceList {
+		typ.Complete()
+	}
+
+	// record all referenced packages as imports
+	list := append(([]*types2.Package)(nil), pkgList[1:]...)
+	sort.Sort(byPath(list))
+	localpkg.SetImports(list)
+
+	// package was imported completely and without errors
+	localpkg.MarkComplete()
+
+	consumed, _ := r.Seek(0, io.SeekCurrent)
+	return int(consumed), localpkg, nil
+}
+
+type iimporter struct {
+	ipath   string
+	version int
+
+	stringData  []byte
+	stringCache map[uint64]string
+	pkgCache    map[uint64]*types2.Package
+
+	declData []byte
+	pkgIndex map[*types2.Package]map[string]uint64
+	typCache map[uint64]types2.Type
+
+	interfaceList []*types2.Interface
+}
+
+func (p *iimporter) doDecl(pkg *types2.Package, name string) {
+	// See if we've already imported this declaration.
+	if obj := pkg.Scope().Lookup(name); obj != nil {
+		return
+	}
+
+	off, ok := p.pkgIndex[pkg][name]
+	if !ok {
+		errorf("%v.%v not in index", pkg, name)
+	}
+
+	r := &importReader{p: p, currPkg: pkg}
+	r.declReader.Reset(p.declData[off:])
+
+	r.obj(name)
+}
+
+func (p *iimporter) stringAt(off uint64) string {
+	if s, ok := p.stringCache[off]; ok {
+		return s
+	}
+
+	slen, n := binary.Uvarint(p.stringData[off:])
+	if n <= 0 {
+		errorf("varint failed")
+	}
+	spos := off + uint64(n)
+	s := string(p.stringData[spos : spos+slen])
+	p.stringCache[off] = s
+	return s
+}
+
+func (p *iimporter) pkgAt(off uint64) *types2.Package {
+	if pkg, ok := p.pkgCache[off]; ok {
+		return pkg
+	}
+	path := p.stringAt(off)
+	errorf("missing package %q in %q", path, p.ipath)
+	return nil
+}
+
+func (p *iimporter) typAt(off uint64, base *types2.Named) types2.Type {
+	if t, ok := p.typCache[off]; ok && (base == nil || !isInterface(t)) {
+		return t
+	}
+
+	if off < predeclReserved {
+		errorf("predeclared type missing from cache: %v", off)
+	}
+
+	r := &importReader{p: p}
+	r.declReader.Reset(p.declData[off-predeclReserved:])
+	t := r.doType(base)
+
+	if base == nil || !isInterface(t) {
+		p.typCache[off] = t
+	}
+	return t
+}
+
+type importReader struct {
+	p          *iimporter
+	declReader bytes.Reader
+	currPkg    *types2.Package
+	prevFile   string
+	prevLine   int64
+	prevColumn int64
+}
+
+func (r *importReader) obj(name string) {
+	tag := r.byte()
+	pos := r.pos()
+
+	switch tag {
+	case 'A':
+		typ := r.typ()
+
+		r.declare(types2.NewTypeName(pos, r.currPkg, name, typ))
+
+	case 'C':
+		typ, val := r.value()
+
+		r.declare(types2.NewConst(pos, r.currPkg, name, typ, val))
+
+	case 'F':
+		sig := r.signature(nil)
+
+		r.declare(types2.NewFunc(pos, r.currPkg, name, sig))
+
+	case 'T':
+		// Types can be recursive. We need to setup a stub
+		// declaration before recursing.
+		obj := types2.NewTypeName(pos, r.currPkg, name, nil)
+		named := types2.NewNamed(obj, nil, nil)
+		r.declare(obj)
+
+		underlying := r.p.typAt(r.uint64(), named).Underlying()
+		named.SetUnderlying(underlying)
+
+		if !isInterface(underlying) {
+			for n := r.uint64(); n > 0; n-- {
+				mpos := r.pos()
+				mname := r.ident()
+				recv := r.param()
+				msig := r.signature(recv)
+
+				named.AddMethod(types2.NewFunc(mpos, r.currPkg, mname, msig))
+			}
+		}
+
+	case 'V':
+		typ := r.typ()
+
+		r.declare(types2.NewVar(pos, r.currPkg, name, typ))
+
+	default:
+		errorf("unexpected tag: %v", tag)
+	}
+}
+
+func (r *importReader) declare(obj types2.Object) {
+	obj.Pkg().Scope().Insert(obj)
+}
+
+func (r *importReader) value() (typ types2.Type, val constant.Value) {
+	typ = r.typ()
+
+	switch b := typ.Underlying().(*types2.Basic); b.Info() & types2.IsConstType {
+	case types2.IsBoolean:
+		val = constant.MakeBool(r.bool())
+
+	case types2.IsString:
+		val = constant.MakeString(r.string())
+
+	case types2.IsInteger:
+		val = r.mpint(b)
+
+	case types2.IsFloat:
+		val = r.mpfloat(b)
+
+	case types2.IsComplex:
+		re := r.mpfloat(b)
+		im := r.mpfloat(b)
+		val = constant.BinaryOp(re, token.ADD, constant.MakeImag(im))
+
+	default:
+		errorf("unexpected type %v", typ) // panics
+		panic("unreachable")
+	}
+
+	return
+}
+
+func intSize(b *types2.Basic) (signed bool, maxBytes uint) {
+	if (b.Info() & types2.IsUntyped) != 0 {
+		return true, 64
+	}
+
+	switch b.Kind() {
+	case types2.Float32, types2.Complex64:
+		return true, 3
+	case types2.Float64, types2.Complex128:
+		return true, 7
+	}
+
+	signed = (b.Info() & types2.IsUnsigned) == 0
+	switch b.Kind() {
+	case types2.Int8, types2.Uint8:
+		maxBytes = 1
+	case types2.Int16, types2.Uint16:
+		maxBytes = 2
+	case types2.Int32, types2.Uint32:
+		maxBytes = 4
+	default:
+		maxBytes = 8
+	}
+
+	return
+}
+
+func (r *importReader) mpint(b *types2.Basic) constant.Value {
+	signed, maxBytes := intSize(b)
+
+	maxSmall := 256 - maxBytes
+	if signed {
+		maxSmall = 256 - 2*maxBytes
+	}
+	if maxBytes == 1 {
+		maxSmall = 256
+	}
+
+	n, _ := r.declReader.ReadByte()
+	if uint(n) < maxSmall {
+		v := int64(n)
+		if signed {
+			v >>= 1
+			if n&1 != 0 {
+				v = ^v
+			}
+		}
+		return constant.MakeInt64(v)
+	}
+
+	v := -n
+	if signed {
+		v = -(n &^ 1) >> 1
+	}
+	if v < 1 || uint(v) > maxBytes {
+		errorf("weird decoding: %v, %v => %v", n, signed, v)
+	}
+
+	buf := make([]byte, v)
+	io.ReadFull(&r.declReader, buf)
+
+	// convert to little endian
+	// TODO(gri) go/constant should have a more direct conversion function
+	//           (e.g., once it supports a big.Float based implementation)
+	for i, j := 0, len(buf)-1; i < j; i, j = i+1, j-1 {
+		buf[i], buf[j] = buf[j], buf[i]
+	}
+
+	x := constant.MakeFromBytes(buf)
+	if signed && n&1 != 0 {
+		x = constant.UnaryOp(token.SUB, x, 0)
+	}
+	return x
+}
+
+func (r *importReader) mpfloat(b *types2.Basic) constant.Value {
+	x := r.mpint(b)
+	if constant.Sign(x) == 0 {
+		return x
+	}
+
+	exp := r.int64()
+	switch {
+	case exp > 0:
+		x = constant.Shift(x, token.SHL, uint(exp))
+	case exp < 0:
+		d := constant.Shift(constant.MakeInt64(1), token.SHL, uint(-exp))
+		x = constant.BinaryOp(x, token.QUO, d)
+	}
+	return x
+}
+
+func (r *importReader) ident() string {
+	return r.string()
+}
+
+func (r *importReader) qualifiedIdent() (*types2.Package, string) {
+	name := r.string()
+	pkg := r.pkg()
+	return pkg, name
+}
+
+func (r *importReader) pos() syntax.Pos {
+	if r.p.version >= 1 {
+		r.posv1()
+	} else {
+		r.posv0()
+	}
+
+	if r.prevFile == "" && r.prevLine == 0 && r.prevColumn == 0 {
+		return syntax.Pos{}
+	}
+	// TODO(gri) fix this
+	// return r.p.fake.pos(r.prevFile, int(r.prevLine), int(r.prevColumn))
+	return syntax.Pos{}
+}
+
+func (r *importReader) posv0() {
+	delta := r.int64()
+	if delta != deltaNewFile {
+		r.prevLine += delta
+	} else if l := r.int64(); l == -1 {
+		r.prevLine += deltaNewFile
+	} else {
+		r.prevFile = r.string()
+		r.prevLine = l
+	}
+}
+
+func (r *importReader) posv1() {
+	delta := r.int64()
+	r.prevColumn += delta >> 1
+	if delta&1 != 0 {
+		delta = r.int64()
+		r.prevLine += delta >> 1
+		if delta&1 != 0 {
+			r.prevFile = r.string()
+		}
+	}
+}
+
+func (r *importReader) typ() types2.Type {
+	return r.p.typAt(r.uint64(), nil)
+}
+
+func isInterface(t types2.Type) bool {
+	_, ok := t.(*types2.Interface)
+	return ok
+}
+
+func (r *importReader) pkg() *types2.Package { return r.p.pkgAt(r.uint64()) }
+func (r *importReader) string() string       { return r.p.stringAt(r.uint64()) }
+
+func (r *importReader) doType(base *types2.Named) types2.Type {
+	switch k := r.kind(); k {
+	default:
+		errorf("unexpected kind tag in %q: %v", r.p.ipath, k)
+		return nil
+
+	case definedType:
+		pkg, name := r.qualifiedIdent()
+		r.p.doDecl(pkg, name)
+		return pkg.Scope().Lookup(name).(*types2.TypeName).Type()
+	case pointerType:
+		return types2.NewPointer(r.typ())
+	case sliceType:
+		return types2.NewSlice(r.typ())
+	case arrayType:
+		n := r.uint64()
+		return types2.NewArray(r.typ(), int64(n))
+	case chanType:
+		dir := chanDir(int(r.uint64()))
+		return types2.NewChan(dir, r.typ())
+	case mapType:
+		return types2.NewMap(r.typ(), r.typ())
+	case signatureType:
+		r.currPkg = r.pkg()
+		return r.signature(nil)
+
+	case structType:
+		r.currPkg = r.pkg()
+
+		fields := make([]*types2.Var, r.uint64())
+		tags := make([]string, len(fields))
+		for i := range fields {
+			fpos := r.pos()
+			fname := r.ident()
+			ftyp := r.typ()
+			emb := r.bool()
+			tag := r.string()
+
+			fields[i] = types2.NewField(fpos, r.currPkg, fname, ftyp, emb)
+			tags[i] = tag
+		}
+		return types2.NewStruct(fields, tags)
+
+	case interfaceType:
+		r.currPkg = r.pkg()
+
+		embeddeds := make([]types2.Type, r.uint64())
+		for i := range embeddeds {
+			_ = r.pos()
+			embeddeds[i] = r.typ()
+		}
+
+		methods := make([]*types2.Func, r.uint64())
+		for i := range methods {
+			mpos := r.pos()
+			mname := r.ident()
+
+			// TODO(mdempsky): Matches bimport.go, but I
+			// don't agree with this.
+			var recv *types2.Var
+			if base != nil {
+				recv = types2.NewVar(syntax.Pos{}, r.currPkg, "", base)
+			}
+
+			msig := r.signature(recv)
+			methods[i] = types2.NewFunc(mpos, r.currPkg, mname, msig)
+		}
+
+		typ := types2.NewInterfaceType(methods, embeddeds)
+		r.p.interfaceList = append(r.p.interfaceList, typ)
+		return typ
+	}
+}
+
+func (r *importReader) kind() itag {
+	return itag(r.uint64())
+}
+
+func (r *importReader) signature(recv *types2.Var) *types2.Signature {
+	params := r.paramList()
+	results := r.paramList()
+	variadic := params.Len() > 0 && r.bool()
+	return types2.NewSignature(recv, params, results, variadic)
+}
+
+func (r *importReader) paramList() *types2.Tuple {
+	xs := make([]*types2.Var, r.uint64())
+	for i := range xs {
+		xs[i] = r.param()
+	}
+	return types2.NewTuple(xs...)
+}
+
+func (r *importReader) param() *types2.Var {
+	pos := r.pos()
+	name := r.ident()
+	typ := r.typ()
+	return types2.NewParam(pos, r.currPkg, name, typ)
+}
+
+func (r *importReader) bool() bool {
+	return r.uint64() != 0
+}
+
+func (r *importReader) int64() int64 {
+	n, err := binary.ReadVarint(&r.declReader)
+	if err != nil {
+		errorf("readVarint: %v", err)
+	}
+	return n
+}
+
+func (r *importReader) uint64() uint64 {
+	n, err := binary.ReadUvarint(&r.declReader)
+	if err != nil {
+		errorf("readUvarint: %v", err)
+	}
+	return n
+}
+
+func (r *importReader) byte() byte {
+	x, err := r.declReader.ReadByte()
+	if err != nil {
+		errorf("declReader.ReadByte: %v", err)
+	}
+	return x
+}

src/cmd/compile/internal/importer/support.go

@@ -0,0 +1,144 @@
+// UNREVIEWED
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements support functionality for iimport.go.
+
+package importer
+
+import (
+	"cmd/compile/internal/types2"
+	"fmt"
+	"go/token"
+	"sync"
+)
+
+func errorf(format string, args ...interface{}) {
+	panic(fmt.Sprintf(format, args...))
+}
+
+const deltaNewFile = -64 // see cmd/compile/internal/gc/bexport.go
+
+// Synthesize a token.Pos
+type fakeFileSet struct {
+	fset  *token.FileSet
+	files map[string]*token.File
+}
+
+func (s *fakeFileSet) pos(file string, line, column int) token.Pos {
+	// TODO(mdempsky): Make use of column.
+
+	// Since we don't know the set of needed file positions, we
+	// reserve maxlines positions per file.
+	const maxlines = 64 * 1024
+	f := s.files[file]
+	if f == nil {
+		f = s.fset.AddFile(file, -1, maxlines)
+		s.files[file] = f
+		// Allocate the fake linebreak indices on first use.
+		// TODO(adonovan): opt: save ~512KB using a more complex scheme?
+		fakeLinesOnce.Do(func() {
+			fakeLines = make([]int, maxlines)
+			for i := range fakeLines {
+				fakeLines[i] = i
+			}
+		})
+		f.SetLines(fakeLines)
+	}
+
+	if line > maxlines {
+		line = 1
+	}
+
+	// Treat the file as if it contained only newlines
+	// and column=1: use the line number as the offset.
+	return f.Pos(line - 1)
+}
+
+var (
+	fakeLines     []int
+	fakeLinesOnce sync.Once
+)
+
+func chanDir(d int) types2.ChanDir {
+	// tag values must match the constants in cmd/compile/internal/gc/go.go
+	switch d {
+	case 1 /* Crecv */ :
+		return types2.RecvOnly
+	case 2 /* Csend */ :
+		return types2.SendOnly
+	case 3 /* Cboth */ :
+		return types2.SendRecv
+	default:
+		errorf("unexpected channel dir %d", d)
+		return 0
+	}
+}
+
+var predeclared = []types2.Type{
+	// basic types
+	types2.Typ[types2.Bool],
+	types2.Typ[types2.Int],
+	types2.Typ[types2.Int8],
+	types2.Typ[types2.Int16],
+	types2.Typ[types2.Int32],
+	types2.Typ[types2.Int64],
+	types2.Typ[types2.Uint],
+	types2.Typ[types2.Uint8],
+	types2.Typ[types2.Uint16],
+	types2.Typ[types2.Uint32],
+	types2.Typ[types2.Uint64],
+	types2.Typ[types2.Uintptr],
+	types2.Typ[types2.Float32],
+	types2.Typ[types2.Float64],
+	types2.Typ[types2.Complex64],
+	types2.Typ[types2.Complex128],
+	types2.Typ[types2.String],
+
+	// basic type aliases
+	types2.Universe.Lookup("byte").Type(),
+	types2.Universe.Lookup("rune").Type(),
+
+	// error
+	types2.Universe.Lookup("error").Type(),
+
+	// untyped types
+	types2.Typ[types2.UntypedBool],
+	types2.Typ[types2.UntypedInt],
+	types2.Typ[types2.UntypedRune],
+	types2.Typ[types2.UntypedFloat],
+	types2.Typ[types2.UntypedComplex],
+	types2.Typ[types2.UntypedString],
+	types2.Typ[types2.UntypedNil],
+
+	// package unsafe
+	types2.Typ[types2.UnsafePointer],
+
+	// invalid type
+	types2.Typ[types2.Invalid], // only appears in packages with errors
+
+	// used internally by gc; never used by this package or in .a files
+	anyType{},
+}
+
+type anyType struct{}
+
+func (t anyType) Underlying() types2.Type { return t }
+func (t anyType) Under() types2.Type      { return t }
+func (t anyType) String() string          { return "any" }
+
+// types2.aType is not exported for now so we need to implemented these here.
+func (anyType) Basic() *types2.Basic         { return nil }
+func (anyType) Array() *types2.Array         { return nil }
+func (anyType) Slice() *types2.Slice         { return nil }
+func (anyType) Struct() *types2.Struct       { return nil }
+func (anyType) Pointer() *types2.Pointer     { return nil }
+func (anyType) Tuple() *types2.Tuple         { return nil }
+func (anyType) Signature() *types2.Signature { return nil }
+func (anyType) Sum() *types2.Sum             { return nil }
+func (anyType) Interface() *types2.Interface { return nil }
+func (anyType) Map() *types2.Map             { return nil }
+func (anyType) Chan() *types2.Chan           { return nil }
+func (anyType) Named() *types2.Named         { return nil }
+func (anyType) TypeParam() *types2.TypeParam { return nil }

src/cmd/compile/internal/importer/testdata/a.go

@@ -0,0 +1,15 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue13566
+
+package a
+
+import "encoding/json"
+
+type A struct {
+	a    *A
+	json json.RawMessage
+}

src/cmd/compile/internal/importer/testdata/b.go

@@ -0,0 +1,12 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue13566
+
+package b
+
+import "./a"
+
+type A a.A

src/cmd/compile/internal/importer/testdata/exports.go

@@ -0,0 +1,89 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is used to generate an object file which
+// serves as test file for gcimporter_test.go.
+
+package exports
+
+import "go/ast"
+
+// Issue 3682: Correctly read dotted identifiers from export data.
+const init1 = 0
+
+func init() {}
+
+const (
+	C0 int = 0
+	C1     = 3.14159265
+	C2     = 2.718281828i
+	C3     = -123.456e-789
+	C4     = +123.456e+789
+	C5     = 1234i
+	C6     = "foo\n"
+	C7     = `bar\n`
+)
+
+type (
+	T1  int
+	T2  [10]int
+	T3  []int
+	T4  *int
+	T5  chan int
+	T6a chan<- int
+	T6b chan (<-chan int)
+	T6c chan<- (chan int)
+	T7  <-chan *ast.File
+	T8  struct{}
+	T9  struct {
+		a    int
+		b, c float32
+		d    []string `go:"tag"`
+	}
+	T10 struct {
+		T8
+		T9
+		_ *T10
+	}
+	T11 map[int]string
+	T12 interface{}
+	T13 interface {
+		m1()
+		m2(int) float32
+	}
+	T14 interface {
+		T12
+		T13
+		m3(x ...struct{}) []T9
+	}
+	T15 func()
+	T16 func(int)
+	T17 func(x int)
+	T18 func() float32
+	T19 func() (x float32)
+	T20 func(...interface{})
+	T21 struct{ next *T21 }
+	T22 struct{ link *T23 }
+	T23 struct{ link *T22 }
+	T24 *T24
+	T25 *T26
+	T26 *T27
+	T27 *T25
+	T28 func(T28) T28
+)
+
+var (
+	V0 int
+	V1         = -991.0
+	V2 float32 = 1.2
+)
+
+func F1()         {}
+func F2(x int)    {}
+func F3() int     { return 0 }
+func F4() float32 { return 0 }
+func F5(a, b, c int, u, v, w struct{ x, y T1 }, more ...interface{}) (p, q, r chan<- T10)
+
+func (p *T1) M1()

src/cmd/compile/internal/importer/testdata/issue15920.go

@@ -0,0 +1,12 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// The underlying type of Error is the underlying type of error.
+// Make sure we can import this again without problems.
+type Error error
+
+func F() Error { return nil }

src/cmd/compile/internal/importer/testdata/issue20046.go

@@ -0,0 +1,10 @@
+// UNREVIEWED
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var V interface {
+	M()
+}

src/cmd/compile/internal/importer/testdata/issue25301.go

@@ -0,0 +1,18 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue25301
+
+type (
+	A = interface {
+		M()
+	}
+	T interface {
+		A
+	}
+	S struct{}
+)
+
+func (S) M() { println("m") }

src/cmd/compile/internal/importer/testdata/issue25596.go

@@ -0,0 +1,14 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue25596
+
+type E interface {
+	M() T
+}
+
+type T interface {
+	E
+}

src/cmd/compile/internal/importer/testdata/p.go

@@ -0,0 +1,14 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue15517
+
+package p
+
+const C = 0
+
+var V int
+
+func F() {}

src/cmd/compile/internal/importer/testdata/versions/test.go

@@ -0,0 +1,29 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To create a test case for a new export format version,
+// build this package with the latest compiler and store
+// the resulting .a file appropriately named in the versions
+// directory. The VersionHandling test will pick it up.
+//
+// In the testdata/versions:
+//
+// go build -o test_go1.$X_$Y.a test.go
+//
+// with $X = Go version and $Y = export format version
+// (add 'b' or 'i' to distinguish between binary and
+// indexed format starting with 1.11 as long as both
+// formats are supported).
+//
+// Make sure this source is extended such that it exercises
+// whatever export format change has taken place.
+
+package test
+
+// Any release before and including Go 1.7 didn't encode
+// the package for a blank struct field.
+type BlankField struct {
+	_ int
+}

src/cmd/compile/internal/types2/api.go

@@ -0,0 +1,426 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package types declares the data types and implements
+// the algorithms for type-checking of Go packages. Use
+// Config.Check to invoke the type checker for a package.
+// Alternatively, create a new type checker with NewChecker
+// and invoke it incrementally by calling Checker.Files.
+//
+// Type-checking consists of several interdependent phases:
+//
+// Name resolution maps each identifier (syntax.Name) in the program to the
+// language object (Object) it denotes.
+// Use Info.{Defs,Uses,Implicits} for the results of name resolution.
+//
+// Constant folding computes the exact constant value (constant.Value)
+// for every expression (syntax.Expr) that is a compile-time constant.
+// Use Info.Types[expr].Value for the results of constant folding.
+//
+// Type inference computes the type (Type) of every expression (syntax.Expr)
+// and checks for compliance with the language specification.
+// Use Info.Types[expr].Type for the results of type inference.
+//
+// For a tutorial, see https://golang.org/s/types-tutorial.
+//
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+)
+
+// An Error describes a type-checking error; it implements the error interface.
+// A "soft" error is an error that still permits a valid interpretation of a
+// package (such as "unused variable"); "hard" errors may lead to unpredictable
+// behavior if ignored.
+type Error struct {
+	Pos  syntax.Pos // error position
+	Msg  string     // default error message, user-friendly
+	Full string     // full error message, for debugging (may contain internal details)
+	Soft bool       // if set, error is "soft"
+}
+
+// Error returns an error string formatted as follows:
+// filename:line:column: message
+func (err Error) Error() string {
+	return fmt.Sprintf("%s: %s", err.Pos, err.Msg)
+}
+
+// FullError returns an error string like Error, buy it may contain
+// type-checker internal details such as subscript indices for type
+// parameters and more. Useful for debugging.
+func (err Error) FullError() string {
+	return fmt.Sprintf("%s: %s", err.Pos, err.Full)
+}
+
+// An Importer resolves import paths to Packages.
+//
+// CAUTION: This interface does not support the import of locally
+// vendored packages. See https://golang.org/s/go15vendor.
+// If possible, external implementations should implement ImporterFrom.
+type Importer interface {
+	// Import returns the imported package for the given import path.
+	// The semantics is like for ImporterFrom.ImportFrom except that
+	// dir and mode are ignored (since they are not present).
+	Import(path string) (*Package, error)
+}
+
+// ImportMode is reserved for future use.
+type ImportMode int
+
+// An ImporterFrom resolves import paths to packages; it
+// supports vendoring per https://golang.org/s/go15vendor.
+// Use go/importer to obtain an ImporterFrom implementation.
+type ImporterFrom interface {
+	// Importer is present for backward-compatibility. Calling
+	// Import(path) is the same as calling ImportFrom(path, "", 0);
+	// i.e., locally vendored packages may not be found.
+	// The types package does not call Import if an ImporterFrom
+	// is present.
+	Importer
+
+	// ImportFrom returns the imported package for the given import
+	// path when imported by a package file located in dir.
+	// If the import failed, besides returning an error, ImportFrom
+	// is encouraged to cache and return a package anyway, if one
+	// was created. This will reduce package inconsistencies and
+	// follow-on type checker errors due to the missing package.
+	// The mode value must be 0; it is reserved for future use.
+	// Two calls to ImportFrom with the same path and dir must
+	// return the same package.
+	ImportFrom(path, dir string, mode ImportMode) (*Package, error)
+}
+
+// A Config specifies the configuration for type checking.
+// The zero value for Config is a ready-to-use default configuration.
+type Config struct {
+	// If IgnoreFuncBodies is set, function bodies are not
+	// type-checked.
+	IgnoreFuncBodies bool
+
+	// If AcceptMethodTypeParams is set, methods may have type parameters.
+	AcceptMethodTypeParams bool
+
+	// If InferFromConstraints is set, constraint type inference is used
+	// if some function type arguments are missing.
+	InferFromConstraints bool
+
+	// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
+	// declares an empty "C" package and errors are omitted for qualified
+	// identifiers referring to package C (which won't find an object).
+	// This feature is intended for the standard library cmd/api tool.
+	//
+	// Caution: Effects may be unpredictable due to follow-on errors.
+	//          Do not use casually!
+	FakeImportC bool
+
+	// If go115UsesCgo is set, the type checker expects the
+	// _cgo_gotypes.go file generated by running cmd/cgo to be
+	// provided as a package source file. Qualified identifiers
+	// referring to package C will be resolved to cgo-provided
+	// declarations within _cgo_gotypes.go.
+	//
+	// It is an error to set both FakeImportC and go115UsesCgo.
+	go115UsesCgo bool
+
+	// If Trace is set, a debug trace is printed to stdout.
+	Trace bool
+
+	// If Error != nil, it is called with each error found
+	// during type checking; err has dynamic type Error.
+	// Secondary errors (for instance, to enumerate all types
+	// involved in an invalid recursive type declaration) have
+	// error strings that start with a '\t' character.
+	// If Error == nil, type-checking stops with the first
+	// error found.
+	Error func(err error)
+
+	// An importer is used to import packages referred to from
+	// import declarations.
+	// If the installed importer implements ImporterFrom, the type
+	// checker calls ImportFrom instead of Import.
+	// The type checker reports an error if an importer is needed
+	// but none was installed.
+	Importer Importer
+
+	// If Sizes != nil, it provides the sizing functions for package unsafe.
+	// Otherwise SizesFor("gc", "amd64") is used instead.
+	Sizes Sizes
+
+	// If DisableUnusedImportCheck is set, packages are not checked
+	// for unused imports.
+	DisableUnusedImportCheck bool
+}
+
+func srcimporter_setUsesCgo(conf *Config) {
+	conf.go115UsesCgo = true
+}
+
+// Info holds result type information for a type-checked package.
+// Only the information for which a map is provided is collected.
+// If the package has type errors, the collected information may
+// be incomplete.
+type Info struct {
+	// Types maps expressions to their types, and for constant
+	// expressions, also their values. Invalid expressions are
+	// omitted.
+	//
+	// For (possibly parenthesized) identifiers denoting built-in
+	// functions, the recorded signatures are call-site specific:
+	// if the call result is not a constant, the recorded type is
+	// an argument-specific signature. Otherwise, the recorded type
+	// is invalid.
+	//
+	// The Types map does not record the type of every identifier,
+	// only those that appear where an arbitrary expression is
+	// permitted. For instance, the identifier f in a selector
+	// expression x.f is found only in the Selections map, the
+	// identifier z in a variable declaration 'var z int' is found
+	// only in the Defs map, and identifiers denoting packages in
+	// qualified identifiers are collected in the Uses map.
+	Types map[syntax.Expr]TypeAndValue
+
+	// Inferred maps calls of parameterized functions that use
+	// type inference to the inferred type arguments and signature
+	// of the function called. The recorded "call" expression may be
+	// an *ast.CallExpr (as in f(x)), or an *ast.IndexExpr (s in f[T]).
+	Inferred map[syntax.Expr]Inferred
+
+	// Defs maps identifiers to the objects they define (including
+	// package names, dots "." of dot-imports, and blank "_" identifiers).
+	// For identifiers that do not denote objects (e.g., the package name
+	// in package clauses, or symbolic variables t in t := x.(type) of
+	// type switch headers), the corresponding objects are nil.
+	//
+	// For an embedded field, Defs returns the field *Var it defines.
+	//
+	// Invariant: Defs[id] == nil || Defs[id].Pos() == id.Pos()
+	Defs map[*syntax.Name]Object
+
+	// Uses maps identifiers to the objects they denote.
+	//
+	// For an embedded field, Uses returns the *TypeName it denotes.
+	//
+	// Invariant: Uses[id].Pos() != id.Pos()
+	Uses map[*syntax.Name]Object
+
+	// Implicits maps nodes to their implicitly declared objects, if any.
+	// The following node and object types may appear:
+	//
+	//     node               declared object
+	//
+	//     *syntax.ImportDecl    *PkgName for imports without renames
+	//     *syntax.CaseClause    type-specific *Var for each type switch case clause (incl. default)
+	//     *syntax.Field         anonymous parameter *Var (incl. unnamed results)
+	//
+	Implicits map[syntax.Node]Object
+
+	// Selections maps selector expressions (excluding qualified identifiers)
+	// to their corresponding selections.
+	Selections map[*syntax.SelectorExpr]*Selection
+
+	// Scopes maps syntax.Nodes to the scopes they define. Package scopes are not
+	// associated with a specific node but with all files belonging to a package.
+	// Thus, the package scope can be found in the type-checked Package object.
+	// Scopes nest, with the Universe scope being the outermost scope, enclosing
+	// the package scope, which contains (one or more) files scopes, which enclose
+	// function scopes which in turn enclose statement and function literal scopes.
+	// Note that even though package-level functions are declared in the package
+	// scope, the function scopes are embedded in the file scope of the file
+	// containing the function declaration.
+	//
+	// The following node types may appear in Scopes:
+	//
+	//     *syntax.File
+	//     *syntax.FuncType
+	//     *syntax.BlockStmt
+	//     *syntax.IfStmt
+	//     *syntax.SwitchStmt
+	//     *syntax.CaseClause
+	//     *syntax.CommClause
+	//     *syntax.ForStmt
+	//
+	Scopes map[syntax.Node]*Scope
+
+	// InitOrder is the list of package-level initializers in the order in which
+	// they must be executed. Initializers referring to variables related by an
+	// initialization dependency appear in topological order, the others appear
+	// in source order. Variables without an initialization expression do not
+	// appear in this list.
+	InitOrder []*Initializer
+}
+
+// TypeOf returns the type of expression e, or nil if not found.
+// Precondition: the Types, Uses and Defs maps are populated.
+//
+func (info *Info) TypeOf(e syntax.Expr) Type {
+	if t, ok := info.Types[e]; ok {
+		return t.Type
+	}
+	if id, _ := e.(*syntax.Name); id != nil {
+		if obj := info.ObjectOf(id); obj != nil {
+			return obj.Type()
+		}
+	}
+	return nil
+}
+
+// ObjectOf returns the object denoted by the specified id,
+// or nil if not found.
+//
+// If id is an embedded struct field, ObjectOf returns the field (*Var)
+// it defines, not the type (*TypeName) it uses.
+//
+// Precondition: the Uses and Defs maps are populated.
+//
+func (info *Info) ObjectOf(id *syntax.Name) Object {
+	if obj := info.Defs[id]; obj != nil {
+		return obj
+	}
+	return info.Uses[id]
+}
+
+// TypeAndValue reports the type and value (for constants)
+// of the corresponding expression.
+type TypeAndValue struct {
+	mode  operandMode
+	Type  Type
+	Value constant.Value
+}
+
+// IsVoid reports whether the corresponding expression
+// is a function call without results.
+func (tv TypeAndValue) IsVoid() bool {
+	return tv.mode == novalue
+}
+
+// IsType reports whether the corresponding expression specifies a type.
+func (tv TypeAndValue) IsType() bool {
+	return tv.mode == typexpr
+}
+
+// IsBuiltin reports whether the corresponding expression denotes
+// a (possibly parenthesized) built-in function.
+func (tv TypeAndValue) IsBuiltin() bool {
+	return tv.mode == builtin
+}
+
+// IsValue reports whether the corresponding expression is a value.
+// Builtins are not considered values. Constant values have a non-
+// nil Value.
+func (tv TypeAndValue) IsValue() bool {
+	switch tv.mode {
+	case constant_, variable, mapindex, value, commaok, commaerr:
+		return true
+	}
+	return false
+}
+
+// IsNil reports whether the corresponding expression denotes the
+// predeclared value nil.
+func (tv TypeAndValue) IsNil() bool {
+	return tv.mode == value && tv.Type == Typ[UntypedNil]
+}
+
+// Addressable reports whether the corresponding expression
+// is addressable (https://golang.org/ref/spec#Address_operators).
+func (tv TypeAndValue) Addressable() bool {
+	return tv.mode == variable
+}
+
+// Assignable reports whether the corresponding expression
+// is assignable to (provided a value of the right type).
+func (tv TypeAndValue) Assignable() bool {
+	return tv.mode == variable || tv.mode == mapindex
+}
+
+// HasOk reports whether the corresponding expression may be
+// used on the rhs of a comma-ok assignment.
+func (tv TypeAndValue) HasOk() bool {
+	return tv.mode == commaok || tv.mode == mapindex
+}
+
+// Inferred reports the inferred type arguments and signature
+// for a parameterized function call that uses type inference.
+type Inferred struct {
+	Targs []Type
+	Sig   *Signature
+}
+
+// An Initializer describes a package-level variable, or a list of variables in case
+// of a multi-valued initialization expression, and the corresponding initialization
+// expression.
+type Initializer struct {
+	Lhs []*Var // var Lhs = Rhs
+	Rhs syntax.Expr
+}
+
+func (init *Initializer) String() string {
+	var buf bytes.Buffer
+	for i, lhs := range init.Lhs {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		buf.WriteString(lhs.Name())
+	}
+	buf.WriteString(" = ")
+	WriteExpr(&buf, init.Rhs)
+	return buf.String()
+}
+
+// Check type-checks a package and returns the resulting package object and
+// the first error if any. Additionally, if info != nil, Check populates each
+// of the non-nil maps in the Info struct.
+//
+// The package is marked as complete if no errors occurred, otherwise it is
+// incomplete. See Config.Error for controlling behavior in the presence of
+// errors.
+//
+// The package is specified by a list of *syntax.Files and corresponding
+// file set, and the package path the package is identified with.
+// The clean path must not be empty or dot (".").
+func (conf *Config) Check(path string, files []*syntax.File, info *Info) (*Package, error) {
+	pkg := NewPackage(path, "")
+	return pkg, NewChecker(conf, pkg, info).Files(files)
+}
+
+// AssertableTo reports whether a value of type V can be asserted to have type T.
+func AssertableTo(V *Interface, T Type) bool {
+	m, _ := (*Checker)(nil).assertableTo(V, T, false)
+	return m == nil
+}
+
+// AssignableTo reports whether a value of type V is assignable to a variable of type T.
+func AssignableTo(V, T Type) bool {
+	x := operand{mode: value, typ: V}
+	return x.assignableTo(nil, T, nil) // check not needed for non-constant x
+}
+
+// ConvertibleTo reports whether a value of type V is convertible to a value of type T.
+func ConvertibleTo(V, T Type) bool {
+	x := operand{mode: value, typ: V}
+	return x.convertibleTo(nil, T) // check not needed for non-constant x
+}
+
+// Implements reports whether type V implements interface T.
+func Implements(V Type, T *Interface) bool {
+	f, _ := MissingMethod(V, T, true)
+	return f == nil
+}
+
+// Identical reports whether x and y are identical types.
+// Receivers of Signature types are ignored.
+func Identical(x, y Type) bool {
+	return (*Checker)(nil).identical(x, y)
+}
+
+// IdenticalIgnoreTags reports whether x and y are identical types if tags are ignored.
+// Receivers of Signature types are ignored.
+func IdenticalIgnoreTags(x, y Type) bool {
+	return (*Checker)(nil).identicalIgnoreTags(x, y)
+}

src/cmd/compile/internal/types2/assignments.go

@@ -0,0 +1,359 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements initialization and assignment checks.
+
+package types2
+
+import "cmd/compile/internal/syntax"
+
+// assignment reports whether x can be assigned to a variable of type T,
+// if necessary by attempting to convert untyped values to the appropriate
+// type. context describes the context in which the assignment takes place.
+// Use T == nil to indicate assignment to an untyped blank identifier.
+// x.mode is set to invalid if the assignment failed.
+func (check *Checker) assignment(x *operand, T Type, context string) {
+	check.singleValue(x)
+
+	switch x.mode {
+	case invalid:
+		return // error reported before
+	case constant_, variable, mapindex, value, commaok, commaerr:
+		// ok
+	default:
+		// we may get here because of other problems (issue #39634, crash 12)
+		check.errorf(x, "cannot assign %s to %s in %s", x, T, context)
+		return
+	}
+
+	if isUntyped(x.typ) {
+		target := T
+		// spec: "If an untyped constant is assigned to a variable of interface
+		// type or the blank identifier, the constant is first converted to type
+		// bool, rune, int, float64, complex128 or string respectively, depending
+		// on whether the value is a boolean, rune, integer, floating-point, complex,
+		// or string constant."
+		if T == nil || IsInterface(T) {
+			if T == nil && x.typ == Typ[UntypedNil] {
+				check.errorf(x, "use of untyped nil in %s", context)
+				x.mode = invalid
+				return
+			}
+			target = Default(x.typ)
+		}
+		check.convertUntyped(x, target)
+		if x.mode == invalid {
+			return
+		}
+	}
+	// x.typ is typed
+
+	// A generic (non-instantiated) function value cannot be assigned to a variable.
+	if sig := x.typ.Signature(); sig != nil && len(sig.tparams) > 0 {
+		check.errorf(x, "cannot use generic function %s without instantiation in %s", x, context)
+	}
+
+	// spec: "If a left-hand side is the blank identifier, any typed or
+	// non-constant value except for the predeclared identifier nil may
+	// be assigned to it."
+	if T == nil {
+		return
+	}
+
+	if reason := ""; !x.assignableTo(check, T, &reason) {
+		if reason != "" {
+			check.errorf(x, "cannot use %s as %s value in %s: %s", x, T, context, reason)
+		} else {
+			check.errorf(x, "cannot use %s as %s value in %s", x, T, context)
+		}
+		x.mode = invalid
+	}
+}
+
+func (check *Checker) initConst(lhs *Const, x *operand) {
+	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return
+	}
+
+	// rhs must be a constant
+	if x.mode != constant_ {
+		check.errorf(x, "%s is not constant", x)
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return
+	}
+	assert(isConstType(x.typ))
+
+	// If the lhs doesn't have a type yet, use the type of x.
+	if lhs.typ == nil {
+		lhs.typ = x.typ
+	}
+
+	check.assignment(x, lhs.typ, "constant declaration")
+	if x.mode == invalid {
+		return
+	}
+
+	lhs.val = x.val
+}
+
+func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
+	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return nil
+	}
+
+	// If the lhs doesn't have a type yet, use the type of x.
+	if lhs.typ == nil {
+		typ := x.typ
+		if isUntyped(typ) {
+			// convert untyped types to default types
+			if typ == Typ[UntypedNil] {
+				check.errorf(x, "use of untyped nil in %s", context)
+				lhs.typ = Typ[Invalid]
+				return nil
+			}
+			typ = Default(typ)
+		}
+		lhs.typ = typ
+	}
+
+	check.assignment(x, lhs.typ, context)
+	if x.mode == invalid {
+		return nil
+	}
+
+	return x.typ
+}
+
+func (check *Checker) assignVar(lhs syntax.Expr, x *operand) Type {
+	if x.mode == invalid || x.typ == Typ[Invalid] {
+		check.useLHS(lhs)
+		return nil
+	}
+
+	// Determine if the lhs is a (possibly parenthesized) identifier.
+	ident, _ := unparen(lhs).(*syntax.Name)
+
+	// Don't evaluate lhs if it is the blank identifier.
+	if ident != nil && ident.Value == "_" {
+		check.recordDef(ident, nil)
+		check.assignment(x, nil, "assignment to _ identifier")
+		if x.mode == invalid {
+			return nil
+		}
+		return x.typ
+	}
+
+	// If the lhs is an identifier denoting a variable v, this assignment
+	// is not a 'use' of v. Remember current value of v.used and restore
+	// after evaluating the lhs via check.expr.
+	var v *Var
+	var v_used bool
+	if ident != nil {
+		if obj := check.lookup(ident.Value); obj != nil {
+			// It's ok to mark non-local variables, but ignore variables
+			// from other packages to avoid potential race conditions with
+			// dot-imported variables.
+			if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
+				v = w
+				v_used = v.used
+			}
+		}
+	}
+
+	var z operand
+	check.expr(&z, lhs)
+	if v != nil {
+		v.used = v_used // restore v.used
+	}
+
+	if z.mode == invalid || z.typ == Typ[Invalid] {
+		return nil
+	}
+
+	// spec: "Each left-hand side operand must be addressable, a map index
+	// expression, or the blank identifier. Operands may be parenthesized."
+	switch z.mode {
+	case invalid:
+		return nil
+	case variable, mapindex:
+		// ok
+	default:
+		if sel, ok := z.expr.(*syntax.SelectorExpr); ok {
+			var op operand
+			check.expr(&op, sel.X)
+			if op.mode == mapindex {
+				check.errorf(&z, "cannot assign to struct field %s in map", ExprString(z.expr))
+				return nil
+			}
+		}
+		check.errorf(&z, "cannot assign to %s", &z)
+		return nil
+	}
+
+	check.assignment(x, z.typ, "assignment")
+	if x.mode == invalid {
+		return nil
+	}
+
+	return x.typ
+}
+
+// If returnPos is valid, initVars is called to type-check the assignment of
+// return expressions, and returnPos is the position of the return statement.
+func (check *Checker) initVars(lhs []*Var, orig_rhs []syntax.Expr, returnPos syntax.Pos) {
+	rhs, commaOk := check.exprList(orig_rhs, len(lhs) == 2 && !returnPos.IsKnown())
+
+	if len(lhs) != len(rhs) {
+		// invalidate lhs
+		for _, obj := range lhs {
+			if obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+		}
+		// don't report an error if we already reported one
+		for _, x := range rhs {
+			if x.mode == invalid {
+				return
+			}
+		}
+		if returnPos.IsKnown() {
+			check.errorf(returnPos, "wrong number of return values (want %d, got %d)", len(lhs), len(rhs))
+			return
+		}
+		check.errorf(rhs[0], "cannot initialize %d variables with %d values", len(lhs), len(rhs))
+		return
+	}
+
+	context := "assignment"
+	if returnPos.IsKnown() {
+		context = "return statement"
+	}
+
+	if commaOk {
+		var a [2]Type
+		for i := range a {
+			a[i] = check.initVar(lhs[i], rhs[i], context)
+		}
+		check.recordCommaOkTypes(orig_rhs[0], a)
+		return
+	}
+
+	for i, lhs := range lhs {
+		check.initVar(lhs, rhs[i], context)
+	}
+}
+
+func (check *Checker) assignVars(lhs, orig_rhs []syntax.Expr) {
+	rhs, commaOk := check.exprList(orig_rhs, len(lhs) == 2)
+
+	if len(lhs) != len(rhs) {
+		check.useLHS(lhs...)
+		// don't report an error if we already reported one
+		for _, x := range rhs {
+			if x.mode == invalid {
+				return
+			}
+		}
+		check.errorf(rhs[0], "cannot assign %d values to %d variables", len(rhs), len(lhs))
+		return
+	}
+
+	if commaOk {
+		var a [2]Type
+		for i := range a {
+			a[i] = check.assignVar(lhs[i], rhs[i])
+		}
+		check.recordCommaOkTypes(orig_rhs[0], a)
+		return
+	}
+
+	for i, lhs := range lhs {
+		check.assignVar(lhs, rhs[i])
+	}
+}
+
+// unpack unpacks a *syntax.ListExpr into a list of syntax.Expr.
+// Helper introduced for the go/types -> types2 port.
+// TODO(gri) Should find a more efficient solution that doesn't
+//           require introduction of a new slice for simple
+//           expressions.
+func unpackExpr(x syntax.Expr) []syntax.Expr {
+	if x, _ := x.(*syntax.ListExpr); x != nil {
+		return x.ElemList
+	}
+	if x != nil {
+		return []syntax.Expr{x}
+	}
+	return nil
+}
+
+func (check *Checker) shortVarDecl(pos syntax.Pos, lhs, rhs []syntax.Expr) {
+	top := len(check.delayed)
+	scope := check.scope
+
+	// collect lhs variables
+	var newVars []*Var
+	var lhsVars = make([]*Var, len(lhs))
+	for i, lhs := range lhs {
+		var obj *Var
+		if ident, _ := lhs.(*syntax.Name); ident != nil {
+			// Use the correct obj if the ident is redeclared. The
+			// variable's scope starts after the declaration; so we
+			// must use Scope.Lookup here and call Scope.Insert
+			// (via check.declare) later.
+			name := ident.Value
+			if alt := scope.Lookup(name); alt != nil {
+				// redeclared object must be a variable
+				if alt, _ := alt.(*Var); alt != nil {
+					obj = alt
+				} else {
+					check.errorf(lhs, "cannot assign to %s", lhs)
+				}
+				check.recordUse(ident, alt)
+			} else {
+				// declare new variable, possibly a blank (_) variable
+				obj = NewVar(ident.Pos(), check.pkg, name, nil)
+				if name != "_" {
+					newVars = append(newVars, obj)
+				}
+				check.recordDef(ident, obj)
+			}
+		} else {
+			check.useLHS(lhs)
+			check.errorf(lhs, "cannot declare %s", lhs)
+		}
+		if obj == nil {
+			obj = NewVar(lhs.Pos(), check.pkg, "_", nil) // dummy variable
+		}
+		lhsVars[i] = obj
+	}
+
+	check.initVars(lhsVars, rhs, nopos)
+
+	// process function literals in rhs expressions before scope changes
+	check.processDelayed(top)
+
+	// declare new variables
+	if len(newVars) > 0 {
+		// spec: "The scope of a constant or variable identifier declared inside
+		// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
+		// for short variable declarations) and ends at the end of the innermost
+		// containing block."
+		scopePos := endPos(rhs[len(rhs)-1])
+		for _, obj := range newVars {
+			check.declare(scope, nil, obj, scopePos) // recordObject already called
+		}
+	} else {
+		check.softErrorf(pos, "no new variables on left side of :=")
+	}
+}

src/cmd/compile/internal/types2/builtins.go

@@ -0,0 +1,777 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of builtin function calls.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"go/constant"
+	"go/token"
+)
+
+// builtin type-checks a call to the built-in specified by id and
+// reports whether the call is valid, with *x holding the result;
+// but x.expr is not set. If the call is invalid, the result is
+// false, and *x is undefined.
+//
+func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (_ bool) {
+	// append is the only built-in that permits the use of ... for the last argument
+	bin := predeclaredFuncs[id]
+	if call.HasDots && id != _Append {
+		//check.invalidOpf(call.Ellipsis, "invalid use of ... with built-in %s", bin.name)
+		check.invalidOpf(call, "invalid use of ... with built-in %s", bin.name)
+		check.use(call.ArgList...)
+		return
+	}
+
+	// For len(x) and cap(x) we need to know if x contains any function calls or
+	// receive operations. Save/restore current setting and set hasCallOrRecv to
+	// false for the evaluation of x so that we can check it afterwards.
+	// Note: We must do this _before_ calling exprList because exprList evaluates
+	//       all arguments.
+	if id == _Len || id == _Cap {
+		defer func(b bool) {
+			check.hasCallOrRecv = b
+		}(check.hasCallOrRecv)
+		check.hasCallOrRecv = false
+	}
+
+	// determine actual arguments
+	var arg func(*operand, int) // TODO(gri) remove use of arg getter in favor of using xlist directly
+	nargs := len(call.ArgList)
+	switch id {
+	default:
+		// make argument getter
+		xlist, _ := check.exprList(call.ArgList, false)
+		arg = func(x *operand, i int) { *x = *xlist[i]; x.typ = expand(x.typ) }
+		nargs = len(xlist)
+		// evaluate first argument, if present
+		if nargs > 0 {
+			arg(x, 0)
+			if x.mode == invalid {
+				return
+			}
+		}
+	case _Make, _New, _Offsetof, _Trace:
+		// arguments require special handling
+	}
+
+	// check argument count
+	{
+		msg := ""
+		if nargs < bin.nargs {
+			msg = "not enough"
+		} else if !bin.variadic && nargs > bin.nargs {
+			msg = "too many"
+		}
+		if msg != "" {
+			check.invalidOpf(call, "%s arguments for %s (expected %d, found %d)", msg, call, bin.nargs, nargs)
+			return
+		}
+	}
+
+	switch id {
+	case _Append:
+		// append(s S, x ...T) S, where T is the element type of S
+		// spec: "The variadic function append appends zero or more values x to s of type
+		// S, which must be a slice type, and returns the resulting slice, also of type S.
+		// The values x are passed to a parameter of type ...T where T is the element type
+		// of S and the respective parameter passing rules apply."
+		S := x.typ
+		var T Type
+		if s := S.Slice(); s != nil {
+			T = s.elem
+		} else {
+			check.invalidArgf(x, "%s is not a slice", x)
+			return
+		}
+
+		// remember arguments that have been evaluated already
+		alist := []operand{*x}
+
+		// spec: "As a special case, append also accepts a first argument assignable
+		// to type []byte with a second argument of string type followed by ... .
+		// This form appends the bytes of the string.
+		if nargs == 2 && call.HasDots && x.assignableTo(check, NewSlice(universeByte), nil) {
+			arg(x, 1)
+			if x.mode == invalid {
+				return
+			}
+			if isString(x.typ) {
+				if check.Types != nil {
+					sig := makeSig(S, S, x.typ)
+					sig.variadic = true
+					check.recordBuiltinType(call.Fun, sig)
+				}
+				x.mode = value
+				x.typ = S
+				break
+			}
+			alist = append(alist, *x)
+			// fallthrough
+		}
+
+		// check general case by creating custom signature
+		sig := makeSig(S, S, NewSlice(T)) // []T required for variadic signature
+		sig.variadic = true
+		var xlist []*operand
+		// convert []operand to []*operand
+		for i := range alist {
+			xlist = append(xlist, &alist[i])
+		}
+		for i := len(alist); i < nargs; i++ {
+			var x operand
+			arg(&x, i)
+			xlist = append(xlist, &x)
+		}
+		check.arguments(call, sig, xlist) // discard result (we know the result type)
+		// ok to continue even if check.arguments reported errors
+
+		x.mode = value
+		x.typ = S
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, sig)
+		}
+
+	case _Cap, _Len:
+		// cap(x)
+		// len(x)
+		mode := invalid
+		var typ Type
+		var val constant.Value
+		switch typ = implicitArrayDeref(optype(x.typ.Under())); t := typ.(type) {
+		case *Basic:
+			if isString(t) && id == _Len {
+				if x.mode == constant_ {
+					mode = constant_
+					val = constant.MakeInt64(int64(len(constant.StringVal(x.val))))
+				} else {
+					mode = value
+				}
+			}
+
+		case *Array:
+			mode = value
+			// spec: "The expressions len(s) and cap(s) are constants
+			// if the type of s is an array or pointer to an array and
+			// the expression s does not contain channel receives or
+			// function calls; in this case s is not evaluated."
+			if !check.hasCallOrRecv {
+				mode = constant_
+				if t.len >= 0 {
+					val = constant.MakeInt64(t.len)
+				} else {
+					val = constant.MakeUnknown()
+				}
+			}
+
+		case *Slice, *Chan:
+			mode = value
+
+		case *Map:
+			if id == _Len {
+				mode = value
+			}
+
+		case *Sum:
+			if t.is(func(t Type) bool {
+				switch t := t.Under().(type) {
+				case *Basic:
+					if isString(t) && id == _Len {
+						return true
+					}
+				case *Array, *Slice, *Chan:
+					return true
+				case *Map:
+					if id == _Len {
+						return true
+					}
+				}
+				return false
+			}) {
+				mode = value
+			}
+		}
+
+		if mode == invalid && typ != Typ[Invalid] {
+			check.invalidArgf(x, "%s for %s", x, bin.name)
+			return
+		}
+
+		x.mode = mode
+		x.typ = Typ[Int]
+		x.val = val
+		if check.Types != nil && mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, typ))
+		}
+
+	case _Close:
+		// close(c)
+		c := x.typ.Chan()
+		if c == nil {
+			check.invalidArgf(x, "%s is not a channel", x)
+			return
+		}
+		if c.dir == RecvOnly {
+			check.invalidArgf(x, "%s must not be a receive-only channel", x)
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, c))
+		}
+
+	case _Complex:
+		// complex(x, y floatT) complexT
+		var y operand
+		arg(&y, 1)
+		if y.mode == invalid {
+			return
+		}
+
+		// convert or check untyped arguments
+		d := 0
+		if isUntyped(x.typ) {
+			d |= 1
+		}
+		if isUntyped(y.typ) {
+			d |= 2
+		}
+		switch d {
+		case 0:
+			// x and y are typed => nothing to do
+		case 1:
+			// only x is untyped => convert to type of y
+			check.convertUntyped(x, y.typ)
+		case 2:
+			// only y is untyped => convert to type of x
+			check.convertUntyped(&y, x.typ)
+		case 3:
+			// x and y are untyped =>
+			// 1) if both are constants, convert them to untyped
+			//    floating-point numbers if possible,
+			// 2) if one of them is not constant (possible because
+			//    it contains a shift that is yet untyped), convert
+			//    both of them to float64 since they must have the
+			//    same type to succeed (this will result in an error
+			//    because shifts of floats are not permitted)
+			if x.mode == constant_ && y.mode == constant_ {
+				toFloat := func(x *operand) {
+					if isNumeric(x.typ) && constant.Sign(constant.Imag(x.val)) == 0 {
+						x.typ = Typ[UntypedFloat]
+					}
+				}
+				toFloat(x)
+				toFloat(&y)
+			} else {
+				check.convertUntyped(x, Typ[Float64])
+				check.convertUntyped(&y, Typ[Float64])
+				// x and y should be invalid now, but be conservative
+				// and check below
+			}
+		}
+		if x.mode == invalid || y.mode == invalid {
+			return
+		}
+
+		// both argument types must be identical
+		if !check.identical(x.typ, y.typ) {
+			check.invalidArgf(x, "mismatched types %s and %s", x.typ, y.typ)
+			return
+		}
+
+		// the argument types must be of floating-point type
+		f := func(x Type) Type {
+			if t := x.Basic(); t != nil {
+				switch t.kind {
+				case Float32:
+					return Typ[Complex64]
+				case Float64:
+					return Typ[Complex128]
+				case UntypedFloat:
+					return Typ[UntypedComplex]
+				}
+			}
+			return nil
+		}
+		resTyp := check.applyTypeFunc(f, x.typ)
+		if resTyp == nil {
+			check.invalidArgf(x, "arguments have type %s, expected floating-point", x.typ)
+			return
+		}
+
+		// if both arguments are constants, the result is a constant
+		if x.mode == constant_ && y.mode == constant_ {
+			x.val = constant.BinaryOp(constant.ToFloat(x.val), token.ADD, constant.MakeImag(constant.ToFloat(y.val)))
+		} else {
+			x.mode = value
+		}
+
+		if check.Types != nil && x.mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ, x.typ))
+		}
+
+		x.typ = resTyp
+
+	case _Copy:
+		// copy(x, y []T) int
+		var dst Type
+		if t := x.typ.Slice(); t != nil {
+			dst = t.elem
+		}
+
+		var y operand
+		arg(&y, 1)
+		if y.mode == invalid {
+			return
+		}
+		var src Type
+		switch t := optype(y.typ.Under()).(type) {
+		case *Basic:
+			if isString(y.typ) {
+				src = universeByte
+			}
+		case *Slice:
+			src = t.elem
+		}
+
+		if dst == nil || src == nil {
+			check.invalidArgf(x, "copy expects slice arguments; found %s and %s", x, &y)
+			return
+		}
+
+		if !check.identical(dst, src) {
+			check.invalidArgf(x, "arguments to copy %s and %s have different element types %s and %s", x, &y, dst, src)
+			return
+		}
+
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(Typ[Int], x.typ, y.typ))
+		}
+		x.mode = value
+		x.typ = Typ[Int]
+
+	case _Delete:
+		// delete(m, k)
+		m := x.typ.Map()
+		if m == nil {
+			check.invalidArgf(x, "%s is not a map", x)
+			return
+		}
+		arg(x, 1) // k
+		if x.mode == invalid {
+			return
+		}
+
+		if !x.assignableTo(check, m.key, nil) {
+			check.invalidArgf(x, "%s is not assignable to %s", x, m.key)
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, m, m.key))
+		}
+
+	case _Imag, _Real:
+		// imag(complexT) floatT
+		// real(complexT) floatT
+
+		// convert or check untyped argument
+		if isUntyped(x.typ) {
+			if x.mode == constant_ {
+				// an untyped constant number can always be considered
+				// as a complex constant
+				if isNumeric(x.typ) {
+					x.typ = Typ[UntypedComplex]
+				}
+			} else {
+				// an untyped non-constant argument may appear if
+				// it contains a (yet untyped non-constant) shift
+				// expression: convert it to complex128 which will
+				// result in an error (shift of complex value)
+				check.convertUntyped(x, Typ[Complex128])
+				// x should be invalid now, but be conservative and check
+				if x.mode == invalid {
+					return
+				}
+			}
+		}
+
+		// the argument must be of complex type
+		f := func(x Type) Type {
+			if t := x.Basic(); t != nil {
+				switch t.kind {
+				case Complex64:
+					return Typ[Float32]
+				case Complex128:
+					return Typ[Float64]
+				case UntypedComplex:
+					return Typ[UntypedFloat]
+				}
+			}
+			return nil
+		}
+		resTyp := check.applyTypeFunc(f, x.typ)
+		if resTyp == nil {
+			check.invalidArgf(x, "argument has type %s, expected complex type", x.typ)
+			return
+		}
+
+		// if the argument is a constant, the result is a constant
+		if x.mode == constant_ {
+			if id == _Real {
+				x.val = constant.Real(x.val)
+			} else {
+				x.val = constant.Imag(x.val)
+			}
+		} else {
+			x.mode = value
+		}
+
+		if check.Types != nil && x.mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ))
+		}
+
+		x.typ = resTyp
+
+	case _Make:
+		// make(T, n)
+		// make(T, n, m)
+		// (no argument evaluated yet)
+		arg0 := call.ArgList[0]
+		T := check.varType(arg0)
+		if T == Typ[Invalid] {
+			return
+		}
+
+		min, max := -1, 10
+		var valid func(t Type) bool
+		valid = func(t Type) bool {
+			var m int
+			switch t := optype(t.Under()).(type) {
+			case *Slice:
+				m = 2
+			case *Map, *Chan:
+				m = 1
+			case *Sum:
+				return t.is(valid)
+			default:
+				return false
+			}
+			if m > min {
+				min = m
+			}
+			if m+1 < max {
+				max = m + 1
+			}
+			return true
+		}
+
+		if !valid(T) {
+			check.invalidArgf(arg0, "cannot make %s; type must be slice, map, or channel", arg0)
+			return
+		}
+		if nargs < min || max < nargs {
+			if min == max {
+				check.errorf(call, "%v expects %d arguments; found %d", call, min, nargs)
+			} else {
+				check.errorf(call, "%v expects %d or %d arguments; found %d", call, min, max, nargs)
+			}
+			return
+		}
+
+		types := []Type{T}
+		var sizes []int64 // constant integer arguments, if any
+		for _, arg := range call.ArgList[1:] {
+			typ, size := check.index(arg, -1) // ok to continue with typ == Typ[Invalid]
+			types = append(types, typ)
+			if size >= 0 {
+				sizes = append(sizes, size)
+			}
+		}
+		if len(sizes) == 2 && sizes[0] > sizes[1] {
+			check.invalidArgf(call.ArgList[1], "length and capacity swapped")
+			// safe to continue
+		}
+		x.mode = value
+		x.typ = T
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, types...))
+		}
+
+	case _New:
+		// new(T)
+		// (no argument evaluated yet)
+		T := check.varType(call.ArgList[0])
+		if T == Typ[Invalid] {
+			return
+		}
+
+		x.mode = value
+		x.typ = &Pointer{base: T}
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, T))
+		}
+
+	case _Panic:
+		// panic(x)
+		// record panic call if inside a function with result parameters
+		// (for use in Checker.isTerminating)
+		if check.sig != nil && check.sig.results.Len() > 0 {
+			// function has result parameters
+			p := check.isPanic
+			if p == nil {
+				// allocate lazily
+				p = make(map[*syntax.CallExpr]bool)
+				check.isPanic = p
+			}
+			p[call] = true
+		}
+
+		check.assignment(x, &emptyInterface, "argument to panic")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, &emptyInterface))
+		}
+
+	case _Print, _Println:
+		// print(x, y, ...)
+		// println(x, y, ...)
+		var params []Type
+		if nargs > 0 {
+			params = make([]Type, nargs)
+			for i := 0; i < nargs; i++ {
+				if i > 0 {
+					arg(x, i) // first argument already evaluated
+				}
+				check.assignment(x, nil, "argument to "+predeclaredFuncs[id].name)
+				if x.mode == invalid {
+					// TODO(gri) "use" all arguments?
+					return
+				}
+				params[i] = x.typ
+			}
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, params...))
+		}
+
+	case _Recover:
+		// recover() interface{}
+		x.mode = value
+		x.typ = &emptyInterface
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ))
+		}
+
+	case _Alignof:
+		// unsafe.Alignof(x T) uintptr
+		if x.typ.TypeParam() != nil {
+			check.invalidOpf(call, "unsafe.Alignof undefined for %s", x)
+			return
+		}
+		check.assignment(x, nil, "argument to unsafe.Alignof")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = constant_
+		x.val = constant.MakeInt64(check.conf.alignof(x.typ))
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Offsetof:
+		// unsafe.Offsetof(x T) uintptr, where x must be a selector
+		// (no argument evaluated yet)
+		arg0 := call.ArgList[0]
+		selx, _ := unparen(arg0).(*syntax.SelectorExpr)
+		if selx == nil {
+			check.invalidArgf(arg0, "%s is not a selector expression", arg0)
+			check.use(arg0)
+			return
+		}
+
+		check.expr(x, selx.X)
+		if x.mode == invalid {
+			return
+		}
+
+		base := derefStructPtr(x.typ)
+		sel := selx.Sel.Value
+		obj, index, indirect := check.lookupFieldOrMethod(base, false, check.pkg, sel)
+		switch obj.(type) {
+		case nil:
+			check.invalidArgf(x, "%s has no single field %s", base, sel)
+			return
+		case *Func:
+			// TODO(gri) Using derefStructPtr may result in methods being found
+			// that don't actually exist. An error either way, but the error
+			// message is confusing. See: https://play.golang.org/p/al75v23kUy ,
+			// but go/types reports: "invalid argument: x.m is a method value".
+			check.invalidArgf(arg0, "%s is a method value", arg0)
+			return
+		}
+		if indirect {
+			check.invalidArgf(x, "field %s is embedded via a pointer in %s", sel, base)
+			return
+		}
+
+		// TODO(gri) Should we pass x.typ instead of base (and indirect report if derefStructPtr indirected)?
+		check.recordSelection(selx, FieldVal, base, obj, index, false)
+
+		offs := check.conf.offsetof(base, index)
+		x.mode = constant_
+		x.val = constant.MakeInt64(offs)
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Sizeof:
+		// unsafe.Sizeof(x T) uintptr
+		if x.typ.TypeParam() != nil {
+			check.invalidOpf(call, "unsafe.Sizeof undefined for %s", x)
+			return
+		}
+		check.assignment(x, nil, "argument to unsafe.Sizeof")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = constant_
+		x.val = constant.MakeInt64(check.conf.sizeof(x.typ))
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Assert:
+		// assert(pred) causes a typechecker error if pred is false.
+		// The result of assert is the value of pred if there is no error.
+		// Note: assert is only available in self-test mode.
+		if x.mode != constant_ || !isBoolean(x.typ) {
+			check.invalidArgf(x, "%s is not a boolean constant", x)
+			return
+		}
+		if x.val.Kind() != constant.Bool {
+			check.errorf(x, "internal error: value of %s should be a boolean constant", x)
+			return
+		}
+		if !constant.BoolVal(x.val) {
+			check.errorf(call, "%v failed", call)
+			// compile-time assertion failure - safe to continue
+		}
+		// result is constant - no need to record signature
+
+	case _Trace:
+		// trace(x, y, z, ...) dumps the positions, expressions, and
+		// values of its arguments. The result of trace is the value
+		// of the first argument.
+		// Note: trace is only available in self-test mode.
+		// (no argument evaluated yet)
+		if nargs == 0 {
+			check.dump("%v: trace() without arguments", posFor(call))
+			x.mode = novalue
+			break
+		}
+		var t operand
+		x1 := x
+		for _, arg := range call.ArgList {
+			check.rawExpr(x1, arg, nil) // permit trace for types, e.g.: new(trace(T))
+			check.dump("%v: %s", posFor(x1), x1)
+			x1 = &t // use incoming x only for first argument
+		}
+		// trace is only available in test mode - no need to record signature
+
+	default:
+		unreachable()
+	}
+
+	return true
+}
+
+// applyTypeFunc applies f to x. If x is a type parameter,
+// the result is a type parameter constrained by an new
+// interface bound. The type bounds for that interface
+// are computed by applying f to each of the type bounds
+// of x. If any of these applications of f return nil,
+// applyTypeFunc returns nil.
+// If x is not a type parameter, the result is f(x).
+func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
+	if tp := x.TypeParam(); tp != nil {
+		// Test if t satisfies the requirements for the argument
+		// type and collect possible result types at the same time.
+		var rtypes []Type
+		if !tp.Bound().is(func(x Type) bool {
+			if r := f(x); r != nil {
+				rtypes = append(rtypes, r)
+				return true
+			}
+			return false
+		}) {
+			return nil
+		}
+
+		// TODO(gri) Would it be ok to return just the one type
+		//           if len(rtypes) == 1? What about top-level
+		//           uses of real() where the result is used to
+		//           define type and initialize a variable?
+
+		// construct a suitable new type parameter
+		tpar := NewTypeName(nopos, nil /* = Universe pkg */, "<type parameter>", nil)
+		ptyp := check.NewTypeParam(tp.ptr, tpar, 0, &emptyInterface) // assigns type to tpar as a side-effect
+		tsum := NewSum(rtypes)
+		ptyp.bound = &Interface{types: tsum, allMethods: markComplete, allTypes: tsum}
+
+		return ptyp
+	}
+
+	return f(x)
+}
+
+// makeSig makes a signature for the given argument and result types.
+// Default types are used for untyped arguments, and res may be nil.
+func makeSig(res Type, args ...Type) *Signature {
+	list := make([]*Var, len(args))
+	for i, param := range args {
+		list[i] = NewVar(nopos, nil, "", Default(param))
+	}
+	params := NewTuple(list...)
+	var result *Tuple
+	if res != nil {
+		assert(!isUntyped(res))
+		result = NewTuple(NewVar(nopos, nil, "", res))
+	}
+	return &Signature{params: params, results: result}
+}
+
+// implicitArrayDeref returns A if typ is of the form *A and A is an array;
+// otherwise it returns typ.
+//
+func implicitArrayDeref(typ Type) Type {
+	if p, ok := typ.(*Pointer); ok {
+		if a := p.base.Array(); a != nil {
+			return a
+		}
+	}
+	return typ
+}
+
+// unparen returns e with any enclosing parentheses stripped.
+func unparen(e syntax.Expr) syntax.Expr {
+	for {
+		p, ok := e.(*syntax.ParenExpr)
+		if !ok {
+			return e
+		}
+		e = p.X
+	}
+}

src/cmd/compile/internal/types2/builtins_test.go

@@ -0,0 +1,219 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2_test
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"testing"
+
+	. "cmd/compile/internal/types2"
+)
+
+var builtinCalls = []struct {
+	name, src, sig string
+}{
+	{"append", `var s []int; _ = append(s)`, `func([]int, ...int) []int`},
+	{"append", `var s []int; _ = append(s, 0)`, `func([]int, ...int) []int`},
+	{"append", `var s []int; _ = (append)(s, 0)`, `func([]int, ...int) []int`},
+	{"append", `var s []byte; _ = ((append))(s, 0)`, `func([]byte, ...byte) []byte`},
+	{"append", `var s []byte; _ = append(s, "foo"...)`, `func([]byte, string...) []byte`},
+	{"append", `type T []byte; var s T; var str string; _ = append(s, str...)`, `func(p.T, string...) p.T`},
+	{"append", `type T []byte; type U string; var s T; var str U; _ = append(s, str...)`, `func(p.T, p.U...) p.T`},
+
+	{"cap", `var s [10]int; _ = cap(s)`, `invalid type`},  // constant
+	{"cap", `var s [10]int; _ = cap(&s)`, `invalid type`}, // constant
+	{"cap", `var s []int64; _ = cap(s)`, `func([]int64) int`},
+	{"cap", `var c chan<-bool; _ = cap(c)`, `func(chan<- bool) int`},
+
+	{"len", `_ = len("foo")`, `invalid type`}, // constant
+	{"len", `var s string; _ = len(s)`, `func(string) int`},
+	{"len", `var s [10]int; _ = len(s)`, `invalid type`},  // constant
+	{"len", `var s [10]int; _ = len(&s)`, `invalid type`}, // constant
+	{"len", `var s []int64; _ = len(s)`, `func([]int64) int`},
+	{"len", `var c chan<-bool; _ = len(c)`, `func(chan<- bool) int`},
+	{"len", `var m map[string]float32; _ = len(m)`, `func(map[string]float32) int`},
+
+	{"close", `var c chan int; close(c)`, `func(chan int)`},
+	{"close", `var c chan<- chan string; close(c)`, `func(chan<- chan string)`},
+
+	{"complex", `_ = complex(1, 0)`, `invalid type`}, // constant
+	{"complex", `var re float32; _ = complex(re, 1.0)`, `func(float32, float32) complex64`},
+	{"complex", `var im float64; _ = complex(1, im)`, `func(float64, float64) complex128`},
+	{"complex", `type F32 float32; var re, im F32; _ = complex(re, im)`, `func(p.F32, p.F32) complex64`},
+	{"complex", `type F64 float64; var re, im F64; _ = complex(re, im)`, `func(p.F64, p.F64) complex128`},
+
+	{"copy", `var src, dst []byte; copy(dst, src)`, `func([]byte, []byte) int`},
+	{"copy", `type T [][]int; var src, dst T; _ = copy(dst, src)`, `func(p.T, p.T) int`},
+	{"copy", `var src string; var dst []byte; copy(dst, src)`, `func([]byte, string) int`},
+	{"copy", `type T string; type U []byte; var src T; var dst U; copy(dst, src)`, `func(p.U, p.T) int`},
+	{"copy", `var dst []byte; copy(dst, "hello")`, `func([]byte, string) int`},
+
+	{"delete", `var m map[string]bool; delete(m, "foo")`, `func(map[string]bool, string)`},
+	{"delete", `type (K string; V int); var m map[K]V; delete(m, "foo")`, `func(map[p.K]p.V, p.K)`},
+
+	{"imag", `_ = imag(1i)`, `invalid type`}, // constant
+	{"imag", `var c complex64; _ = imag(c)`, `func(complex64) float32`},
+	{"imag", `var c complex128; _ = imag(c)`, `func(complex128) float64`},
+	{"imag", `type C64 complex64; var c C64; _ = imag(c)`, `func(p.C64) float32`},
+	{"imag", `type C128 complex128; var c C128; _ = imag(c)`, `func(p.C128) float64`},
+
+	{"real", `_ = real(1i)`, `invalid type`}, // constant
+	{"real", `var c complex64; _ = real(c)`, `func(complex64) float32`},
+	{"real", `var c complex128; _ = real(c)`, `func(complex128) float64`},
+	{"real", `type C64 complex64; var c C64; _ = real(c)`, `func(p.C64) float32`},
+	{"real", `type C128 complex128; var c C128; _ = real(c)`, `func(p.C128) float64`},
+
+	{"make", `_ = make([]int, 10)`, `func([]int, int) []int`},
+	{"make", `type T []byte; _ = make(T, 10, 20)`, `func(p.T, int, int) p.T`},
+
+	// issue #37349
+	{"make", `              _ = make([]int, 0   )`, `func([]int, int) []int`},
+	{"make", `var l    int; _ = make([]int, l   )`, `func([]int, int) []int`},
+	{"make", `              _ = make([]int, 0, 0)`, `func([]int, int, int) []int`},
+	{"make", `var l    int; _ = make([]int, l, 0)`, `func([]int, int, int) []int`},
+	{"make", `var    c int; _ = make([]int, 0, c)`, `func([]int, int, int) []int`},
+	{"make", `var l, c int; _ = make([]int, l, c)`, `func([]int, int, int) []int`},
+
+	// issue #37393
+	{"make", `                _ = make([]int       , 0   )`, `func([]int, int) []int`},
+	{"make", `var l    byte ; _ = make([]int8      , l   )`, `func([]int8, byte) []int8`},
+	{"make", `                _ = make([]int16     , 0, 0)`, `func([]int16, int, int) []int16`},
+	{"make", `var l    int16; _ = make([]string    , l, 0)`, `func([]string, int16, int) []string`},
+	{"make", `var    c int32; _ = make([]float64   , 0, c)`, `func([]float64, int, int32) []float64`},
+	{"make", `var l, c uint ; _ = make([]complex128, l, c)`, `func([]complex128, uint, uint) []complex128`},
+
+	{"new", `_ = new(int)`, `func(int) *int`},
+	{"new", `type T struct{}; _ = new(T)`, `func(p.T) *p.T`},
+
+	{"panic", `panic(0)`, `func(interface{})`},
+	{"panic", `panic("foo")`, `func(interface{})`},
+
+	{"print", `print()`, `func()`},
+	{"print", `print(0)`, `func(int)`},
+	{"print", `print(1, 2.0, "foo", true)`, `func(int, float64, string, bool)`},
+
+	{"println", `println()`, `func()`},
+	{"println", `println(0)`, `func(int)`},
+	{"println", `println(1, 2.0, "foo", true)`, `func(int, float64, string, bool)`},
+
+	{"recover", `recover()`, `func() interface{}`},
+	{"recover", `_ = recover()`, `func() interface{}`},
+
+	{"Alignof", `_ = unsafe.Alignof(0)`, `invalid type`},                 // constant
+	{"Alignof", `var x struct{}; _ = unsafe.Alignof(x)`, `invalid type`}, // constant
+
+	{"Offsetof", `var x struct{f bool}; _ = unsafe.Offsetof(x.f)`, `invalid type`},           // constant
+	{"Offsetof", `var x struct{_ int; f bool}; _ = unsafe.Offsetof((&x).f)`, `invalid type`}, // constant
+
+	{"Sizeof", `_ = unsafe.Sizeof(0)`, `invalid type`},                 // constant
+	{"Sizeof", `var x struct{}; _ = unsafe.Sizeof(x)`, `invalid type`}, // constant
+
+	{"assert", `assert(true)`, `invalid type`},                                    // constant
+	{"assert", `type B bool; const pred B = 1 < 2; assert(pred)`, `invalid type`}, // constant
+
+	// no tests for trace since it produces output as a side-effect
+}
+
+func TestBuiltinSignatures(t *testing.T) {
+	DefPredeclaredTestFuncs()
+
+	seen := map[string]bool{"trace": true} // no test for trace built-in; add it manually
+	for _, call := range builtinCalls {
+		testBuiltinSignature(t, call.name, call.src, call.sig)
+		seen[call.name] = true
+	}
+
+	// make sure we didn't miss one
+	for _, name := range Universe.Names() {
+		if _, ok := Universe.Lookup(name).(*Builtin); ok && !seen[name] {
+			t.Errorf("missing test for %s", name)
+		}
+	}
+	for _, name := range Unsafe.Scope().Names() {
+		if _, ok := Unsafe.Scope().Lookup(name).(*Builtin); ok && !seen[name] {
+			t.Errorf("missing test for unsafe.%s", name)
+		}
+	}
+}
+
+func testBuiltinSignature(t *testing.T, name, src0, want string) {
+	src := fmt.Sprintf(`package p; import "unsafe"; type _ unsafe.Pointer /* use unsafe */; func _() { %s }`, src0)
+	f, err := parseSrc("", src)
+	if err != nil {
+		t.Errorf("%s: %s", src0, err)
+		return
+	}
+
+	conf := Config{Importer: defaultImporter()}
+	uses := make(map[*syntax.Name]Object)
+	types := make(map[syntax.Expr]TypeAndValue)
+	_, err = conf.Check(f.PkgName.Value, []*syntax.File{f}, &Info{Uses: uses, Types: types})
+	if err != nil {
+		t.Errorf("%s: %s", src0, err)
+		return
+	}
+
+	// find called function
+	n := 0
+	var fun syntax.Expr
+	for x := range types {
+		if call, _ := x.(*syntax.CallExpr); call != nil {
+			fun = call.Fun
+			n++
+		}
+	}
+	if n != 1 {
+		t.Errorf("%s: got %d CallExprs; want 1", src0, n)
+		return
+	}
+
+	// check recorded types for fun and descendents (may be parenthesized)
+	for {
+		// the recorded type for the built-in must match the wanted signature
+		typ := types[fun].Type
+		if typ == nil {
+			t.Errorf("%s: no type recorded for %s", src0, ExprString(fun))
+			return
+		}
+		if got := typ.String(); got != want {
+			t.Errorf("%s: got type %s; want %s", src0, got, want)
+			return
+		}
+
+		// called function must be a (possibly parenthesized, qualified)
+		// identifier denoting the expected built-in
+		switch p := fun.(type) {
+		case *syntax.Name:
+			obj := uses[p]
+			if obj == nil {
+				t.Errorf("%s: no object found for %s", src0, p.Value)
+				return
+			}
+			bin, _ := obj.(*Builtin)
+			if bin == nil {
+				t.Errorf("%s: %s does not denote a built-in", src0, p.Value)
+				return
+			}
+			if bin.Name() != name {
+				t.Errorf("%s: got built-in %s; want %s", src0, bin.Name(), name)
+				return
+			}
+			return // we're done
+
+		case *syntax.ParenExpr:
+			fun = p.X // unpack
+
+		case *syntax.SelectorExpr:
+			// built-in from package unsafe - ignore details
+			return // we're done
+
+		default:
+			t.Errorf("%s: invalid function call", src0)
+			return
+		}
+	}
+}

src/cmd/compile/internal/types2/call.go

@@ -0,0 +1,808 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of call and selector expressions.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"strings"
+	"unicode"
+)
+
+// funcInst type-checks a function instantiaton inst and returns the result in x.
+// The operand x must be the evaluation of inst.X and its type must be a signature.
+func (check *Checker) funcInst(x *operand, inst *syntax.IndexExpr) {
+	args, ok := check.exprOrTypeList(unpackExpr(inst.Index))
+	if ok && len(args) > 0 && args[0].mode != typexpr {
+		check.errorf(args[0], "%s is not a type", args[0])
+		ok = false
+	}
+	if !ok {
+		x.mode = invalid
+		x.expr = inst
+		return
+	}
+
+	// check number of type arguments
+	n := len(args)
+	sig := x.typ.(*Signature)
+	if !check.conf.InferFromConstraints && n != len(sig.tparams) || n > len(sig.tparams) {
+		check.errorf(args[n-1], "got %d type arguments but want %d", n, len(sig.tparams))
+		x.mode = invalid
+		x.expr = inst
+		return
+	}
+
+	// collect types
+	targs := make([]Type, n)
+	poslist := make([]syntax.Pos, n)
+	for i, a := range args {
+		if a.mode != typexpr {
+			// error was reported earlier
+			x.mode = invalid
+			x.expr = inst
+			return
+		}
+		targs[i] = a.typ
+		poslist[i] = a.Pos()
+	}
+
+	// if we don't have enough type arguments, use constraint type inference
+	var inferred bool
+	if n < len(sig.tparams) {
+		var failed int
+		targs, failed = check.inferB(sig.tparams, targs)
+		if targs == nil {
+			// error was already reported
+			x.mode = invalid
+			x.expr = inst
+			return
+		}
+		if failed >= 0 {
+			// at least one type argument couldn't be inferred
+			assert(targs[failed] == nil)
+			tpar := sig.tparams[failed]
+			check.errorf(inst, "cannot infer %s (%s) (%s)", tpar.name, tpar.pos, targs)
+			x.mode = invalid
+			x.expr = inst
+			return
+		}
+		// all type arguments were inferred sucessfully
+		if debug {
+			for _, targ := range targs {
+				assert(targ != nil)
+			}
+		}
+		//check.dump("### inferred targs = %s", targs)
+		n = len(targs)
+		inferred = true
+	}
+	assert(n == len(sig.tparams))
+
+	// instantiate function signature
+	for i, typ := range targs {
+		// some positions may be missing if types are inferred
+		var pos syntax.Pos
+		if i < len(poslist) {
+			pos = poslist[i]
+		}
+		check.ordinaryType(pos, typ)
+	}
+	res := check.instantiate(x.Pos(), sig, targs, poslist).(*Signature)
+	assert(res.tparams == nil) // signature is not generic anymore
+	if inferred {
+		check.recordInferred(inst, targs, res)
+	}
+	x.typ = res
+	x.mode = value
+	x.expr = inst
+}
+
+func (check *Checker) call(x *operand, call *syntax.CallExpr) exprKind {
+	check.exprOrType(x, call.Fun)
+
+	switch x.mode {
+	case invalid:
+		check.use(call.ArgList...)
+		x.expr = call
+		return statement
+
+	case typexpr:
+		// conversion
+		T := x.typ
+		x.mode = invalid
+		switch n := len(call.ArgList); n {
+		case 0:
+			check.errorf(call, "missing argument in conversion to %s", T)
+		case 1:
+			check.expr(x, call.ArgList[0])
+			if x.mode != invalid {
+				if t := T.Interface(); t != nil {
+					check.completeInterface(nopos, t)
+					if t.IsConstraint() {
+						check.errorf(call, "cannot use interface %s in conversion (contains type list or is comparable)", T)
+						break
+					}
+				}
+				check.conversion(x, T)
+			}
+		default:
+			check.use(call.ArgList...)
+			check.errorf(call.ArgList[n-1], "too many arguments in conversion to %s", T)
+		}
+		x.expr = call
+		return conversion
+
+	case builtin:
+		id := x.id
+		if !check.builtin(x, call, id) {
+			x.mode = invalid
+		}
+		x.expr = call
+		// a non-constant result implies a function call
+		if x.mode != invalid && x.mode != constant_ {
+			check.hasCallOrRecv = true
+		}
+		return predeclaredFuncs[id].kind
+
+	default:
+		// function/method call
+		cgocall := x.mode == cgofunc
+
+		sig := x.typ.Signature()
+		if sig == nil {
+			check.invalidOpf(x, "cannot call non-function %s", x)
+			x.mode = invalid
+			x.expr = call
+			return statement
+		}
+
+		// evaluate arguments
+		args, ok := check.exprOrTypeList(call.ArgList)
+		if !ok {
+			x.mode = invalid
+			x.expr = call
+			return expression
+		}
+
+		sig = check.arguments(call, sig, args)
+
+		// determine result
+		switch sig.results.Len() {
+		case 0:
+			x.mode = novalue
+		case 1:
+			if cgocall {
+				x.mode = commaerr
+			} else {
+				x.mode = value
+			}
+			x.typ = sig.results.vars[0].typ // unpack tuple
+		default:
+			x.mode = value
+			x.typ = sig.results
+		}
+		x.expr = call
+		check.hasCallOrRecv = true
+
+		// if type inference failed, a parametrized result must be invalidated
+		// (operands cannot have a parametrized type)
+		if x.mode == value && len(sig.tparams) > 0 && isParameterized(sig.tparams, x.typ) {
+			x.mode = invalid
+		}
+
+		return statement
+	}
+}
+
+// exprOrTypeList returns a list of operands and reports an error if the
+// list contains a mix of values and types (ignoring invalid operands).
+// TODO(gri) Now we can split this into exprList and typeList.
+func (check *Checker) exprOrTypeList(elist []syntax.Expr) (xlist []*operand, ok bool) {
+	ok = true
+
+	switch len(elist) {
+	case 0:
+		// nothing to do
+
+	case 1:
+		// single (possibly comma-ok) value or type, or function returning multiple values
+		e := elist[0]
+		var x operand
+		check.multiExprOrType(&x, e)
+		if t, ok := x.typ.(*Tuple); ok && x.mode != invalid && x.mode != typexpr {
+			// multiple values
+			xlist = make([]*operand, t.Len())
+			for i, v := range t.vars {
+				xlist[i] = &operand{mode: value, expr: e, typ: v.typ}
+			}
+			break
+		}
+
+		check.instantiatedOperand(&x)
+
+		// exactly one (possibly invalid or comma-ok) value or type
+		xlist = []*operand{&x}
+
+	default:
+		// multiple (possibly invalid) values or types
+		xlist = make([]*operand, len(elist))
+		ntypes := 0
+		for i, e := range elist {
+			var x operand
+			check.exprOrType(&x, e)
+			xlist[i] = &x
+			switch x.mode {
+			case invalid:
+				ntypes = len(xlist) // make 'if' condition fail below (no additional error in this case)
+			case typexpr:
+				ntypes++
+				check.instantiatedOperand(&x)
+			}
+		}
+		if 0 < ntypes && ntypes < len(xlist) {
+			check.errorf(xlist[0], "mix of value and type expressions")
+			ok = false
+		}
+	}
+
+	return
+}
+
+func (check *Checker) exprList(elist []syntax.Expr, allowCommaOk bool) (xlist []*operand, commaOk bool) {
+	switch len(elist) {
+	case 0:
+		// nothing to do
+
+	case 1:
+		// single (possibly comma-ok) value, or function returning multiple values
+		e := elist[0]
+		var x operand
+		check.multiExpr(&x, e)
+		if t, ok := x.typ.(*Tuple); ok && x.mode != invalid {
+			// multiple values
+			xlist = make([]*operand, t.Len())
+			for i, v := range t.vars {
+				xlist[i] = &operand{mode: value, expr: e, typ: v.typ}
+			}
+			break
+		}
+
+		// exactly one (possibly invalid or comma-ok) value
+		xlist = []*operand{&x}
+		if allowCommaOk && (x.mode == mapindex || x.mode == commaok || x.mode == commaerr) {
+			x.mode = value
+			xlist = append(xlist, &operand{mode: value, expr: e, typ: Typ[UntypedBool]})
+			commaOk = true
+		}
+
+	default:
+		// multiple (possibly invalid) values
+		xlist = make([]*operand, len(elist))
+		for i, e := range elist {
+			var x operand
+			check.expr(&x, e)
+			xlist[i] = &x
+		}
+	}
+
+	return
+}
+
+func (check *Checker) arguments(call *syntax.CallExpr, sig *Signature, args []*operand) (rsig *Signature) {
+	rsig = sig
+
+	// TODO(gri) try to eliminate this extra verification loop
+	for _, a := range args {
+		switch a.mode {
+		case typexpr:
+			check.errorf(a, "%s used as value", a)
+			return
+		case invalid:
+			return
+		}
+	}
+
+	// Function call argument/parameter count requirements
+	//
+	//               | standard call    | dotdotdot call |
+	// --------------+------------------+----------------+
+	// standard func | nargs == npars   | invalid        |
+	// --------------+------------------+----------------+
+	// variadic func | nargs >= npars-1 | nargs == npars |
+	// --------------+------------------+----------------+
+
+	nargs := len(args)
+	npars := sig.params.Len()
+	ddd := call.HasDots
+
+	// set up parameters
+	sig_params := sig.params // adjusted for variadic functions (may be nil for empty parameter lists!)
+	adjusted := false        // indicates if sig_params is different from t.params
+	if sig.variadic {
+		if ddd {
+			// variadic_func(a, b, c...)
+			if len(call.ArgList) == 1 && nargs > 1 {
+				// f()... is not permitted if f() is multi-valued
+				//check.errorf(call.Ellipsis, "cannot use ... with %d-valued %s", nargs, call.ArgList[0])
+				check.errorf(call, "cannot use ... with %d-valued %s", nargs, call.ArgList[0])
+				return
+			}
+		} else {
+			// variadic_func(a, b, c)
+			if nargs >= npars-1 {
+				// Create custom parameters for arguments: keep
+				// the first npars-1 parameters and add one for
+				// each argument mapping to the ... parameter.
+				vars := make([]*Var, npars-1) // npars > 0 for variadic functions
+				copy(vars, sig.params.vars)
+				last := sig.params.vars[npars-1]
+				typ := last.typ.(*Slice).elem
+				for len(vars) < nargs {
+					vars = append(vars, NewParam(last.pos, last.pkg, last.name, typ))
+				}
+				sig_params = NewTuple(vars...) // possibly nil!
+				adjusted = true
+				npars = nargs
+			} else {
+				// nargs < npars-1
+				npars-- // for correct error message below
+			}
+		}
+	} else {
+		if ddd {
+			// standard_func(a, b, c...)
+			//check.errorf(call.Ellipsis, "cannot use ... in call to non-variadic %s", call.Fun)
+			check.errorf(call, "cannot use ... in call to non-variadic %s", call.Fun)
+			return
+		}
+		// standard_func(a, b, c)
+	}
+
+	// check argument count
+	switch {
+	case nargs < npars:
+		check.errorf(call, "not enough arguments in call to %s", call.Fun)
+		return
+	case nargs > npars:
+		check.errorf(args[npars], "too many arguments in call to %s", call.Fun) // report at first extra argument
+		return
+	}
+
+	// infer type arguments and instantiate signature if necessary
+	if len(sig.tparams) > 0 {
+		// TODO(gri) provide position information for targs so we can feed
+		//           it to the instantiate call for better error reporting
+		targs, failed := check.infer(sig.tparams, sig_params, args)
+		if targs == nil {
+			return // error already reported
+		}
+		if failed >= 0 {
+			// Some type arguments couldn't be inferred. Use
+			// bounds type inference to try to make progress.
+			if check.conf.InferFromConstraints {
+				targs, failed = check.inferB(sig.tparams, targs)
+				if targs == nil {
+					return // error already reported
+				}
+			}
+			if failed >= 0 {
+				// at least one type argument couldn't be inferred
+				assert(targs[failed] == nil)
+				tpar := sig.tparams[failed]
+				// TODO(gri) here we'd like to use the position of the call's ')'
+				check.errorf(call.Pos(), "cannot infer %s (%s) (%s)", tpar.name, tpar.pos, targs)
+				return
+			}
+		}
+		// all type arguments were inferred sucessfully
+		if debug {
+			for _, targ := range targs {
+				assert(targ != nil)
+			}
+		}
+		//check.dump("### inferred targs = %s", targs)
+
+		// compute result signature
+		rsig = check.instantiate(call.Pos(), sig, targs, nil).(*Signature)
+		assert(rsig.tparams == nil) // signature is not generic anymore
+		check.recordInferred(call, targs, rsig)
+
+		// Optimization: Only if the parameter list was adjusted do we
+		// need to compute it from the adjusted list; otherwise we can
+		// simply use the result signature's parameter list.
+		if adjusted {
+			sig_params = check.subst(call.Pos(), sig_params, makeSubstMap(sig.tparams, targs)).(*Tuple)
+		} else {
+			sig_params = rsig.params
+		}
+	}
+
+	// check arguments
+	for i, a := range args {
+		check.assignment(a, sig_params.vars[i].typ, "argument")
+	}
+
+	return
+}
+
+var cgoPrefixes = [...]string{
+	"_Ciconst_",
+	"_Cfconst_",
+	"_Csconst_",
+	"_Ctype_",
+	"_Cvar_", // actually a pointer to the var
+	"_Cfpvar_fp_",
+	"_Cfunc_",
+	"_Cmacro_", // function to evaluate the expanded expression
+}
+
+func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
+	// these must be declared before the "goto Error" statements
+	var (
+		obj      Object
+		index    []int
+		indirect bool
+	)
+
+	sel := e.Sel.Value
+	// If the identifier refers to a package, handle everything here
+	// so we don't need a "package" mode for operands: package names
+	// can only appear in qualified identifiers which are mapped to
+	// selector expressions.
+	if ident, ok := e.X.(*syntax.Name); ok {
+		obj := check.lookup(ident.Value)
+		if pname, _ := obj.(*PkgName); pname != nil {
+			assert(pname.pkg == check.pkg)
+			check.recordUse(ident, pname)
+			pname.used = true
+			pkg := pname.imported
+
+			var exp Object
+			funcMode := value
+			if pkg.cgo {
+				// cgo special cases C.malloc: it's
+				// rewritten to _CMalloc and does not
+				// support two-result calls.
+				if sel == "malloc" {
+					sel = "_CMalloc"
+				} else {
+					funcMode = cgofunc
+				}
+				for _, prefix := range cgoPrefixes {
+					// cgo objects are part of the current package (in file
+					// _cgo_gotypes.go). Use regular lookup.
+					_, exp = check.scope.LookupParent(prefix+sel, check.pos)
+					if exp != nil {
+						break
+					}
+				}
+				if exp == nil {
+					check.errorf(e.Sel, "%s not declared by package C", sel)
+					goto Error
+				}
+				check.objDecl(exp, nil)
+			} else {
+				exp = pkg.scope.Lookup(sel)
+				if exp == nil {
+					if !pkg.fake {
+						check.errorf(e.Sel, "%s not declared by package %s", sel, pkg.name)
+					}
+					goto Error
+				}
+				if !exp.Exported() {
+					check.errorf(e.Sel, "%s not exported by package %s", sel, pkg.name)
+					// ok to continue
+				}
+			}
+			check.recordUse(e.Sel, exp)
+
+			// Simplified version of the code for *syntax.Names:
+			// - imported objects are always fully initialized
+			switch exp := exp.(type) {
+			case *Const:
+				assert(exp.Val() != nil)
+				x.mode = constant_
+				x.typ = exp.typ
+				x.val = exp.val
+			case *TypeName:
+				x.mode = typexpr
+				x.typ = exp.typ
+			case *Var:
+				x.mode = variable
+				x.typ = exp.typ
+				if pkg.cgo && strings.HasPrefix(exp.name, "_Cvar_") {
+					x.typ = x.typ.(*Pointer).base
+				}
+			case *Func:
+				x.mode = funcMode
+				x.typ = exp.typ
+				if pkg.cgo && strings.HasPrefix(exp.name, "_Cmacro_") {
+					x.mode = value
+					x.typ = x.typ.(*Signature).results.vars[0].typ
+				}
+			case *Builtin:
+				x.mode = builtin
+				x.typ = exp.typ
+				x.id = exp.id
+			default:
+				check.dump("%v: unexpected object %v", posFor(e.Sel), exp)
+				unreachable()
+			}
+			x.expr = e
+			return
+		}
+	}
+
+	check.exprOrType(x, e.X)
+	if x.mode == invalid {
+		goto Error
+	}
+
+	check.instantiatedOperand(x)
+
+	obj, index, indirect = check.lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
+	if obj == nil {
+		switch {
+		case index != nil:
+			// TODO(gri) should provide actual type where the conflict happens
+			check.errorf(e.Sel, "ambiguous selector %s.%s", x.expr, sel)
+		case indirect:
+			check.errorf(e.Sel, "cannot call pointer method %s on %s", sel, x.typ)
+		default:
+			var why string
+			if tpar := x.typ.TypeParam(); tpar != nil {
+				// Type parameter bounds don't specify fields, so don't mention "field".
+				switch obj := tpar.Bound().obj.(type) {
+				case nil:
+					why = check.sprintf("type bound for %s has no method %s", x.typ, sel)
+				case *TypeName:
+					why = check.sprintf("interface %s has no method %s", obj.name, sel)
+				}
+			} else {
+				why = check.sprintf("type %s has no field or method %s", x.typ, sel)
+			}
+
+			// Check if capitalization of sel matters and provide better error message in that case.
+			if len(sel) > 0 {
+				var changeCase string
+				if r := rune(sel[0]); unicode.IsUpper(r) {
+					changeCase = string(unicode.ToLower(r)) + sel[1:]
+				} else {
+					changeCase = string(unicode.ToUpper(r)) + sel[1:]
+				}
+				if obj, _, _ = check.lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, changeCase); obj != nil {
+					why += ", but does have " + changeCase
+				}
+			}
+
+			check.errorf(e.Sel, "%s.%s undefined (%s)", x.expr, sel, why)
+
+		}
+		goto Error
+	}
+
+	// methods may not have a fully set up signature yet
+	if m, _ := obj.(*Func); m != nil {
+		// check.dump("### found method %s", m)
+		check.objDecl(m, nil)
+		// If m has a parameterized receiver type, infer the type parameter
+		// values from the actual receiver provided and then substitute the
+		// type parameters in the signature accordingly.
+		// TODO(gri) factor this code out
+		sig := m.typ.(*Signature)
+		if len(sig.rparams) > 0 {
+			//check.dump("### recv typ = %s", x.typ)
+			//check.dump("### method = %s rparams = %s tparams = %s", m, sig.rparams, sig.tparams)
+			// The method may have a pointer receiver, but the actually provided receiver
+			// may be a (hopefully addressable) non-pointer value, or vice versa. Here we
+			// only care about inferring receiver type parameters; to make the inference
+			// work, match up pointer-ness of receiver and argument.
+			arg := x
+			if ptrRecv := isPointer(sig.recv.typ); ptrRecv != isPointer(arg.typ) {
+				copy := *arg
+				if ptrRecv {
+					copy.typ = NewPointer(arg.typ)
+				} else {
+					copy.typ = arg.typ.(*Pointer).base
+				}
+				arg = &copy
+			}
+			targs, failed := check.infer(sig.rparams, NewTuple(sig.recv), []*operand{arg})
+			//check.dump("### inferred targs = %s", targs)
+			if failed >= 0 {
+				// We may reach here if there were other errors (see issue #40056).
+				// check.infer will report a follow-up error.
+				// TODO(gri) avoid the follow-up error or provide better explanation.
+				goto Error
+			}
+			// Don't modify m. Instead - for now - make a copy of m and use that instead.
+			// (If we modify m, some tests will fail; possibly because the m is in use.)
+			// TODO(gri) investigate and provide a correct explanation here
+			copy := *m
+			copy.typ = check.subst(e.Pos(), m.typ, makeSubstMap(sig.rparams, targs))
+			obj = &copy
+		}
+		// TODO(gri) we also need to do substitution for parameterized interface methods
+		//           (this breaks code in testdata/linalg.go2 at the moment)
+		//           12/20/2019: Is this TODO still correct?
+	}
+
+	if x.mode == typexpr {
+		// method expression
+		m, _ := obj.(*Func)
+		if m == nil {
+			// TODO(gri) should check if capitalization of sel matters and provide better error message in that case
+			check.errorf(e.Sel, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
+			goto Error
+		}
+
+		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)
+
+		// the receiver type becomes the type of the first function
+		// argument of the method expression's function type
+		var params []*Var
+		sig := m.typ.(*Signature)
+		if sig.params != nil {
+			params = sig.params.vars
+		}
+		x.mode = value
+		x.typ = &Signature{
+			tparams:  sig.tparams,
+			params:   NewTuple(append([]*Var{NewVar(nopos, check.pkg, "_", x.typ)}, params...)...),
+			results:  sig.results,
+			variadic: sig.variadic,
+		}
+
+		check.addDeclDep(m)
+
+	} else {
+		// regular selector
+		switch obj := obj.(type) {
+		case *Var:
+			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
+			if x.mode == variable || indirect {
+				x.mode = variable
+			} else {
+				x.mode = value
+			}
+			x.typ = obj.typ
+
+		case *Func:
+			// TODO(gri) If we needed to take into account the receiver's
+			// addressability, should we report the type &(x.typ) instead?
+			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)
+
+			// TODO(gri) The verification pass below is disabled for now because
+			//           method sets don't match method lookup in some cases.
+			//           For instance, if we made a copy above when creating a
+			//           custom method for a parameterized received type, the
+			//           method set method doesn't match (no copy there). There
+			///          may be other situations.
+			disabled := true
+			if !disabled && debug {
+				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
+				// TODO(gri) This only works because we call LookupFieldOrMethod
+				// _before_ calling NewMethodSet: LookupFieldOrMethod completes
+				// any incomplete interfaces so they are available to NewMethodSet
+				// (which assumes that interfaces have been completed already).
+				typ := x.typ
+				if x.mode == variable {
+					// If typ is not an (unnamed) pointer or an interface,
+					// use *typ instead, because the method set of *typ
+					// includes the methods of typ.
+					// Variables are addressable, so we can always take their
+					// address.
+					if _, ok := typ.(*Pointer); !ok && !IsInterface(typ) {
+						typ = &Pointer{base: typ}
+					}
+				}
+				// If we created a synthetic pointer type above, we will throw
+				// away the method set computed here after use.
+				// TODO(gri) Method set computation should probably always compute
+				// both, the value and the pointer receiver method set and represent
+				// them in a single structure.
+				// TODO(gri) Consider also using a method set cache for the lifetime
+				// of checker once we rely on MethodSet lookup instead of individual
+				// lookup.
+				mset := NewMethodSet(typ)
+				if m := mset.Lookup(check.pkg, sel); m == nil || m.obj != obj {
+					check.dump("%v: (%s).%v -> %s", posFor(e), typ, obj.name, m)
+					check.dump("%s\n", mset)
+					// Caution: MethodSets are supposed to be used externally
+					// only (after all interface types were completed). It's
+					// now possible that we get here incorrectly. Not urgent
+					// to fix since we only run this code in debug mode.
+					// TODO(gri) fix this eventually.
+					panic("method sets and lookup don't agree")
+				}
+			}
+
+			x.mode = value
+
+			// remove receiver
+			sig := *obj.typ.(*Signature)
+			sig.recv = nil
+			x.typ = &sig
+
+			check.addDeclDep(obj)
+
+		default:
+			unreachable()
+		}
+	}
+
+	// everything went well
+	x.expr = e
+	return
+
+Error:
+	x.mode = invalid
+	x.expr = e
+}
+
+// use type-checks each argument.
+// Useful to make sure expressions are evaluated
+// (and variables are "used") in the presence of other errors.
+// The arguments may be nil.
+// TODO(gri) make this accept a []syntax.Expr and use an unpack function when we have a ListExpr?
+func (check *Checker) use(arg ...syntax.Expr) {
+	var x operand
+	for _, e := range arg {
+		// Certain AST fields may legally be nil (e.g., the ast.SliceExpr.High field).
+		if e == nil {
+			continue
+		}
+		if l, _ := e.(*syntax.ListExpr); l != nil {
+			check.use(l.ElemList...)
+			continue
+		}
+		check.rawExpr(&x, e, nil)
+	}
+}
+
+// useLHS is like use, but doesn't "use" top-level identifiers.
+// It should be called instead of use if the arguments are
+// expressions on the lhs of an assignment.
+// The arguments must not be nil.
+func (check *Checker) useLHS(arg ...syntax.Expr) {
+	var x operand
+	for _, e := range arg {
+		// If the lhs is an identifier denoting a variable v, this assignment
+		// is not a 'use' of v. Remember current value of v.used and restore
+		// after evaluating the lhs via check.rawExpr.
+		var v *Var
+		var v_used bool
+		if ident, _ := unparen(e).(*syntax.Name); ident != nil {
+			// never type-check the blank name on the lhs
+			if ident.Value == "_" {
+				continue
+			}
+			if _, obj := check.scope.LookupParent(ident.Value, nopos); obj != nil {
+				// It's ok to mark non-local variables, but ignore variables
+				// from other packages to avoid potential race conditions with
+				// dot-imported variables.
+				if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
+					v = w
+					v_used = v.used
+				}
+			}
+		}
+		check.rawExpr(&x, e, nil)
+		if v != nil {
+			v.used = v_used // restore v.used
+		}
+	}
+}
+
+// instantiatedOperand reports an error of x is an uninstantiated (generic) type and sets x.typ to Typ[Invalid].
+func (check *Checker) instantiatedOperand(x *operand) {
+	if x.mode == typexpr && isGeneric(x.typ) {
+		check.errorf(x, "cannot use generic type %s without instantiation", x.typ)
+		x.typ = Typ[Invalid]
+	}
+}

src/cmd/compile/internal/types2/check.go

@@ -0,0 +1,449 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements the Check function, which drives type-checking.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"errors"
+	"fmt"
+	"go/constant"
+)
+
+var nopos syntax.Pos
+
+// debugging/development support
+const debug = true // leave on during development
+
+// If forceStrict is set, the type-checker enforces additional
+// rules not specified by the Go 1 spec, but which will
+// catch guaranteed run-time errors if the respective
+// code is executed. In other words, programs passing in
+// strict mode are Go 1 compliant, but not all Go 1 programs
+// will pass in strict mode. The additional rules are:
+//
+// - A type assertion x.(T) where T is an interface type
+//   is invalid if any (statically known) method that exists
+//   for both x and T have different signatures.
+//
+const forceStrict = false
+
+// If methodTypeParamsOk is set, type parameters are
+// permitted in method declarations (in interfaces, too).
+// Generalization and experimental feature.
+const methodTypeParamsOk = true
+
+// exprInfo stores information about an untyped expression.
+type exprInfo struct {
+	isLhs bool // expression is lhs operand of a shift with delayed type-check
+	mode  operandMode
+	typ   *Basic
+	val   constant.Value // constant value; or nil (if not a constant)
+}
+
+// A context represents the context within which an object is type-checked.
+type context struct {
+	decl          *declInfo                 // package-level declaration whose init expression/function body is checked
+	scope         *Scope                    // top-most scope for lookups
+	pos           syntax.Pos                // if valid, identifiers are looked up as if at position pos (used by Eval)
+	iota          constant.Value            // value of iota in a constant declaration; nil otherwise
+	sig           *Signature                // function signature if inside a function; nil otherwise
+	isPanic       map[*syntax.CallExpr]bool // set of panic call expressions (used for termination check)
+	hasLabel      bool                      // set if a function makes use of labels (only ~1% of functions); unused outside functions
+	hasCallOrRecv bool                      // set if an expression contains a function call or channel receive operation
+}
+
+// lookup looks up name in the current context and returns the matching object, or nil.
+func (ctxt *context) lookup(name string) Object {
+	_, obj := ctxt.scope.LookupParent(name, ctxt.pos)
+	return obj
+}
+
+// An importKey identifies an imported package by import path and source directory
+// (directory containing the file containing the import). In practice, the directory
+// may always be the same, or may not matter. Given an (import path, directory), an
+// importer must always return the same package (but given two different import paths,
+// an importer may still return the same package by mapping them to the same package
+// paths).
+type importKey struct {
+	path, dir string
+}
+
+// A Checker maintains the state of the type checker.
+// It must be created with NewChecker.
+type Checker struct {
+	// package information
+	// (initialized by NewChecker, valid for the life-time of checker)
+	conf *Config
+	pkg  *Package
+	*Info
+	nextId uint64                      // unique Id for type parameters (first valid Id is 1)
+	objMap map[Object]*declInfo        // maps package-level objects and (non-interface) methods to declaration info
+	impMap map[importKey]*Package      // maps (import path, source directory) to (complete or fake) package
+	posMap map[*Interface][]syntax.Pos // maps interface types to lists of embedded interface positions
+	typMap map[string]*Named           // maps an instantiated named type hash to a *Named type
+	pkgCnt map[string]int              // counts number of imported packages with a given name (for better error messages)
+
+	// information collected during type-checking of a set of package files
+	// (initialized by Files, valid only for the duration of check.Files;
+	// maps and lists are allocated on demand)
+	files            []*syntax.File                     // package files
+	unusedDotImports map[*Scope]map[*Package]syntax.Pos // positions of unused dot-imported packages for each file scope
+
+	firstErr error                    // first error encountered
+	methods  map[*TypeName][]*Func    // maps package scope type names to associated non-blank (non-interface) methods
+	untyped  map[syntax.Expr]exprInfo // map of expressions without final type
+	delayed  []func()                 // stack of delayed action segments; segments are processed in FIFO order
+	finals   []func()                 // list of final actions; processed at the end of type-checking the current set of files
+	objPath  []Object                 // path of object dependencies during type inference (for cycle reporting)
+
+	// context within which the current object is type-checked
+	// (valid only for the duration of type-checking a specific object)
+	context
+
+	// debugging
+	indent int // indentation for tracing
+}
+
+// addUnusedImport adds the position of a dot-imported package
+// pkg to the map of dot imports for the given file scope.
+func (check *Checker) addUnusedDotImport(scope *Scope, pkg *Package, pos syntax.Pos) {
+	mm := check.unusedDotImports
+	if mm == nil {
+		mm = make(map[*Scope]map[*Package]syntax.Pos)
+		check.unusedDotImports = mm
+	}
+	m := mm[scope]
+	if m == nil {
+		m = make(map[*Package]syntax.Pos)
+		mm[scope] = m
+	}
+	m[pkg] = pos
+}
+
+// addDeclDep adds the dependency edge (check.decl -> to) if check.decl exists
+func (check *Checker) addDeclDep(to Object) {
+	from := check.decl
+	if from == nil {
+		return // not in a package-level init expression
+	}
+	if _, found := check.objMap[to]; !found {
+		return // to is not a package-level object
+	}
+	from.addDep(to)
+}
+
+func (check *Checker) rememberUntyped(e syntax.Expr, lhs bool, mode operandMode, typ *Basic, val constant.Value) {
+	m := check.untyped
+	if m == nil {
+		m = make(map[syntax.Expr]exprInfo)
+		check.untyped = m
+	}
+	m[e] = exprInfo{lhs, mode, typ, val}
+}
+
+// later pushes f on to the stack of actions that will be processed later;
+// either at the end of the current statement, or in case of a local constant
+// or variable declaration, before the constant or variable is in scope
+// (so that f still sees the scope before any new declarations).
+func (check *Checker) later(f func()) {
+	check.delayed = append(check.delayed, f)
+}
+
+// atEnd adds f to the list of actions processed at the end
+// of type-checking, before initialization order computation.
+// Actions added by atEnd are processed after any actions
+// added by later.
+func (check *Checker) atEnd(f func()) {
+	check.finals = append(check.finals, f)
+}
+
+// push pushes obj onto the object path and returns its index in the path.
+func (check *Checker) push(obj Object) int {
+	check.objPath = append(check.objPath, obj)
+	return len(check.objPath) - 1
+}
+
+// pop pops and returns the topmost object from the object path.
+func (check *Checker) pop() Object {
+	i := len(check.objPath) - 1
+	obj := check.objPath[i]
+	check.objPath[i] = nil
+	check.objPath = check.objPath[:i]
+	return obj
+}
+
+// NewChecker returns a new Checker instance for a given package.
+// Package files may be added incrementally via checker.Files.
+func NewChecker(conf *Config, pkg *Package, info *Info) *Checker {
+	// make sure we have a configuration
+	if conf == nil {
+		conf = new(Config)
+	}
+
+	// make sure we have an info struct
+	if info == nil {
+		info = new(Info)
+	}
+
+	return &Checker{
+		conf:   conf,
+		pkg:    pkg,
+		Info:   info,
+		nextId: 1,
+		objMap: make(map[Object]*declInfo),
+		impMap: make(map[importKey]*Package),
+		posMap: make(map[*Interface][]syntax.Pos),
+		typMap: make(map[string]*Named),
+		pkgCnt: make(map[string]int),
+	}
+}
+
+// initFiles initializes the files-specific portion of checker.
+// The provided files must all belong to the same package.
+func (check *Checker) initFiles(files []*syntax.File) {
+	// start with a clean slate (check.Files may be called multiple times)
+	check.files = nil
+	check.unusedDotImports = nil
+
+	check.firstErr = nil
+	check.methods = nil
+	check.untyped = nil
+	check.delayed = nil
+	check.finals = nil
+
+	// determine package name and collect valid files
+	pkg := check.pkg
+	for _, file := range files {
+		switch name := file.PkgName.Value; pkg.name {
+		case "":
+			if name != "_" {
+				pkg.name = name
+			} else {
+				check.errorf(file.PkgName, "invalid package name _")
+			}
+			fallthrough
+
+		case name:
+			check.files = append(check.files, file)
+
+		default:
+			check.errorf(file, "package %s; expected %s", name, pkg.name)
+			// ignore this file
+		}
+	}
+}
+
+// A bailout panic is used for early termination.
+type bailout struct{}
+
+func (check *Checker) handleBailout(err *error) {
+	switch p := recover().(type) {
+	case nil, bailout:
+		// normal return or early exit
+		*err = check.firstErr
+	default:
+		// re-panic
+		panic(p)
+	}
+}
+
+// Files checks the provided files as part of the checker's package.
+func (check *Checker) Files(files []*syntax.File) error { return check.checkFiles(files) }
+
+var errBadCgo = errors.New("cannot use FakeImportC and go115UsesCgo together")
+
+func (check *Checker) checkFiles(files []*syntax.File) (err error) {
+	if check.conf.FakeImportC && check.conf.go115UsesCgo {
+		return errBadCgo
+	}
+
+	defer check.handleBailout(&err)
+
+	print := func(msg string) {
+		if check.conf.Trace {
+			fmt.Println(msg)
+		}
+	}
+
+	print("== initFiles ==")
+	check.initFiles(files)
+
+	print("== collectObjects ==")
+	check.collectObjects()
+
+	print("== packageObjects ==")
+	check.packageObjects()
+
+	print("== processDelayed ==")
+	check.processDelayed(0) // incl. all functions
+	check.processFinals()
+
+	print("== initOrder ==")
+	check.initOrder()
+
+	if !check.conf.DisableUnusedImportCheck {
+		print("== unusedImports ==")
+		check.unusedImports()
+	}
+
+	print("== recordUntyped ==")
+	check.recordUntyped()
+
+	if check.Info != nil {
+		print("== sanitizeInfo ==")
+		sanitizeInfo(check.Info)
+	}
+
+	check.pkg.complete = true
+	return
+}
+
+// processDelayed processes all delayed actions pushed after top.
+func (check *Checker) processDelayed(top int) {
+	// If each delayed action pushes a new action, the
+	// stack will continue to grow during this loop.
+	// However, it is only processing functions (which
+	// are processed in a delayed fashion) that may
+	// add more actions (such as nested functions), so
+	// this is a sufficiently bounded process.
+	for i := top; i < len(check.delayed); i++ {
+		check.delayed[i]() // may append to check.delayed
+	}
+	assert(top <= len(check.delayed)) // stack must not have shrunk
+	check.delayed = check.delayed[:top]
+}
+
+func (check *Checker) processFinals() {
+	n := len(check.finals)
+	for _, f := range check.finals {
+		f() // must not append to check.finals
+	}
+	if len(check.finals) != n {
+		panic("internal error: final action list grew")
+	}
+}
+
+func (check *Checker) recordUntyped() {
+	if !debug && check.Types == nil {
+		return // nothing to do
+	}
+
+	for x, info := range check.untyped {
+		if debug && isTyped(info.typ) {
+			check.dump("%v: %s (type %s) is typed", posFor(x), x, info.typ)
+			unreachable()
+		}
+		check.recordTypeAndValue(x, info.mode, info.typ, info.val)
+	}
+}
+
+func (check *Checker) recordTypeAndValue(x syntax.Expr, mode operandMode, typ Type, val constant.Value) {
+	assert(x != nil)
+	assert(typ != nil)
+	if mode == invalid {
+		return // omit
+	}
+	if mode == constant_ {
+		assert(val != nil)
+		assert(typ == Typ[Invalid] || isConstType(typ))
+	}
+	if m := check.Types; m != nil {
+		m[x] = TypeAndValue{mode, typ, val}
+	}
+}
+
+func (check *Checker) recordBuiltinType(f syntax.Expr, sig *Signature) {
+	// f must be a (possibly parenthesized) identifier denoting a built-in
+	// (built-ins in package unsafe always produce a constant result and
+	// we don't record their signatures, so we don't see qualified idents
+	// here): record the signature for f and possible children.
+	for {
+		check.recordTypeAndValue(f, builtin, sig, nil)
+		switch p := f.(type) {
+		case *syntax.Name:
+			return // we're done
+		case *syntax.ParenExpr:
+			f = p.X
+		default:
+			unreachable()
+		}
+	}
+}
+
+func (check *Checker) recordCommaOkTypes(x syntax.Expr, a [2]Type) {
+	assert(x != nil)
+	if a[0] == nil || a[1] == nil {
+		return
+	}
+	assert(isTyped(a[0]) && isTyped(a[1]) && (isBoolean(a[1]) || a[1] == universeError))
+	if m := check.Types; m != nil {
+		for {
+			tv := m[x]
+			assert(tv.Type != nil) // should have been recorded already
+			pos := x.Pos()
+			tv.Type = NewTuple(
+				NewVar(pos, check.pkg, "", a[0]),
+				NewVar(pos, check.pkg, "", a[1]),
+			)
+			m[x] = tv
+			// if x is a parenthesized expression (p.X), update p.X
+			p, _ := x.(*syntax.ParenExpr)
+			if p == nil {
+				break
+			}
+			x = p.X
+		}
+	}
+}
+
+func (check *Checker) recordInferred(call syntax.Expr, targs []Type, sig *Signature) {
+	assert(call != nil)
+	assert(sig != nil)
+	if m := check.Inferred; m != nil {
+		m[call] = Inferred{targs, sig}
+	}
+}
+
+func (check *Checker) recordDef(id *syntax.Name, obj Object) {
+	assert(id != nil)
+	if m := check.Defs; m != nil {
+		m[id] = obj
+	}
+}
+
+func (check *Checker) recordUse(id *syntax.Name, obj Object) {
+	assert(id != nil)
+	assert(obj != nil)
+	if m := check.Uses; m != nil {
+		m[id] = obj
+	}
+}
+
+func (check *Checker) recordImplicit(node syntax.Node, obj Object) {
+	assert(node != nil)
+	assert(obj != nil)
+	if m := check.Implicits; m != nil {
+		m[node] = obj
+	}
+}
+
+func (check *Checker) recordSelection(x *syntax.SelectorExpr, kind SelectionKind, recv Type, obj Object, index []int, indirect bool) {
+	assert(obj != nil && (recv == nil || len(index) > 0))
+	check.recordUse(x.Sel, obj)
+	if m := check.Selections; m != nil {
+		m[x] = &Selection{kind, recv, obj, index, indirect}
+	}
+}
+
+func (check *Checker) recordScope(node syntax.Node, scope *Scope) {
+	assert(node != nil)
+	assert(scope != nil)
+	if m := check.Scopes; m != nil {
+		m[node] = scope
+	}
+}

src/cmd/compile/internal/types2/check_test.go

@@ -0,0 +1,269 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a typechecker test harness. The packages specified
+// in tests are typechecked. Error messages reported by the typechecker are
+// compared against the error messages expected in the test files.
+//
+// Expected errors are indicated in the test files by putting a comment
+// of the form /* ERROR "rx" */ immediately following an offending token.
+// The harness will verify that an error matching the regular expression
+// rx is reported at that source position. Consecutive comments may be
+// used to indicate multiple errors for the same token position.
+//
+// For instance, the following test file indicates that a "not declared"
+// error should be reported for the undeclared variable x:
+//
+//	package p
+//	func f() {
+//		_ = x /* ERROR "not declared" */ + 1
+//	}
+
+// TODO(gri) Also collect strict mode errors of the form /* STRICT ... */
+//           and test against strict mode.
+
+package types2_test
+
+import (
+	"cmd/compile/internal/syntax"
+	"flag"
+	"fmt"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+
+	. "cmd/compile/internal/types2"
+)
+
+var (
+	haltOnError = flag.Bool("halt", false, "halt on error")
+	listErrors  = flag.Bool("errlist", false, "list errors")
+	testFiles   = flag.String("files", "", "space-separated list of test files")
+)
+
+func parseFiles(t *testing.T, filenames []string) ([]*syntax.File, []error) {
+	var files []*syntax.File
+	var errlist []error
+	errh := func(err error) { errlist = append(errlist, err) }
+	for _, filename := range filenames {
+		file, err := syntax.ParseFile(filename, errh, nil, syntax.AllowGenerics)
+		if file == nil {
+			t.Fatalf("%s: %s", filename, err)
+		}
+		files = append(files, file)
+	}
+	return files, errlist
+}
+
+func unpackError(err error) syntax.Error {
+	switch err := err.(type) {
+	case syntax.Error:
+		return err
+	case Error:
+		return syntax.Error{Pos: err.Pos, Msg: err.Msg}
+	default:
+		return syntax.Error{Msg: err.Error()}
+	}
+}
+
+func delta(x, y uint) uint {
+	switch {
+	case x < y:
+		return y - x
+	case x > y:
+		return x - y
+	default:
+		return 0
+	}
+}
+
+func checkFiles(t *testing.T, sources []string, colDelta uint, trace bool) {
+	// parse files and collect parser errors
+	files, errlist := parseFiles(t, sources)
+
+	pkgName := "<no package>"
+	if len(files) > 0 {
+		pkgName = files[0].PkgName.Value
+	}
+
+	if *listErrors && len(errlist) > 0 {
+		t.Errorf("--- %s:", pkgName)
+		for _, err := range errlist {
+			t.Error(err)
+		}
+	}
+
+	// typecheck and collect typechecker errors
+	var conf Config
+	conf.AcceptMethodTypeParams = true
+	conf.InferFromConstraints = true
+	// special case for importC.src
+	if len(sources) == 1 && strings.HasSuffix(sources[0], "importC.src") {
+		conf.FakeImportC = true
+	}
+	conf.Trace = trace
+	conf.Importer = defaultImporter()
+	conf.Error = func(err error) {
+		if *haltOnError {
+			defer panic(err)
+		}
+		if *listErrors {
+			t.Error(err)
+			return
+		}
+		// Ignore secondary error messages starting with "\t";
+		// they are clarifying messages for a primary error.
+		if !strings.Contains(err.Error(), ": \t") {
+			errlist = append(errlist, err)
+		}
+	}
+	conf.Check(pkgName, files, nil)
+
+	if *listErrors {
+		return
+	}
+
+	// collect expected errors
+	errmap := make(map[string]map[uint][]syntax.Error)
+	for _, filename := range sources {
+		f, err := os.Open(filename)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if m := syntax.ErrorMap(f); len(m) > 0 {
+			errmap[filename] = m
+		}
+		f.Close()
+	}
+
+	// match against found errors
+	for _, err := range errlist {
+		got := unpackError(err)
+
+		// find list of errors for the respective error line
+		filename := got.Pos.Base().Filename()
+		filemap := errmap[filename]
+		var line uint
+		var list []syntax.Error
+		if filemap != nil {
+			line = got.Pos.Line()
+			list = filemap[line]
+		}
+		// list may be nil
+
+		// one of errors in list should match the current error
+		index := -1 // list index of matching message, if any
+		for i, want := range list {
+			rx, err := regexp.Compile(want.Msg)
+			if err != nil {
+				t.Errorf("%s:%d:%d: %v", filename, line, want.Pos.Col(), err)
+				continue
+			}
+			if rx.MatchString(got.Msg) {
+				index = i
+				break
+			}
+		}
+		if index < 0 {
+			t.Errorf("%s: no error expected: %q", got.Pos, got.Msg)
+			continue
+		}
+
+		// column position must be within expected colDelta
+		want := list[index]
+		if delta(got.Pos.Col(), want.Pos.Col()) > colDelta {
+			t.Errorf("%s: got col = %d; want %d", got.Pos, got.Pos.Col(), want.Pos.Col())
+		}
+
+		// eliminate from list
+		if n := len(list) - 1; n > 0 {
+			// not the last entry - swap in last element and shorten list by 1
+			list[index] = list[n]
+			filemap[line] = list[:n]
+		} else {
+			// last entry - remove list from filemap
+			delete(filemap, line)
+		}
+
+		// if filemap is empty, eliminate from errmap
+		if len(filemap) == 0 {
+			delete(errmap, filename)
+		}
+	}
+
+	// there should be no expected errors left
+	if len(errmap) > 0 {
+		t.Errorf("--- %s: unreported errors:", pkgName)
+		for filename, filemap := range errmap {
+			for line, list := range filemap {
+				for _, err := range list {
+					t.Errorf("%s:%d:%d: %s", filename, line, err.Pos.Col(), err.Msg)
+				}
+			}
+		}
+	}
+}
+
+// TestCheck is for manual testing of selected input files, provided with -files.
+func TestCheck(t *testing.T) {
+	if *testFiles == "" {
+		return
+	}
+	testenv.MustHaveGoBuild(t)
+	DefPredeclaredTestFuncs()
+	checkFiles(t, strings.Split(*testFiles, " "), 0, testing.Verbose())
+}
+
+// TODO(gri) Enable once we have added the testdata tests.
+// func TestTestdata(t *testing.T)  { DefPredeclaredTestFuncs(); testDir(t, 75, "testdata") } // TODO(gri) narrow column tolerance
+func TestExamples(t *testing.T)  { testDir(t, 0, "examples") }
+func TestFixedbugs(t *testing.T) { testDir(t, 0, "fixedbugs") }
+
+func testDir(t *testing.T, colDelta uint, dir string) {
+	testenv.MustHaveGoBuild(t)
+
+	fis, err := ioutil.ReadDir(dir)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	for count, fi := range fis {
+		path := filepath.Join(dir, fi.Name())
+
+		// if fi is a directory, its files make up a single package
+		if fi.IsDir() {
+			if testing.Verbose() {
+				fmt.Printf("%3d %s\n", count, path)
+			}
+			fis, err := ioutil.ReadDir(path)
+			if err != nil {
+				t.Error(err)
+				continue
+			}
+			files := make([]string, len(fis))
+			for i, fi := range fis {
+				// if fi is a directory, checkFiles below will complain
+				files[i] = filepath.Join(path, fi.Name())
+				if testing.Verbose() {
+					fmt.Printf("\t%s\n", files[i])
+				}
+			}
+			checkFiles(t, files, colDelta, false)
+			continue
+		}
+
+		// otherwise, fi is a stand-alone file
+		if testing.Verbose() {
+			fmt.Printf("%3d %s\n", count, path)
+		}
+		checkFiles(t, []string{path}, colDelta, false)
+	}
+}

src/cmd/compile/internal/types2/conversions.go

@@ -0,0 +1,164 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of conversions.
+
+package types2
+
+import "go/constant"
+
+// Conversion type-checks the conversion T(x).
+// The result is in x.
+func (check *Checker) conversion(x *operand, T Type) {
+	constArg := x.mode == constant_
+
+	var ok bool
+	switch {
+	case constArg && isConstType(T):
+		// constant conversion
+		switch t := T.Basic(); {
+		case representableConst(x.val, check, t, &x.val):
+			ok = true
+		case isInteger(x.typ) && isString(t):
+			codepoint := int64(-1)
+			if i, ok := constant.Int64Val(x.val); ok {
+				codepoint = i
+			}
+			// If codepoint < 0 the absolute value is too large (or unknown) for
+			// conversion. This is the same as converting any other out-of-range
+			// value - let string(codepoint) do the work.
+			x.val = constant.MakeString(string(rune(codepoint)))
+			ok = true
+		}
+	case x.convertibleTo(check, T):
+		// non-constant conversion
+		x.mode = value
+		ok = true
+	}
+
+	if !ok {
+		check.errorf(x, "cannot convert %s to %s", x, T)
+		x.mode = invalid
+		return
+	}
+
+	// The conversion argument types are final. For untyped values the
+	// conversion provides the type, per the spec: "A constant may be
+	// given a type explicitly by a constant declaration or conversion,...".
+	if isUntyped(x.typ) {
+		final := T
+		// - For conversions to interfaces, use the argument's default type.
+		// - For conversions of untyped constants to non-constant types, also
+		//   use the default type (e.g., []byte("foo") should report string
+		//   not []byte as type for the constant "foo").
+		// - Keep untyped nil for untyped nil arguments.
+		// - For integer to string conversions, keep the argument type.
+		//   (See also the TODO below.)
+		if IsInterface(T) || constArg && !isConstType(T) {
+			final = Default(x.typ)
+		} else if isInteger(x.typ) && isString(T) {
+			final = x.typ
+		}
+		check.updateExprType(x.expr, final, true)
+	}
+
+	x.typ = T
+}
+
+// TODO(gri) convertibleTo checks if T(x) is valid. It assumes that the type
+// of x is fully known, but that's not the case for say string(1<<s + 1.0):
+// Here, the type of 1<<s + 1.0 will be UntypedFloat which will lead to the
+// (correct!) refusal of the conversion. But the reported error is essentially
+// "cannot convert untyped float value to string", yet the correct error (per
+// the spec) is that we cannot shift a floating-point value: 1 in 1<<s should
+// be converted to UntypedFloat because of the addition of 1.0. Fixing this
+// is tricky because we'd have to run updateExprType on the argument first.
+// (Issue #21982.)
+
+// convertibleTo reports whether T(x) is valid.
+// The check parameter may be nil if convertibleTo is invoked through an
+// exported API call, i.e., when all methods have been type-checked.
+func (x *operand) convertibleTo(check *Checker, T Type) bool {
+	// "x is assignable to T"
+	if x.assignableTo(check, T, nil) {
+		return true
+	}
+
+	// "x's type and T have identical underlying types if tags are ignored"
+	V := x.typ
+	Vu := V.Under()
+	Tu := T.Under()
+	if check.identicalIgnoreTags(Vu, Tu) {
+		return true
+	}
+
+	// "x's type and T are unnamed pointer types and their pointer base types
+	// have identical underlying types if tags are ignored"
+	if V, ok := V.(*Pointer); ok {
+		if T, ok := T.(*Pointer); ok {
+			if check.identicalIgnoreTags(V.base.Under(), T.base.Under()) {
+				return true
+			}
+		}
+	}
+
+	// "x's type and T are both integer or floating point types"
+	if isIntegerOrFloat(V) && isIntegerOrFloat(T) {
+		return true
+	}
+
+	// "x's type and T are both complex types"
+	if isComplex(V) && isComplex(T) {
+		return true
+	}
+
+	// "x is an integer or a slice of bytes or runes and T is a string type"
+	if (isInteger(V) || isBytesOrRunes(Vu)) && isString(T) {
+		return true
+	}
+
+	// "x is a string and T is a slice of bytes or runes"
+	if isString(V) && isBytesOrRunes(Tu) {
+		return true
+	}
+
+	// package unsafe:
+	// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"
+	if (isPointer(Vu) || isUintptr(Vu)) && isUnsafePointer(T) {
+		return true
+	}
+	// "and vice versa"
+	if isUnsafePointer(V) && (isPointer(Tu) || isUintptr(Tu)) {
+		return true
+	}
+
+	return false
+}
+
+func isUintptr(typ Type) bool {
+	t := typ.Basic()
+	return t != nil && t.kind == Uintptr
+}
+
+func isUnsafePointer(typ Type) bool {
+	// TODO(gri): Is this typ.Basic() instead of typ.(*Basic) correct?
+	//            (The former calls typ.Under(), while the latter doesn't.)
+	//            The spec does not say so, but gc claims it is. See also
+	//            issue 6326.
+	t := typ.Basic()
+	return t != nil && t.kind == UnsafePointer
+}
+
+func isPointer(typ Type) bool {
+	return typ.Pointer() != nil
+}
+
+func isBytesOrRunes(typ Type) bool {
+	if s := typ.Slice(); s != nil {
+		t := s.elem.Basic()
+		return t != nil && (t.kind == Byte || t.kind == Rune)
+	}
+	return false
+}

src/cmd/compile/internal/types2/decl.go

@@ -0,0 +1,981 @@
+// UNREVIEWED
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+)
+
+func (check *Checker) reportAltDecl(obj Object) {
+	if pos := obj.Pos(); pos.IsKnown() {
+		// We use "other" rather than "previous" here because
+		// the first declaration seen may not be textually
+		// earlier in the source.
+		check.errorf(pos, "\tother declaration of %s", obj.Name()) // secondary error, \t indented
+	}
+}
+
+func (check *Checker) declare(scope *Scope, id *syntax.Name, obj Object, pos syntax.Pos) {
+	// spec: "The blank identifier, represented by the underscore
+	// character _, may be used in a declaration like any other
+	// identifier but the declaration does not introduce a new
+	// binding."
+	if obj.Name() != "_" {
+		if alt := scope.Insert(obj); alt != nil {
+			check.errorf(obj.Pos(), "%s redeclared in this block", obj.Name())
+			check.reportAltDecl(alt)
+			return
+		}
+		obj.setScopePos(pos)
+	}
+	if id != nil {
+		check.recordDef(id, obj)
+	}
+}
+
+// pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
+func pathString(path []Object) string {
+	var s string
+	for i, p := range path {
+		if i > 0 {
+			s += "->"
+		}
+		s += p.Name()
+	}
+	return s
+}
+
+// objDecl type-checks the declaration of obj in its respective (file) context.
+// For the meaning of def, see Checker.definedType, in typexpr.go.
+func (check *Checker) objDecl(obj Object, def *Named) {
+	if check.conf.Trace && obj.Type() == nil {
+		if check.indent == 0 {
+			fmt.Println() // empty line between top-level objects for readability
+		}
+		check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
+		check.indent++
+		defer func() {
+			check.indent--
+			check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
+		}()
+	}
+
+	// Checking the declaration of obj means inferring its type
+	// (and possibly its value, for constants).
+	// An object's type (and thus the object) may be in one of
+	// three states which are expressed by colors:
+	//
+	// - an object whose type is not yet known is painted white (initial color)
+	// - an object whose type is in the process of being inferred is painted grey
+	// - an object whose type is fully inferred is painted black
+	//
+	// During type inference, an object's color changes from white to grey
+	// to black (pre-declared objects are painted black from the start).
+	// A black object (i.e., its type) can only depend on (refer to) other black
+	// ones. White and grey objects may depend on white and black objects.
+	// A dependency on a grey object indicates a cycle which may or may not be
+	// valid.
+	//
+	// When objects turn grey, they are pushed on the object path (a stack);
+	// they are popped again when they turn black. Thus, if a grey object (a
+	// cycle) is encountered, it is on the object path, and all the objects
+	// it depends on are the remaining objects on that path. Color encoding
+	// is such that the color value of a grey object indicates the index of
+	// that object in the object path.
+
+	// During type-checking, white objects may be assigned a type without
+	// traversing through objDecl; e.g., when initializing constants and
+	// variables. Update the colors of those objects here (rather than
+	// everywhere where we set the type) to satisfy the color invariants.
+	if obj.color() == white && obj.Type() != nil {
+		obj.setColor(black)
+		return
+	}
+
+	switch obj.color() {
+	case white:
+		assert(obj.Type() == nil)
+		// All color values other than white and black are considered grey.
+		// Because black and white are < grey, all values >= grey are grey.
+		// Use those values to encode the object's index into the object path.
+		obj.setColor(grey + color(check.push(obj)))
+		defer func() {
+			check.pop().setColor(black)
+		}()
+
+	case black:
+		assert(obj.Type() != nil)
+		return
+
+	default:
+		// Color values other than white or black are considered grey.
+		fallthrough
+
+	case grey:
+		// We have a cycle.
+		// In the existing code, this is marked by a non-nil type
+		// for the object except for constants and variables whose
+		// type may be non-nil (known), or nil if it depends on the
+		// not-yet known initialization value.
+		// In the former case, set the type to Typ[Invalid] because
+		// we have an initialization cycle. The cycle error will be
+		// reported later, when determining initialization order.
+		// TODO(gri) Report cycle here and simplify initialization
+		// order code.
+		switch obj := obj.(type) {
+		case *Const:
+			if check.cycle(obj) || obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+
+		case *Var:
+			if check.cycle(obj) || obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+
+		case *TypeName:
+			if check.cycle(obj) {
+				// break cycle
+				// (without this, calling underlying()
+				// below may lead to an endless loop
+				// if we have a cycle for a defined
+				// (*Named) type)
+				obj.typ = Typ[Invalid]
+			}
+
+		case *Func:
+			if check.cycle(obj) {
+				// Don't set obj.typ to Typ[Invalid] here
+				// because plenty of code type-asserts that
+				// functions have a *Signature type. Grey
+				// functions have their type set to an empty
+				// signature which makes it impossible to
+				// initialize a variable with the function.
+			}
+
+		default:
+			unreachable()
+		}
+		assert(obj.Type() != nil)
+		return
+	}
+
+	d := check.objMap[obj]
+	if d == nil {
+		check.dump("%v: %s should have been declared", obj.Pos(), obj)
+		unreachable()
+	}
+
+	// save/restore current context and setup object context
+	defer func(ctxt context) {
+		check.context = ctxt
+	}(check.context)
+	check.context = context{
+		scope: d.file,
+	}
+
+	// Const and var declarations must not have initialization
+	// cycles. We track them by remembering the current declaration
+	// in check.decl. Initialization expressions depending on other
+	// consts, vars, or functions, add dependencies to the current
+	// check.decl.
+	switch obj := obj.(type) {
+	case *Const:
+		check.decl = d // new package-level const decl
+		check.constDecl(obj, d.vtyp, d.init)
+	case *Var:
+		check.decl = d // new package-level var decl
+		check.varDecl(obj, d.lhs, d.vtyp, d.init)
+	case *TypeName:
+		// invalid recursive types are detected via path
+		check.typeDecl(obj, d.tdecl, def)
+		check.collectMethods(obj) // methods can only be added to top-level types
+	case *Func:
+		// functions may be recursive - no need to track dependencies
+		check.funcDecl(obj, d)
+	default:
+		unreachable()
+	}
+}
+
+// cycle checks if the cycle starting with obj is valid and
+// reports an error if it is not.
+func (check *Checker) cycle(obj Object) (isCycle bool) {
+	// The object map contains the package scope objects and the non-interface methods.
+	if debug {
+		info := check.objMap[obj]
+		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
+		isPkgObj := obj.Parent() == check.pkg.scope
+		if isPkgObj != inObjMap {
+			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
+			unreachable()
+		}
+	}
+
+	// Count cycle objects.
+	assert(obj.color() >= grey)
+	start := obj.color() - grey // index of obj in objPath
+	cycle := check.objPath[start:]
+	nval := 0 // number of (constant or variable) values in the cycle
+	ndef := 0 // number of type definitions in the cycle
+	for _, obj := range cycle {
+		switch obj := obj.(type) {
+		case *Const, *Var:
+			nval++
+		case *TypeName:
+			// Determine if the type name is an alias or not. For
+			// package-level objects, use the object map which
+			// provides syntactic information (which doesn't rely
+			// on the order in which the objects are set up). For
+			// local objects, we can rely on the order, so use
+			// the object's predicate.
+			// TODO(gri) It would be less fragile to always access
+			// the syntactic information. We should consider storing
+			// this information explicitly in the object.
+			var alias bool
+			if d := check.objMap[obj]; d != nil {
+				alias = d.tdecl.Alias // package-level object
+			} else {
+				alias = obj.IsAlias() // function local object
+			}
+			if !alias {
+				ndef++
+			}
+		case *Func:
+			// ignored for now
+		default:
+			unreachable()
+		}
+	}
+
+	if check.conf.Trace {
+		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
+		check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
+		defer func() {
+			if isCycle {
+				check.trace(obj.Pos(), "=> error: cycle is invalid")
+			}
+		}()
+	}
+
+	// A cycle involving only constants and variables is invalid but we
+	// ignore them here because they are reported via the initialization
+	// cycle check.
+	if nval == len(cycle) {
+		return false
+	}
+
+	// A cycle involving only types (and possibly functions) must have at least
+	// one type definition to be permitted: If there is no type definition, we
+	// have a sequence of alias type names which will expand ad infinitum.
+	if nval == 0 && ndef > 0 {
+		return false // cycle is permitted
+	}
+
+	check.cycleError(cycle)
+
+	return true
+}
+
+type typeInfo uint
+
+// validType verifies that the given type does not "expand" infinitely
+// producing a cycle in the type graph. Cycles are detected by marking
+// defined types.
+// (Cycles involving alias types, as in "type A = [10]A" are detected
+// earlier, via the objDecl cycle detection mechanism.)
+func (check *Checker) validType(typ Type, path []Object) typeInfo {
+	const (
+		unknown typeInfo = iota
+		marked
+		valid
+		invalid
+	)
+
+	switch t := typ.(type) {
+	case *Array:
+		return check.validType(t.elem, path)
+
+	case *Struct:
+		for _, f := range t.fields {
+			if check.validType(f.typ, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Interface:
+		for _, etyp := range t.embeddeds {
+			if check.validType(etyp, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Named:
+		// don't touch the type if it is from a different package or the Universe scope
+		// (doing so would lead to a race condition - was issue #35049)
+		if t.obj.pkg != check.pkg {
+			return valid
+		}
+
+		// don't report a 2nd error if we already know the type is invalid
+		// (e.g., if a cycle was detected earlier, via Checker.underlying).
+		if t.underlying == Typ[Invalid] {
+			t.info = invalid
+			return invalid
+		}
+
+		switch t.info {
+		case unknown:
+			t.info = marked
+			t.info = check.validType(t.orig, append(path, t.obj)) // only types of current package added to path
+		case marked:
+			// cycle detected
+			for i, tn := range path {
+				if t.obj.pkg != check.pkg {
+					panic("internal error: type cycle via package-external type")
+				}
+				if tn == t.obj {
+					check.cycleError(path[i:])
+					t.info = invalid
+					return t.info
+				}
+			}
+			panic("internal error: cycle start not found")
+		}
+		return t.info
+
+	case *instance:
+		return check.validType(t.expand(), path)
+	}
+
+	return valid
+}
+
+// cycleError reports a declaration cycle starting with
+// the object in cycle that is "first" in the source.
+func (check *Checker) cycleError(cycle []Object) {
+	// TODO(gri) Should we start with the last (rather than the first) object in the cycle
+	//           since that is the earliest point in the source where we start seeing the
+	//           cycle? That would be more consistent with other error messages.
+	i := firstInSrc(cycle)
+	obj := cycle[i]
+	check.errorf(obj.Pos(), "illegal cycle in declaration of %s", obj.Name())
+	for range cycle {
+		check.errorf(obj.Pos(), "\t%s refers to", obj.Name()) // secondary error, \t indented
+		i++
+		if i >= len(cycle) {
+			i = 0
+		}
+		obj = cycle[i]
+	}
+	check.errorf(obj.Pos(), "\t%s", obj.Name())
+}
+
+// TODO(gri) This functionality should probably be with the Pos implementation.
+func cmpPos(p, q syntax.Pos) int {
+	// TODO(gri) is RelFilename correct here?
+	pname := p.RelFilename()
+	qname := q.RelFilename()
+	switch {
+	case pname < qname:
+		return -1
+	case pname > qname:
+		return +1
+	}
+
+	pline := p.Line()
+	qline := q.Line()
+	switch {
+	case pline < qline:
+		return -1
+	case pline > qline:
+		return +1
+	}
+
+	pcol := p.Col()
+	qcol := q.Col()
+	switch {
+	case pcol < qcol:
+		return -1
+	case pcol > qcol:
+		return +1
+	}
+
+	return 0
+}
+
+// firstInSrc reports the index of the object with the "smallest"
+// source position in path. path must not be empty.
+func firstInSrc(path []Object) int {
+	fst, pos := 0, path[0].Pos()
+	for i, t := range path[1:] {
+		if cmpPos(t.Pos(), pos) < 0 {
+			fst, pos = i+1, t.Pos()
+		}
+	}
+	return fst
+}
+
+func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr) {
+	assert(obj.typ == nil)
+
+	// use the correct value of iota
+	defer func(iota constant.Value) { check.iota = iota }(check.iota)
+	check.iota = obj.val
+
+	// provide valid constant value under all circumstances
+	obj.val = constant.MakeUnknown()
+
+	// determine type, if any
+	if typ != nil {
+		t := check.typ(typ)
+		if !isConstType(t) {
+			// don't report an error if the type is an invalid C (defined) type
+			// (issue #22090)
+			if t.Under() != Typ[Invalid] {
+				check.errorf(typ, "invalid constant type %s", t)
+			}
+			obj.typ = Typ[Invalid]
+			return
+		}
+		obj.typ = t
+	}
+
+	// check initialization
+	var x operand
+	if init != nil {
+		check.expr(&x, init)
+	}
+	check.initConst(obj, &x)
+}
+
+func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
+	assert(obj.typ == nil)
+
+	// determine type, if any
+	if typ != nil {
+		obj.typ = check.varType(typ)
+		// We cannot spread the type to all lhs variables if there
+		// are more than one since that would mark them as checked
+		// (see Checker.objDecl) and the assignment of init exprs,
+		// if any, would not be checked.
+		//
+		// TODO(gri) If we have no init expr, we should distribute
+		// a given type otherwise we need to re-evalate the type
+		// expr for each lhs variable, leading to duplicate work.
+	}
+
+	// check initialization
+	if init == nil {
+		if typ == nil {
+			// error reported before by arityMatch
+			obj.typ = Typ[Invalid]
+		}
+		return
+	}
+
+	if lhs == nil || len(lhs) == 1 {
+		assert(lhs == nil || lhs[0] == obj)
+		var x operand
+		check.expr(&x, init)
+		check.initVar(obj, &x, "variable declaration")
+		return
+	}
+
+	if debug {
+		// obj must be one of lhs
+		found := false
+		for _, lhs := range lhs {
+			if obj == lhs {
+				found = true
+				break
+			}
+		}
+		if !found {
+			panic("inconsistent lhs")
+		}
+	}
+
+	// We have multiple variables on the lhs and one init expr.
+	// Make sure all variables have been given the same type if
+	// one was specified, otherwise they assume the type of the
+	// init expression values (was issue #15755).
+	if typ != nil {
+		for _, lhs := range lhs {
+			lhs.typ = obj.typ
+		}
+	}
+
+	check.initVars(lhs, []syntax.Expr{init}, nopos)
+}
+
+// Under returns the expanded underlying type of n0; possibly by following
+// forward chains of named types. If an underlying type is found, resolve
+// the chain by setting the underlying type for each defined type in the
+// chain before returning it. If no underlying type is found or a cycle
+// is detected, the result is Typ[Invalid]. If a cycle is detected and
+// n0.check != nil, the cycle is reported.
+func (n0 *Named) Under() Type {
+	u := n0.underlying
+	if u == nil {
+		return Typ[Invalid]
+	}
+
+	// If the underlying type of a defined type is not a defined
+	// type, then that is the desired underlying type.
+	n := u.Named()
+	if n == nil {
+		return u // common case
+	}
+
+	// Otherwise, follow the forward chain.
+	seen := map[*Named]int{n0: 0}
+	path := []Object{n0.obj}
+	for {
+		u = n.underlying
+		if u == nil {
+			u = Typ[Invalid]
+			break
+		}
+		n1 := u.Named()
+		if n1 == nil {
+			break // end of chain
+		}
+
+		seen[n] = len(seen)
+		path = append(path, n.obj)
+		n = n1
+
+		if i, ok := seen[n]; ok {
+			// cycle
+			if n0.check != nil {
+				n0.check.cycleError(path[i:])
+			}
+			u = Typ[Invalid]
+			break
+		}
+	}
+
+	for n := range seen {
+		// We should never have to update the underlying type of an imported type;
+		// those underlying types should have been resolved during the import.
+		// Also, doing so would lead to a race condition (was issue #31749).
+		// Do this check always, not just in debug more (it's cheap).
+		if n0.check != nil && n.obj.pkg != n0.check.pkg {
+			panic("internal error: imported type with unresolved underlying type")
+		}
+		n.underlying = u
+	}
+
+	return u
+}
+
+func (n *Named) setUnderlying(typ Type) {
+	if n != nil {
+		n.underlying = typ
+	}
+}
+
+func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named) {
+	assert(obj.typ == nil)
+
+	check.later(func() {
+		check.validType(obj.typ, nil)
+	})
+
+	alias := tdecl.Alias
+	if alias && tdecl.TParamList != nil {
+		// The parser will ensure this but we may still get an invalid AST.
+		// Complain and continue as regular type definition.
+		check.errorf(tdecl, "generic type cannot be alias")
+		alias = false
+	}
+
+	if alias {
+		// type alias declaration
+
+		obj.typ = Typ[Invalid]
+		obj.typ = check.anyType(tdecl.Type)
+
+	} else {
+		// defined type declaration
+
+		named := &Named{check: check, obj: obj}
+		def.setUnderlying(named)
+		obj.typ = named // make sure recursive type declarations terminate
+
+		if tdecl.TParamList != nil {
+			check.openScope(tdecl, "type parameters")
+			defer check.closeScope()
+			named.tparams = check.collectTypeParams(tdecl.TParamList)
+		}
+
+		// determine underlying type of named
+		named.orig = check.definedType(tdecl.Type, named)
+
+		// The underlying type of named may be itself a named type that is
+		// incomplete:
+		//
+		//	type (
+		//		A B
+		//		B *C
+		//		C A
+		//	)
+		//
+		// The type of C is the (named) type of A which is incomplete,
+		// and which has as its underlying type the named type B.
+		// Determine the (final, unnamed) underlying type by resolving
+		// any forward chain.
+		// TODO(gri) Investigate if we can just use named.origin here
+		//           and rely on lazy computation of the underlying type.
+		named.underlying = named.Under()
+	}
+
+}
+
+func (check *Checker) collectTypeParams(list []*syntax.Field) (tparams []*TypeName) {
+	// Type parameter lists should not be empty. The parser will
+	// complain but we still may get an incorrect AST: ignore it.
+	if len(list) == 0 {
+		return
+	}
+
+	// Declare type parameters up-front, with empty interface as type bound.
+	// The scope of type parameters starts at the beginning of the type parameter
+	// list (so we can have mutually recursive parameterized interfaces).
+	for _, f := range list {
+		tparams = check.declareTypeParam(tparams, f.Name)
+	}
+
+	var bound Type
+	for i, j := 0, 0; i < len(list); i = j {
+		f := list[i]
+		ftype := f.Type
+
+		// determine the range of type parameters list[i:j] with identical type bound
+		// (declared as in (type a, b, c B))
+		j = i + 1
+		for j < len(list) && list[j].Type == ftype {
+			j++
+		}
+
+		// this should never be the case, but be careful
+		if ftype == nil {
+			continue
+		}
+
+		// If the type bound expects exactly one type argument, permit leaving
+		// it away and use the corresponding type parameter as implicit argument.
+		// This allows us to write (type p b(p), q b(q), r b(r)) as (type p, q, r b).
+		// Enabled if enableImplicitTParam is set.
+		const enableImplicitTParam = false
+
+		// The predeclared identifier "any" is visible only as a constraint
+		// in a type parameter list. Look for it before general constraint
+		// resolution.
+		if tident, _ := f.Type.(*syntax.Name); tident != nil && tident.Value == "any" && check.lookup("any") == nil {
+			bound = universeAny
+		} else if enableImplicitTParam {
+			bound = check.anyType(f.Type)
+		} else {
+			bound = check.typ(f.Type)
+		}
+
+		// type bound must be an interface
+		// TODO(gri) We should delay the interface check because
+		//           we may not have a complete interface yet:
+		//           type C(type T C) interface {}
+		//           (issue #39724).
+		if _, ok := bound.Under().(*Interface); ok {
+			if enableImplicitTParam && isGeneric(bound) {
+				base := bound.(*Named) // only a *Named type can be generic
+				if j-i != 1 || len(base.tparams) != 1 {
+					// TODO(gri) make this error message better
+					check.errorf(ftype, "cannot use generic type %s without instantiation (more than one type parameter)", bound)
+					bound = Typ[Invalid]
+					continue
+				}
+				// We have exactly one type parameter.
+				// "Manually" instantiate the bound with each type
+				// parameter the bound applies to.
+				// TODO(gri) this code (in more general form) is also in
+				// checker.typInternal for the *ast.CallExpr case. Factor?
+				typ := new(instance)
+				typ.check = check
+				typ.pos = ftype.Pos()
+				typ.base = base
+				typ.targs = []Type{tparams[i].typ}
+				typ.poslist = []syntax.Pos{f.Name.Pos()}
+				// Make sure we check instantiation works at least once
+				// and that the resulting type is valid.
+				check.atEnd(func() {
+					check.validType(typ.expand(), nil)
+				})
+				// update bound and recorded type
+				bound = typ
+				check.recordTypeAndValue(ftype, typexpr, typ, nil)
+			}
+			// set the type bounds
+			for i < j {
+				tparams[i].typ.(*TypeParam).bound = bound
+				i++
+			}
+		} else if bound != Typ[Invalid] {
+			check.errorf(f.Type, "%s is not an interface", bound)
+		}
+	}
+
+	return
+}
+
+func (check *Checker) declareTypeParam(tparams []*TypeName, name *syntax.Name) []*TypeName {
+	var ptr bool
+	nstr := name.Value
+	if len(nstr) > 0 && nstr[0] == '*' {
+		ptr = true
+		nstr = nstr[1:]
+	}
+	tpar := NewTypeName(name.Pos(), check.pkg, nstr, nil)
+	check.NewTypeParam(ptr, tpar, len(tparams), &emptyInterface) // assigns type to tpar as a side-effect
+	check.declare(check.scope, name, tpar, check.scope.pos)      // TODO(gri) check scope position
+	tparams = append(tparams, tpar)
+
+	if check.conf.Trace {
+		check.trace(name.Pos(), "type param = %v", tparams[len(tparams)-1])
+	}
+
+	return tparams
+}
+
+func (check *Checker) collectMethods(obj *TypeName) {
+	// get associated methods
+	// (Checker.collectObjects only collects methods with non-blank names;
+	// Checker.resolveBaseTypeName ensures that obj is not an alias name
+	// if it has attached methods.)
+	methods := check.methods[obj]
+	if methods == nil {
+		return
+	}
+	delete(check.methods, obj)
+	assert(!check.objMap[obj].tdecl.Alias) // don't use TypeName.IsAlias (requires fully set up object)
+
+	// use an objset to check for name conflicts
+	var mset objset
+
+	// spec: "If the base type is a struct type, the non-blank method
+	// and field names must be distinct."
+	base := obj.typ.Named() // shouldn't fail but be conservative
+	if base != nil {
+		if t, _ := base.underlying.(*Struct); t != nil {
+			for _, fld := range t.fields {
+				if fld.name != "_" {
+					assert(mset.insert(fld) == nil)
+				}
+			}
+		}
+
+		// Checker.Files may be called multiple times; additional package files
+		// may add methods to already type-checked types. Add pre-existing methods
+		// so that we can detect redeclarations.
+		for _, m := range base.methods {
+			assert(m.name != "_")
+			assert(mset.insert(m) == nil)
+		}
+	}
+
+	// add valid methods
+	for _, m := range methods {
+		// spec: "For a base type, the non-blank names of methods bound
+		// to it must be unique."
+		assert(m.name != "_")
+		if alt := mset.insert(m); alt != nil {
+			switch alt.(type) {
+			case *Var:
+				check.errorf(m.pos, "field and method with the same name %s", m.name)
+			case *Func:
+				check.errorf(m.pos, "method %s already declared for %s", m.name, obj)
+			default:
+				unreachable()
+			}
+			check.reportAltDecl(alt)
+			continue
+		}
+
+		if base != nil {
+			base.methods = append(base.methods, m)
+		}
+	}
+}
+
+func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
+	assert(obj.typ == nil)
+
+	// func declarations cannot use iota
+	assert(check.iota == nil)
+
+	sig := new(Signature)
+	obj.typ = sig // guard against cycles
+
+	// Avoid cycle error when referring to method while type-checking the signature.
+	// This avoids a nuisance in the best case (non-parameterized receiver type) and
+	// since the method is not a type, we get an error. If we have a parameterized
+	// receiver type, instantiating the receiver type leads to the instantiation of
+	// its methods, and we don't want a cycle error in that case.
+	// TODO(gri) review if this is correct and/or whether we still need this?
+	saved := obj.color_
+	obj.color_ = black
+	fdecl := decl.fdecl
+	check.funcType(sig, fdecl.Recv, fdecl.TParamList, fdecl.Type)
+	obj.color_ = saved
+
+	// function body must be type-checked after global declarations
+	// (functions implemented elsewhere have no body)
+	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
+		check.later(func() {
+			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
+		})
+	}
+}
+
+func (check *Checker) declStmt(list []syntax.Decl) {
+	pkg := check.pkg
+
+	first := -1                // index of first ConstDecl in the current group, or -1
+	var last *syntax.ConstDecl // last ConstDecl with init expressions, or nil
+	for index, decl := range list {
+		if _, ok := decl.(*syntax.ConstDecl); !ok {
+			first = -1 // we're not in a constant declaration
+		}
+
+		switch s := decl.(type) {
+		case *syntax.ConstDecl:
+			top := len(check.delayed)
+
+			// iota is the index of the current constDecl within the group
+			if first < 0 || list[index-1].(*syntax.ConstDecl).Group != s.Group {
+				first = index
+				last = nil
+			}
+			iota := constant.MakeInt64(int64(index - first))
+
+			// determine which initialization expressions to use
+			inherited := true
+			switch {
+			case s.Type != nil || s.Values != nil:
+				last = s
+				inherited = false
+			case last == nil:
+				last = new(syntax.ConstDecl) // make sure last exists
+				inherited = false
+			}
+
+			// declare all constants
+			lhs := make([]*Const, len(s.NameList))
+			values := unpackExpr(last.Values)
+			for i, name := range s.NameList {
+				obj := NewConst(name.Pos(), pkg, name.Value, nil, iota)
+				lhs[i] = obj
+
+				var init syntax.Expr
+				if i < len(values) {
+					init = values[i]
+				}
+
+				check.constDecl(obj, last.Type, init)
+			}
+
+			// Constants must always have init values.
+			check.arity(s.Pos(), s.NameList, values, true, inherited)
+
+			// process function literals in init expressions before scope changes
+			check.processDelayed(top)
+
+			// spec: "The scope of a constant or variable identifier declared
+			// inside a function begins at the end of the ConstSpec or VarSpec
+			// (ShortVarDecl for short variable declarations) and ends at the
+			// end of the innermost containing block."
+			scopePos := endPos(s)
+			for i, name := range s.NameList {
+				check.declare(check.scope, name, lhs[i], scopePos)
+			}
+
+		case *syntax.VarDecl:
+			top := len(check.delayed)
+
+			lhs0 := make([]*Var, len(s.NameList))
+			for i, name := range s.NameList {
+				lhs0[i] = NewVar(name.Pos(), pkg, name.Value, nil)
+			}
+
+			// initialize all variables
+			values := unpackExpr(s.Values)
+			for i, obj := range lhs0 {
+				var lhs []*Var
+				var init syntax.Expr
+				switch len(values) {
+				case len(s.NameList):
+					// lhs and rhs match
+					init = values[i]
+				case 1:
+					// rhs is expected to be a multi-valued expression
+					lhs = lhs0
+					init = values[0]
+				default:
+					if i < len(values) {
+						init = values[i]
+					}
+				}
+				check.varDecl(obj, lhs, s.Type, init)
+				if len(values) == 1 {
+					// If we have a single lhs variable we are done either way.
+					// If we have a single rhs expression, it must be a multi-
+					// valued expression, in which case handling the first lhs
+					// variable will cause all lhs variables to have a type
+					// assigned, and we are done as well.
+					if debug {
+						for _, obj := range lhs0 {
+							assert(obj.typ != nil)
+						}
+					}
+					break
+				}
+			}
+
+			// If we have no type, we must have values.
+			if s.Type == nil || values != nil {
+				check.arity(s.Pos(), s.NameList, values, false, false)
+			}
+
+			// process function literals in init expressions before scope changes
+			check.processDelayed(top)
+
+			// declare all variables
+			// (only at this point are the variable scopes (parents) set)
+			scopePos := endPos(s) // see constant declarations
+			for i, name := range s.NameList {
+				// see constant declarations
+				check.declare(check.scope, name, lhs0[i], scopePos)
+			}
+
+		case *syntax.TypeDecl:
+			obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
+			// spec: "The scope of a type identifier declared inside a function
+			// begins at the identifier in the TypeSpec and ends at the end of
+			// the innermost containing block."
+			scopePos := s.Name.Pos()
+			check.declare(check.scope, s.Name, obj, scopePos)
+			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
+			obj.setColor(grey + color(check.push(obj)))
+			check.typeDecl(obj, s, nil)
+			check.pop().setColor(black)
+
+		default:
+			check.invalidASTf(s, "unknown syntax.Decl node %T", s)
+		}
+	}
+}

src/cmd/compile/internal/types2/errors.go

@@ -0,0 +1,159 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements various error reporters.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+func unimplemented() {
+	panic("unimplemented")
+}
+
+func assert(p bool) {
+	if !p {
+		panic("assertion failed")
+	}
+}
+
+func unreachable() {
+	panic("unreachable")
+}
+
+func (check *Checker) qualifier(pkg *Package) string {
+	// Qualify the package unless it's the package being type-checked.
+	if pkg != check.pkg {
+		// If the same package name was used by multiple packages, display the full path.
+		if check.pkgCnt[pkg.name] > 1 {
+			return strconv.Quote(pkg.path)
+		}
+		return pkg.name
+	}
+	return ""
+}
+
+func (check *Checker) sprintf(format string, args ...interface{}) string {
+	for i, arg := range args {
+		switch a := arg.(type) {
+		case nil:
+			arg = "<nil>"
+		case operand:
+			panic("internal error: should always pass *operand")
+		case *operand:
+			arg = operandString(a, check.qualifier)
+		case syntax.Pos:
+			arg = a.String()
+		case syntax.Expr:
+			arg = ExprString(a)
+		case Object:
+			arg = ObjectString(a, check.qualifier)
+		case Type:
+			arg = TypeString(a, check.qualifier)
+		}
+		args[i] = arg
+	}
+	return fmt.Sprintf(format, args...)
+}
+
+func (check *Checker) trace(pos syntax.Pos, format string, args ...interface{}) {
+	fmt.Printf("%s:\t%s%s\n",
+		pos,
+		strings.Repeat(".  ", check.indent),
+		check.sprintf(format, args...),
+	)
+}
+
+// dump is only needed for debugging
+func (check *Checker) dump(format string, args ...interface{}) {
+	fmt.Println(check.sprintf(format, args...))
+}
+
+func (check *Checker) err(pos syntax.Pos, msg string, soft bool) {
+	// Cheap trick: Don't report errors with messages containing
+	// "invalid operand" or "invalid type" as those tend to be
+	// follow-on errors which don't add useful information. Only
+	// exclude them if these strings are not at the beginning,
+	// and only if we have at least one error already reported.
+	if check.firstErr != nil && (strings.Index(msg, "invalid operand") > 0 || strings.Index(msg, "invalid type") > 0) {
+		return
+	}
+
+	err := Error{pos, stripAnnotations(msg), msg, soft}
+	if check.firstErr == nil {
+		check.firstErr = err
+	}
+
+	if check.conf.Trace {
+		check.trace(pos, "ERROR: %s", msg)
+	}
+
+	f := check.conf.Error
+	if f == nil {
+		panic(bailout{}) // report only first error
+	}
+	f(err)
+}
+
+type poser interface {
+	Pos() syntax.Pos
+}
+
+func (check *Checker) error(at poser, msg string) {
+	check.err(posFor(at), msg, false)
+}
+
+func (check *Checker) errorf(at poser, format string, args ...interface{}) {
+	check.err(posFor(at), check.sprintf(format, args...), false)
+}
+
+func (check *Checker) softErrorf(at poser, format string, args ...interface{}) {
+	check.err(posFor(at), check.sprintf(format, args...), true)
+}
+
+func (check *Checker) invalidASTf(at poser, format string, args ...interface{}) {
+	check.errorf(at, "invalid AST: "+format, args...)
+}
+
+func (check *Checker) invalidArgf(at poser, format string, args ...interface{}) {
+	check.errorf(at, "invalid argument: "+format, args...)
+}
+
+func (check *Checker) invalidOpf(at poser, format string, args ...interface{}) {
+	check.errorf(at, "invalid operation: "+format, args...)
+}
+
+// posFor reports the left (= start) position of at.
+func posFor(at poser) syntax.Pos {
+	switch x := at.(type) {
+	case *operand:
+		if x.expr != nil {
+			return startPos(x.expr)
+		}
+	case syntax.Node:
+		return startPos(x)
+	}
+	return at.Pos()
+}
+
+// stripAnnotations removes internal (type) annotations from s.
+func stripAnnotations(s string) string {
+	var b strings.Builder
+	for _, r := range s {
+		// strip #'s and subscript digits
+		if r != instanceMarker && !('₀' <= r && r < '₀'+10) { // '₀' == U+2080
+			b.WriteRune(r)
+		}
+	}
+	if b.Len() < len(s) {
+		return b.String()
+	}
+	return s
+}

src/cmd/compile/internal/types2/errors_test.go

@@ -0,0 +1,26 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import "testing"
+
+func TestStripAnnotations(t *testing.T) {
+	for _, test := range []struct {
+		in, want string
+	}{
+		{"", ""},
+		{"   ", "   "},
+		{"foo", "foo"},
+		{"foo₀", "foo"},
+		{"foo(T₀)", "foo(T)"},
+		{"#foo(T₀)", "foo(T)"},
+	} {
+		got := stripAnnotations(test.in)
+		if got != test.want {
+			t.Errorf("%q: got %q; want %q", test.in, got, test.want)
+		}
+	}
+}

src/cmd/compile/internal/types2/example_test.go

@@ -0,0 +1,324 @@
+// UNREVIEWED
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Only run where builders (build.golang.org) have
+// access to compiled packages for import.
+//
+// +build !arm,!arm64
+
+package types2_test
+
+// This file shows examples of basic usage of the go/types API.
+//
+// To locate a Go package, use (*go/build.Context).Import.
+// To load, parse, and type-check a complete Go program
+// from source, use golang.org/x/tools/go/loader.
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/types2"
+	"fmt"
+	"log"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+// ExampleScope prints the tree of Scopes of a package created from a
+// set of parsed files.
+func ExampleScope() {
+	// Parse the source files for a package.
+	var files []*syntax.File
+	for _, file := range []struct{ name, input string }{
+		{"main.go", `
+package main
+import "fmt"
+func main() {
+	freezing := FToC(-18)
+	fmt.Println(freezing, Boiling) }
+`},
+		{"celsius.go", `
+package main
+import "fmt"
+type Celsius float64
+func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
+func FToC(f float64) Celsius { return Celsius(f - 32 / 9 * 5) }
+const Boiling Celsius = 100
+func Unused() { {}; {{ var x int; _ = x }} } // make sure empty block scopes get printed
+`},
+	} {
+		f, err := parseSrc(file.name, file.input)
+		if err != nil {
+			log.Fatal(err)
+		}
+		files = append(files, f)
+	}
+
+	// Type-check a package consisting of these files.
+	// Type information for the imported "fmt" package
+	// comes from $GOROOT/pkg/$GOOS_$GOOARCH/fmt.a.
+	conf := types2.Config{Importer: defaultImporter()}
+	pkg, err := conf.Check("temperature", files, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the tree of scopes.
+	// For determinism, we redact addresses.
+	var buf bytes.Buffer
+	pkg.Scope().WriteTo(&buf, 0, true)
+	rx := regexp.MustCompile(` 0x[a-fA-F0-9]*`)
+	fmt.Println(rx.ReplaceAllString(buf.String(), ""))
+
+	// Output:
+	// package "temperature" scope {
+	// .  const temperature.Boiling temperature.Celsius
+	// .  type temperature.Celsius float64
+	// .  func temperature.FToC(f float64) temperature.Celsius
+	// .  func temperature.Unused()
+	// .  func temperature.main()
+	// .  main.go scope {
+	// .  .  package fmt
+	// .  .  function scope {
+	// .  .  .  var freezing temperature.Celsius
+	// .  .  }
+	// .  }
+	// .  celsius.go scope {
+	// .  .  package fmt
+	// .  .  function scope {
+	// .  .  .  var c temperature.Celsius
+	// .  .  }
+	// .  .  function scope {
+	// .  .  .  var f float64
+	// .  .  }
+	// .  .  function scope {
+	// .  .  .  block scope {
+	// .  .  .  }
+	// .  .  .  block scope {
+	// .  .  .  .  block scope {
+	// .  .  .  .  .  var x int
+	// .  .  .  .  }
+	// .  .  .  }
+	// .  .  }
+	// .  }
+	// }
+}
+
+// ExampleMethodSet prints the method sets of various types.
+func ExampleMethodSet() {
+	// Parse a single source file.
+	const input = `
+package temperature
+import "fmt"
+type Celsius float64
+func (c Celsius) String() string  { return fmt.Sprintf("%g°C", c) }
+func (c *Celsius) SetF(f float64) { *c = Celsius(f - 32 / 9 * 5) }
+
+type S struct { I; m int }
+type I interface { m() byte }
+`
+	f, err := parseSrc("celsius.go", input)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Type-check a package consisting of this file.
+	// Type information for the imported packages
+	// comes from $GOROOT/pkg/$GOOS_$GOOARCH/fmt.a.
+	conf := types2.Config{Importer: defaultImporter()}
+	pkg, err := conf.Check("temperature", []*syntax.File{f}, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the method sets of Celsius and *Celsius.
+	celsius := pkg.Scope().Lookup("Celsius").Type()
+	for _, t := range []types2.Type{celsius, types2.NewPointer(celsius)} {
+		fmt.Printf("Method set of %s:\n", t)
+		mset := types2.NewMethodSet(t)
+		for i := 0; i < mset.Len(); i++ {
+			fmt.Println(mset.At(i))
+		}
+		fmt.Println()
+	}
+
+	// Print the method set of S.
+	styp := pkg.Scope().Lookup("S").Type()
+	fmt.Printf("Method set of %s:\n", styp)
+	fmt.Println(types2.NewMethodSet(styp))
+
+	// Output:
+	// Method set of temperature.Celsius:
+	// method (temperature.Celsius) String() string
+	//
+	// Method set of *temperature.Celsius:
+	// method (*temperature.Celsius) SetF(f float64)
+	// method (*temperature.Celsius) String() string
+	//
+	// Method set of temperature.S:
+	// MethodSet {}
+}
+
+// ExampleInfo prints various facts recorded by the type checker in a
+// types2.Info struct: definitions of and references to each named object,
+// and the type, value, and mode of every expression in the package.
+func ExampleInfo() {
+	// Parse a single source file.
+	const input = `
+package fib
+
+type S string
+
+var a, b, c = len(b), S(c), "hello"
+
+func fib(x int) int {
+	if x < 2 {
+		return x
+	}
+	return fib(x-1) - fib(x-2)
+}`
+	f, err := parseSrc("fib.go", input)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Type-check the package.
+	// We create an empty map for each kind of input
+	// we're interested in, and Check populates them.
+	info := types2.Info{
+		Types: make(map[syntax.Expr]types2.TypeAndValue),
+		Defs:  make(map[*syntax.Name]types2.Object),
+		Uses:  make(map[*syntax.Name]types2.Object),
+	}
+	var conf types2.Config
+	pkg, err := conf.Check("fib", []*syntax.File{f}, &info)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print package-level variables in initialization order.
+	fmt.Printf("InitOrder: %v\n\n", info.InitOrder)
+
+	// For each named object, print the line and
+	// column of its definition and each of its uses.
+	fmt.Println("Defs and Uses of each named object:")
+	usesByObj := make(map[types2.Object][]string)
+	for id, obj := range info.Uses {
+		posn := id.Pos()
+		lineCol := fmt.Sprintf("%d:%d", posn.Line(), posn.Col())
+		usesByObj[obj] = append(usesByObj[obj], lineCol)
+	}
+	var items []string
+	for obj, uses := range usesByObj {
+		sort.Strings(uses)
+		item := fmt.Sprintf("%s:\n  defined at %s\n  used at %s",
+			types2.ObjectString(obj, types2.RelativeTo(pkg)),
+			obj.Pos(),
+			strings.Join(uses, ", "))
+		items = append(items, item)
+	}
+	sort.Strings(items) // sort by line:col, in effect
+	fmt.Println(strings.Join(items, "\n"))
+	fmt.Println()
+
+	// TODO(gri) Enable once positions are updated/verified
+	// fmt.Println("Types and Values of each expression:")
+	// items = nil
+	// for expr, tv := range info.Types {
+	// 	var buf bytes.Buffer
+	// 	posn := expr.Pos()
+	// 	tvstr := tv.Type.String()
+	// 	if tv.Value != nil {
+	// 		tvstr += " = " + tv.Value.String()
+	// 	}
+	// 	// line:col | expr | mode : type = value
+	// 	fmt.Fprintf(&buf, "%2d:%2d | %-19s | %-7s : %s",
+	// 		posn.Line(), posn.Col(), types2.ExprString(expr),
+	// 		mode(tv), tvstr)
+	// 	items = append(items, buf.String())
+	// }
+	// sort.Strings(items)
+	// fmt.Println(strings.Join(items, "\n"))
+
+	// Output:
+	// InitOrder: [c = "hello" b = S(c) a = len(b)]
+	//
+	// Defs and Uses of each named object:
+	// builtin len:
+	//   defined at <unknown position>
+	//   used at 6:15
+	// func fib(x int) int:
+	//   defined at fib.go:8:6
+	//   used at 12:20, 12:9
+	// type S string:
+	//   defined at fib.go:4:6
+	//   used at 6:23
+	// type int:
+	//   defined at <unknown position>
+	//   used at 8:12, 8:17
+	// type string:
+	//   defined at <unknown position>
+	//   used at 4:8
+	// var b S:
+	//   defined at fib.go:6:8
+	//   used at 6:19
+	// var c string:
+	//   defined at fib.go:6:11
+	//   used at 6:25
+	// var x int:
+	//   defined at fib.go:8:10
+	//   used at 10:10, 12:13, 12:24, 9:5
+
+	// TODO(gri) Enable once positions are updated/verified
+	// Types and Values of each expression:
+	//  4: 8 | string              | type    : string
+	//  6:15 | len                 | builtin : func(string) int
+	//  6:15 | len(b)              | value   : int
+	//  6:19 | b                   | var     : fib.S
+	//  6:23 | S                   | type    : fib.S
+	//  6:23 | S(c)                | value   : fib.S
+	//  6:25 | c                   | var     : string
+	//  6:29 | "hello"             | value   : string = "hello"
+	//  8:12 | int                 | type    : int
+	//  8:17 | int                 | type    : int
+	//  9: 5 | x                   | var     : int
+	//  9: 5 | x < 2               | value   : untyped bool
+	//  9: 9 | 2                   | value   : int = 2
+	// 10:10 | x                   | var     : int
+	// 12: 9 | fib                 | value   : func(x int) int
+	// 12: 9 | fib(x - 1)          | value   : int
+	// 12: 9 | fib(x - 1) - fib(x - 2) | value   : int
+	// 12:13 | x                   | var     : int
+	// 12:13 | x - 1               | value   : int
+	// 12:15 | 1                   | value   : int = 1
+	// 12:20 | fib                 | value   : func(x int) int
+	// 12:20 | fib(x - 2)          | value   : int
+	// 12:24 | x                   | var     : int
+	// 12:24 | x - 2               | value   : int
+	// 12:26 | 2                   | value   : int = 2
+}
+
+func mode(tv types2.TypeAndValue) string {
+	switch {
+	case tv.IsVoid():
+		return "void"
+	case tv.IsType():
+		return "type"
+	case tv.IsBuiltin():
+		return "builtin"
+	case tv.IsNil():
+		return "nil"
+	case tv.Assignable():
+		if tv.Addressable() {
+			return "var"
+		}
+		return "mapindex"
+	case tv.IsValue():
+		return "value"
+	default:
+		return "unknown"
+	}
+}

src/cmd/compile/internal/types2/examples/functions.go2

@@ -0,0 +1,216 @@
+// UNREVIEWED
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of type-parameterized functions.
+
+package p
+
+// Reverse is a generic function that takes a []T argument and
+// reverses that slice in place.
+func Reverse[T any](list []T) {
+	i := 0
+	j := len(list)-1
+	for i < j {
+		list[i], list[j] = list[j], list[i]
+		i++
+		j--
+	}
+}
+
+func _() {
+	// Reverse can be called with an explicit type argument.
+	Reverse[int](nil)
+	Reverse[string]([]string{"foo", "bar"})
+	Reverse[struct{x, y int}]([]struct{x, y int}{{1, 2}, {2, 3}, {3, 4}})
+
+	// Since the type parameter is used for an incoming argument,
+	// it can be inferred from the provided argument's type.
+	Reverse([]string{"foo", "bar"})
+	Reverse([]struct{x, y int}{{1, 2}, {2, 3}, {3, 4}})
+
+	// But the incoming argument must have a type, even if it's a
+	// default type. An untyped nil won't work.
+	// Reverse(nil) // this won't type-check
+
+	// A typed nil will work, though.
+	Reverse([]int(nil))
+}
+
+// Certain functions, such as the built-in `new` could be written using
+// type parameters.
+func new[T any]() *T {
+	var x T
+	return &x
+}
+
+// When calling our own `new`, we need to pass the type parameter
+// explicitly since there is no (value) argument from which the
+// result type could be inferred. We don't try to infer the
+// result type from the assignment to keep things simple and
+// easy to understand.
+var _ = new[int]()
+var _ *float64 = new[float64]() // the result type is indeed *float64
+
+// A function may have multiple type parameters, of course.
+func foo[A, B, C any](a A, b []B, c *C) B {
+	// do something here
+	return b[0]
+}
+
+// As before, we can pass type parameters explicitly.
+var s = foo[int, string, float64](1, []string{"first"}, new[float64]())
+
+// Or we can use type inference.
+var _ float64 = foo(42, []float64{1.0}, &s)
+
+// Type inference works in a straight-forward manner even
+// for variadic functions.
+func variadic[A, B any](A, B, ...B) int
+
+// var _ = variadic(1) // ERROR not enough arguments
+var _ = variadic(1, 2.3)
+var _ = variadic(1, 2.3, 3.4, 4.5)
+var _ = variadic[int, float64](1, 2.3, 3.4, 4)
+
+// Type inference also works in recursive function calls where
+// the inferred type is the type parameter of the caller.
+func f1[T any](x T) {
+	f1(x)
+}
+
+func f2a[T any](x, y T) {
+	f2a(x, y)
+}
+
+func f2b[T any](x, y T) {
+	f2b(y, x)
+}
+
+func g2a[P, Q any](x P, y Q) {
+	g2a(x, y)
+}
+
+func g2b[P, Q any](x P, y Q) {
+	g2b(y, x)
+}
+
+// Here's an example of a recursive function call with variadic
+// arguments and type inference inferring the type parameter of
+// the caller (i.e., itself).
+func max[T interface{ type int }](x ...T) T {
+	var x0 T
+	if len(x) > 0 {
+		x0 = x[0]
+	}
+	if len(x) > 1 {
+		x1 := max(x[1:]...)
+		if x1 > x0 {
+			return x1
+		}
+	}
+	return x0
+}
+
+// When inferring channel types, the channel direction is ignored
+// for the purpose of type inference. Once the type has been in-
+// fered, the usual parameter passing rules are applied.
+// Thus even if a type can be inferred successfully, the function
+// call may not be valid.
+
+func fboth[T any](chan T)
+func frecv[T any](<-chan T)
+func fsend[T any](chan<- T)
+
+func _() {
+	var both chan int
+	var recv <-chan int
+	var send chan<-int
+
+	fboth(both)
+	fboth(recv /* ERROR cannot use */ )
+	fboth(send /* ERROR cannot use */ )
+
+	frecv(both)
+	frecv(recv)
+	frecv(send /* ERROR cannot use */ )
+
+	fsend(both)
+	fsend(recv /* ERROR cannot use */)
+	fsend(send)
+}
+
+func ffboth[T any](func(chan T))
+func ffrecv[T any](func(<-chan T))
+func ffsend[T any](func(chan<- T))
+
+func _() {
+	var both func(chan int)
+	var recv func(<-chan int)
+	var send func(chan<- int)
+
+	ffboth(both)
+	ffboth(recv /* ERROR cannot use */ )
+	ffboth(send /* ERROR cannot use */ )
+
+	ffrecv(both /* ERROR cannot use */ )
+	ffrecv(recv)
+	ffrecv(send /* ERROR cannot use */ )
+
+	ffsend(both /* ERROR cannot use */ )
+	ffsend(recv /* ERROR cannot use */ )
+	ffsend(send)
+}
+
+// When inferring elements of unnamed composite parameter types,
+// if the arguments are defined types, use their underlying types.
+// Even though the matching types are not exactly structurally the
+// same (one is a type literal, the other a named type), because
+// assignment is permitted, parameter passing is permitted as well,
+// so type inference should be able to handle these cases well.
+
+func g1[T any]([]T)
+func g2[T any]([]T, T)
+func g3[T any](*T, ...T)
+
+func _() {
+	type intSlize []int
+	g1([]int{})
+	g1(intSlize{})
+	g2(nil, 0)
+
+	type myString string
+	var s1 string
+	g3(nil, "1", myString("2"), "3")
+	g3(&s1, "1", myString /* ERROR does not match */ ("2"), "3")
+	_ = s1
+
+	type myStruct struct{x int}
+	var s2 myStruct
+	g3(nil, struct{x int}{}, myStruct{})
+	g3(&s2, struct{x int}{}, myStruct{})
+	g3(nil, myStruct{}, struct{x int}{})
+	g3(&s2, myStruct{}, struct{x int}{})
+}
+
+// Here's a realistic example.
+
+func append[T any](s []T, t ...T) []T
+
+func _() {
+	var f func()
+	type Funcs []func()
+	var funcs Funcs
+	_ = append(funcs, f)
+}
+
+// Generic type declarations cannot have empty type parameter lists
+// (that would indicate a slice type). Thus, generic functions cannot
+// have empty type parameter lists, either. This is a syntax error.
+
+func h[] /* ERROR empty type parameter list */ ()
+
+func _() {
+	h[] /* ERROR operand */ ()
+}

src/cmd/compile/internal/types2/examples/methods.go2

@@ -0,0 +1,97 @@
+// UNREVIEWED
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of methods on type-parameterized types.
+
+package p
+
+// Parameterized types may have methods.
+type T1[A any] struct{ a A }
+
+// When declaring a method for a parameterized type, the "instantiated"
+// receiver type acts as an implicit declaration of the type parameters
+// for the receiver type. In the example below, method m1 on type T1 has
+// the receiver type T1[A] which declares the type parameter A for use
+// with this method. That is, within the method m1, A stands for the
+// actual type argument provided to an instantiated T1.
+func (t T1[A]) m1() A { return t.a }
+
+// For instance, if T1 is instantiated with the type int, the type
+// parameter A in m1 assumes that type (int) as well and we can write
+// code like this:
+var x T1[int]
+var _ int = x.m1()
+
+// Because the type parameter provided to a parameterized receiver type
+// is declared through that receiver declaration, it must be an identifier.
+// It cannot possibly be some other type because the receiver type is not
+// instantiated with concrete types, it is standing for the parameterized
+// receiver type.
+func (t T1[[ /* ERROR must be an identifier */ ]int]) m2() {}
+
+// Note that using what looks like a predeclared identifier, say int,
+// as type parameter in this situation is deceptive and considered bad
+// style. In m3 below, int is the name of the local receiver type parameter
+// and it shadows the predeclared identifier int which then cannot be used
+// anymore as expected.
+// This is no different from locally redelaring a predeclared identifier
+// and usually should be avoided. There are some notable exceptions; e.g.,
+// sometimes it makes sense to use the identifier "copy" which happens to
+// also be the name of a predeclared built-in function.
+func (t T1[int]) m3() { var _ int = 42 /* ERROR cannot convert 42 .* to int */ }
+
+// The names of the type parameters used in a parameterized receiver
+// type don't have to match the type parameter names in the the declaration
+// of the type used for the receiver. In our example, even though T1 is
+// declared with type parameter named A, methods using that receiver type
+// are free to use their own name for that type parameter. That is, the
+// name of type parameters is always local to the declaration where they
+// are introduced. In our example we can write a method m2 and use the
+// name X instead of A for the type parameter w/o any difference.
+func (t T1[X]) m4() X { return t.a }
+
+// If the receiver type is parameterized, type parameters must always be
+// provided: this simply follows from the general rule that a parameterized
+// type must be instantiated before it can be used. A method receiver
+// declaration using a parameterized receiver type is no exception. It is
+// simply that such receiver type expressions perform two tasks simultaneously:
+// they declare the (local) type parameters and then use them to instantiate
+// the receiver type. Forgetting to provide a type parameter leads to an error.
+func (t T1 /* ERROR generic type .* without instantiation */ ) m5() {}
+
+// However, sometimes we don't need the type parameter, and thus it is
+// inconvenient to have to choose a name. Since the receiver type expression
+// serves as a declaration for its type parameters, we are free to choose the
+// blank identifier:
+func (t T1[_]) m6() {}
+
+// Naturally, these rules apply to any number of type parameters on the receiver
+// type. Here are some more complex examples.
+type T2[A, B, C any] struct {
+        a A
+        b B
+        c C
+}
+
+// Naming of the type parameters is local and has no semantic impact:
+func (t T2[A, B, C]) m1() (A, B, C) { return t.a, t.b, t.c }
+func (t T2[C, B, A]) m2() (C, B, A) { return t.a, t.b, t.c }
+func (t T2[X, Y, Z]) m3() (X, Y, Z) { return t.a, t.b, t.c }
+
+// Type parameters may be left blank if they are not needed:
+func (t T2[A, _, C]) m4() (A, C) { return t.a, t.c }
+func (t T2[_, _, X]) m5() X { return t.c }
+func (t T2[_, _, _]) m6() {}
+
+// As usual, blank names may be used for any object which we don't care about
+// using later. For instance, we may write an unnamed method with a receiver
+// that cannot be accessed:
+func (_ T2[_, _, _]) _() int { return 42 }
+
+// Because a receiver parameter list is simply a parameter list, we can
+// leave the receiver argument away for receiver types.
+type T0 struct{}
+func (T0) _() {}
+func (T1[A]) _() {}

src/cmd/compile/internal/types2/examples/types.go2

@@ -0,0 +1,261 @@
+// UNREVIEWED
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file shows some examples of generic types.
+
+package p
+
+// List is just what it says - a slice of E elements.
+type List[E any] []E
+
+// A generic (parameterized) type must always be instantiated
+// before it can be used to designate the type of a variable
+// (including a struct field, or function parameter); though
+// for the latter cases, the provided type may be another type
+// parameter. So:
+var _ List[byte] = []byte{}
+
+// A generic binary tree might be declared as follows.
+type Tree[E any] struct {
+	left, right *Tree[E]
+	payload E
+}
+
+// A simple instantiation of Tree:
+var root1 Tree[int]
+
+// The actual type parameter provided may be a generic type itself:
+var root2 Tree[List[int]]
+
+// A couple of more complex examples.
+// We don't need extra parentheses around the element type of the slices on
+// the right (unlike when we use ()'s rather than []'s for type parameters).
+var _ List[List[int]] = []List[int]{}
+var _ List[List[List[Tree[int]]]] = []List[List[Tree[int]]]{}
+
+// Type parameters act like type aliases when used in generic types
+// in the sense that we can "emulate" a specific type instantiation
+// with type aliases.
+type T1[P any] struct {
+	f P
+}
+
+type T2[P any] struct {
+	f struct {
+		g P
+	}
+}
+
+var x1 T1[struct{ g int }]
+var x2 T2[int]
+
+func _() {
+	// This assignment is invalid because the types of x1, x2 are T1(...)
+	// and T2(...) respectively, which are two different defined types.
+	x1 = x2 // ERROR assignment
+
+	// This assignment is valid because the types of x1.f and x2.f are
+	// both struct { g int }; the type parameters act like type aliases
+	// and their actual names don't come into play here.
+	x1.f = x2.f
+}
+
+// We can verify this behavior using type aliases instead:
+type T1a struct {
+	f A1
+}
+type A1 = struct { g int }
+
+type T2a struct {
+	f struct {
+		g A2
+	}
+}
+type A2 = int
+
+var x1a T1a
+var x2a T2a
+
+func _() {
+	x1a = x2a // ERROR assignment
+	x1a.f = x2a.f
+}
+
+// Another interesting corner case are generic types that don't use
+// their type arguments. For instance:
+type T[P any] struct{}
+
+var xint T[int]
+var xbool T[bool]
+
+// Are these two variables of the same type? After all, their underlying
+// types are identical. We consider them to be different because each type
+// instantiation creates a new named type, in this case T<int> and T<bool>
+// even if their underlying types are identical. This is sensible because
+// we might still have methods that have different signatures or behave
+// differently depending on the type arguments, and thus we can't possibly
+// consider such types identical. Consequently:
+func _() {
+	xint = xbool // ERROR assignment
+}
+
+// Generic types cannot be used without instantiation.
+var _ T // ERROR cannot use generic type T
+
+// In type context, generic (parameterized) types cannot be parenthesized before
+// being instantiated. See also NOTES entry from 12/4/2019.
+var _ (T /* ERROR cannot use generic type T */ )[ /* ERROR unexpected \[ */ int]
+
+// All types may be parameterized, including interfaces.
+type I1[T any] interface{
+	m1(T)
+}
+
+// Generic interfaces may be embedded as one would expect.
+type I2 interface {
+	I1(int)     // method!
+	I1[string]  // embedded I1
+}
+
+func _() {
+	var x I2
+	x.I1(0)
+	x.m1("foo")
+}
+
+type I0 interface {
+	m0()
+}
+
+type I3 interface {
+	I0
+	I1[bool]
+	m(string)
+}
+
+func _() {
+	var x I3
+	x.m0()
+	x.m1(true)
+	x.m("foo")
+}
+
+type _ struct {
+	( /* ERROR cannot parenthesize */ int8)
+	( /* ERROR cannot parenthesize */ *int16)
+	*( /* ERROR cannot parenthesize */ int32)
+	List[int]
+
+	int8 /* ERROR int8 redeclared */
+	* /* ERROR int16 redeclared */ int16
+	List /* ERROR List redeclared */ [int]
+}
+
+// It's possible to declare local types whose underlying types
+// are type parameters. As with ordinary type definitions, the
+// types underlying properties are "inherited" but the methods
+// are not.
+func _[T interface{ m(); type int }]() {
+	type L T
+	var x L
+
+	// m is not defined on L (it is not "inherited" from
+	// its underlying type).
+	x.m /* ERROR x.m undefined */ ()
+
+	// But the properties of T, such that as that it supports
+	// the operations of the types given by its type bound,
+	// are also the properties of L.
+	x++
+	_ = x - x
+
+	// On the other hand, if we define a local alias for T,
+	// that alias stands for T as expected.
+	type A = T
+	var y A
+	y.m()
+	_ = y < 0
+}
+
+// As a special case, an explicit type argument may be omitted
+// from a type parameter bound if the type bound expects exactly
+// one type argument. In that case, the type argument is the
+// respective type parameter to which the type bound applies.
+// Note: We may not permit this syntactic sugar at first.
+// Note: This is now disabled. All examples below are adjusted.
+type Adder[T any] interface {
+	Add(T) T
+}
+
+// We don't need to explicitly instantiate the Adder bound
+// if we have exactly one type parameter.
+func Sum[T Adder[T]](list []T) T {
+	var sum T
+	for _, x := range list {
+		sum = sum.Add(x)
+	}
+	return sum
+}
+
+// Valid and invalid variations.
+type B0 interface {}
+type B1[_ any] interface{}
+type B2[_, _ any] interface{}
+
+func _[T1 B0]()
+func _[T1 B1[T1]]()
+func _[T1 B2 /* ERROR cannot use generic type .* without instantiation */ ]()
+
+func _[T1, T2 B0]()
+func _[T1 B1[T1], T2 B1[T2]]()
+func _[T1, T2 B2 /* ERROR cannot use generic type .* without instantiation */ ]()
+
+func _[T1 B0, T2 B1[T2]]() // here B1 applies to T2
+
+// When the type argument is left away, the type bound is
+// instantiated for each type parameter with that type
+// parameter.
+// Note: We may not permit this syntactic sugar at first.
+func _[A Adder[A], B Adder[B], C Adder[A]]() {
+	var a A // A's type bound is Adder[A]
+	a = a.Add(a)
+	var b B // B's type bound is Adder[B]
+	b = b.Add(b)
+	var c C // C's type bound is Adder[A]
+	a = c.Add(a)
+}
+
+// The type of variables (incl. parameters and return values) cannot
+// be an interface with type constraints or be/embed comparable.
+type I interface {
+	type int
+}
+
+var (
+	_ interface /* ERROR contains type constraints */ {type int}
+	_ I /* ERROR contains type constraints */
+)
+
+func _(I /* ERROR contains type constraints */ )
+func _(x, y, z I /* ERROR contains type constraints */ )
+func _() I /* ERROR contains type constraints */
+
+func _() {
+	var _ I /* ERROR contains type constraints */
+}
+
+type C interface {
+	comparable
+}
+
+var _ comparable /* ERROR comparable */
+var _ C /* ERROR comparable */
+
+func _(_ comparable /* ERROR comparable */ , _ C /* ERROR comparable */ )
+
+func _() {
+	var _ comparable /* ERROR comparable */
+	var _ C /* ERROR comparable */
+}

src/cmd/compile/internal/types2/exprstring.go

@@ -0,0 +1,287 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements printing of expressions.
+
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+)
+
+// ExprString returns the (possibly shortened) string representation for x.
+// Shortened representations are suitable for user interfaces but may not
+// necessarily follow Go syntax.
+func ExprString(x syntax.Expr) string {
+	var buf bytes.Buffer
+	WriteExpr(&buf, x)
+	return buf.String()
+}
+
+// WriteExpr writes the (possibly shortened) string representation for x to buf.
+// Shortened representations are suitable for user interfaces but may not
+// necessarily follow Go syntax.
+func WriteExpr(buf *bytes.Buffer, x syntax.Expr) {
+	// The AST preserves source-level parentheses so there is
+	// no need to introduce them here to correct for different
+	// operator precedences. (This assumes that the AST was
+	// generated by a Go parser.)
+
+	// TODO(gri): This assumption is not correct - we need to recreate
+	//            parentheses in expressions.
+
+	switch x := x.(type) {
+	default:
+		buf.WriteString("(ast: bad expr)") // nil, syntax.BadExpr, syntax.KeyValueExpr
+
+	case *syntax.Name:
+		buf.WriteString(x.Value)
+
+	case *syntax.DotsType:
+		buf.WriteString("...")
+		if x.Elem != nil {
+			WriteExpr(buf, x.Elem)
+		}
+
+	case *syntax.BasicLit:
+		buf.WriteString(x.Value)
+
+	case *syntax.FuncLit:
+		buf.WriteByte('(')
+		WriteExpr(buf, x.Type)
+		buf.WriteString(" literal)") // shortened
+
+	case *syntax.CompositeLit:
+		buf.WriteByte('(')
+		WriteExpr(buf, x.Type)
+		buf.WriteString(" literal)") // shortened
+
+	case *syntax.ParenExpr:
+		buf.WriteByte('(')
+		WriteExpr(buf, x.X)
+		buf.WriteByte(')')
+
+	case *syntax.SelectorExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteByte('.')
+		buf.WriteString(x.Sel.Value)
+
+	case *syntax.IndexExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteByte('[')
+		WriteExpr(buf, x.Index) // x.Index may be a *ListExpr
+		buf.WriteByte(']')
+
+	case *syntax.SliceExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteByte('[')
+		if x.Index[0] != nil {
+			WriteExpr(buf, x.Index[0])
+		}
+		buf.WriteByte(':')
+		if x.Index[1] != nil {
+			WriteExpr(buf, x.Index[1])
+		}
+		if x.Full {
+			buf.WriteByte(':')
+			if x.Index[2] != nil {
+				WriteExpr(buf, x.Index[2])
+			}
+		}
+		buf.WriteByte(']')
+
+	case *syntax.AssertExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteString(".(")
+		WriteExpr(buf, x.Type)
+		buf.WriteByte(')')
+
+	case *syntax.CallExpr:
+		WriteExpr(buf, x.Fun)
+		buf.WriteByte('(')
+		writeExprList(buf, x.ArgList)
+		if x.HasDots {
+			buf.WriteString("...")
+		}
+		buf.WriteByte(')')
+
+	case *syntax.ListExpr:
+		writeExprList(buf, x.ElemList)
+
+	case *syntax.Operation:
+		// TODO(gri) This would be simpler if x.X == nil meant unary expression.
+		if x.Y == nil {
+			// unary expression
+			buf.WriteString(x.Op.String())
+			WriteExpr(buf, x.X)
+		} else {
+			// binary expression
+			WriteExpr(buf, x.X)
+			buf.WriteByte(' ')
+			buf.WriteString(x.Op.String())
+			buf.WriteByte(' ')
+			WriteExpr(buf, x.Y)
+		}
+
+		// case *ast.StarExpr:
+		// 	buf.WriteByte('*')
+		// 	WriteExpr(buf, x.X)
+
+		// case *ast.UnaryExpr:
+		// 	buf.WriteString(x.Op.String())
+		// 	WriteExpr(buf, x.X)
+
+		// case *ast.BinaryExpr:
+		// 	WriteExpr(buf, x.X)
+		// 	buf.WriteByte(' ')
+		// 	buf.WriteString(x.Op.String())
+		// 	buf.WriteByte(' ')
+		// 	WriteExpr(buf, x.Y)
+
+	case *syntax.ArrayType:
+		if x.Len == nil {
+			buf.WriteString("[...]")
+		} else {
+			buf.WriteByte('[')
+			WriteExpr(buf, x.Len)
+			buf.WriteByte(']')
+		}
+		WriteExpr(buf, x.Elem)
+
+	case *syntax.SliceType:
+		buf.WriteString("[]")
+		WriteExpr(buf, x.Elem)
+
+	case *syntax.StructType:
+		buf.WriteString("struct{")
+		writeFieldList(buf, x.FieldList, "; ", false)
+		buf.WriteByte('}')
+
+	case *syntax.FuncType:
+		buf.WriteString("func")
+		writeSigExpr(buf, x)
+
+	case *syntax.InterfaceType:
+		// separate type list types from method list
+		// TODO(gri) we can get rid of this extra code if writeExprList does the separation
+		var types []syntax.Expr
+		var methods []*syntax.Field
+		for _, f := range x.MethodList {
+			if f.Name != nil && f.Name.Value == "type" {
+				// type list type
+				types = append(types, f.Type)
+			} else {
+				// method or embedded interface
+				methods = append(methods, f)
+			}
+		}
+
+		buf.WriteString("interface{")
+		writeFieldList(buf, methods, "; ", true)
+		if len(types) > 0 {
+			if len(methods) > 0 {
+				buf.WriteString("; ")
+			}
+			buf.WriteString("type ")
+			writeExprList(buf, types)
+		}
+		buf.WriteByte('}')
+
+	case *syntax.MapType:
+		buf.WriteString("map[")
+		WriteExpr(buf, x.Key)
+		buf.WriteByte(']')
+		WriteExpr(buf, x.Value)
+
+	case *syntax.ChanType:
+		var s string
+		switch x.Dir {
+		case syntax.SendOnly:
+			s = "chan<- "
+		case syntax.RecvOnly:
+			s = "<-chan "
+		default:
+			s = "chan "
+		}
+		buf.WriteString(s)
+		if e, _ := x.Elem.(*syntax.ChanType); x.Dir != syntax.SendOnly && e != nil && e.Dir == syntax.RecvOnly {
+			// don't print chan (<-chan T) as chan <-chan T (but chan<- <-chan T is ok)
+			buf.WriteByte('(')
+			WriteExpr(buf, x.Elem)
+			buf.WriteByte(')')
+		} else {
+			WriteExpr(buf, x.Elem)
+		}
+	}
+}
+
+func writeSigExpr(buf *bytes.Buffer, sig *syntax.FuncType) {
+	buf.WriteByte('(')
+	writeFieldList(buf, sig.ParamList, ", ", false)
+	buf.WriteByte(')')
+
+	res := sig.ResultList
+	n := len(res)
+	if n == 0 {
+		// no result
+		return
+	}
+
+	buf.WriteByte(' ')
+	if n == 1 && res[0].Name == nil {
+		// single unnamed result
+		WriteExpr(buf, res[0].Type)
+		return
+	}
+
+	// multiple or named result(s)
+	buf.WriteByte('(')
+	writeFieldList(buf, res, ", ", false)
+	buf.WriteByte(')')
+}
+
+func writeFieldList(buf *bytes.Buffer, list []*syntax.Field, sep string, iface bool) {
+	for i := 0; i < len(list); {
+		f := list[i]
+		if i > 0 {
+			buf.WriteString(sep)
+		}
+
+		// if we don't have a name, we have an embedded type
+		if f.Name == nil {
+			WriteExpr(buf, f.Type)
+			i++
+			continue
+		}
+
+		// types of interface methods consist of signatures only
+		if sig, _ := f.Type.(*syntax.FuncType); sig != nil && iface {
+			buf.WriteString(f.Name.Value)
+			writeSigExpr(buf, sig)
+			i++
+			continue
+		}
+
+		// write the type only once for a sequence of fields with the same type
+		t := f.Type
+		buf.WriteString(f.Name.Value)
+		for i++; i < len(list) && list[i].Type == t; i++ {
+			buf.WriteString(", ")
+			buf.WriteString(list[i].Name.Value)
+		}
+		buf.WriteByte(' ')
+		WriteExpr(buf, t)
+	}
+}
+
+func writeExprList(buf *bytes.Buffer, list []syntax.Expr) {
+	for i, x := range list {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		WriteExpr(buf, x)
+	}
+}

src/cmd/compile/internal/types2/exprstring_test.go

@@ -0,0 +1,97 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2_test
+
+import (
+	"testing"
+
+	"cmd/compile/internal/syntax"
+	. "cmd/compile/internal/types2"
+)
+
+var testExprs = []testEntry{
+	// basic type literals
+	dup("x"),
+	dup("true"),
+	dup("42"),
+	dup("3.1415"),
+	dup("2.71828i"),
+	dup(`'a'`),
+	dup(`"foo"`),
+	dup("`bar`"),
+
+	// func and composite literals
+	{"func(){}", "(func() literal)"},
+	{"func(x int) complex128 {}", "(func(x int) complex128 literal)"},
+	{"[]int{1, 2, 3}", "([]int literal)"},
+
+	// non-type expressions
+	dup("(x)"),
+	dup("x.f"),
+	dup("a[i]"),
+
+	dup("s[:]"),
+	dup("s[i:]"),
+	dup("s[:j]"),
+	dup("s[i:j]"),
+	dup("s[:j:k]"),
+	dup("s[i:j:k]"),
+
+	dup("x.(T)"),
+
+	dup("x.([10]int)"),
+	dup("x.([...]int)"),
+
+	dup("x.(struct{})"),
+	dup("x.(struct{x int; y, z float32; E})"),
+
+	dup("x.(func())"),
+	dup("x.(func(x int))"),
+	dup("x.(func() int)"),
+	dup("x.(func(x, y int, z float32) (r int))"),
+	dup("x.(func(a, b, c int))"),
+	dup("x.(func(x ...T))"),
+
+	dup("x.(interface{})"),
+	dup("x.(interface{m(); n(x int); E})"),
+	dup("x.(interface{m(); n(x int) T; E; F})"),
+
+	dup("x.(map[K]V)"),
+
+	dup("x.(chan E)"),
+	dup("x.(<-chan E)"),
+	dup("x.(chan<- chan int)"),
+	dup("x.(chan<- <-chan int)"),
+	dup("x.(<-chan chan int)"),
+	dup("x.(chan (<-chan int))"),
+
+	dup("f()"),
+	dup("f(x)"),
+	dup("int(x)"),
+	dup("f(x, x + y)"),
+	dup("f(s...)"),
+	dup("f(a, s...)"),
+
+	dup("*x"),
+	dup("&x"),
+	dup("x + y"),
+	dup("x + y << (2 * s)"),
+}
+
+func TestExprString(t *testing.T) {
+	for _, test := range testExprs {
+		src := "package p; var _ = " + test.src
+		f, err := parseSrc("expr", src)
+		if err != nil {
+			t.Errorf("%s: %s", test.src, err)
+			continue
+		}
+		x := f.DeclList[0].(*syntax.VarDecl).Values
+		if got := ExprString(x); got != test.str {
+			t.Errorf("%s: got %s, want %s", test.src, got, test.str)
+		}
+	}
+}

src/cmd/compile/internal/types2/fixedbugs/issue39634.go2

@@ -0,0 +1,92 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Examples adjusted to match new [T any] syntax for type parameters.
+// Also, previously permitted empty type parameter lists and instantiations
+// are now syntax errors.
+
+package p
+
+// crash 1
+type nt1[_ any]interface{g /* ERROR undeclared name */ }
+type ph1[e nt1[e],g(d /* ERROR undeclared name */ )]s /* ERROR undeclared name */
+func(*ph1[e,e /* ERROR redeclared */ ])h(d /* ERROR undeclared name */ )
+
+// crash 2
+// Disabled: empty []'s are now syntax errors. This example leads to too many follow-on errors.
+// type Numeric2 interface{t2 /* ERROR not a type */ }
+// func t2[T Numeric2](s[]T){0 /* ERROR not a type */ []{s /* ERROR cannot index */ [0][0]}}
+
+// crash 3
+type t3 *interface{ t3.p /* ERROR no field or method p */ }
+
+// crash 4
+type Numeric4 interface{t4 /* ERROR not a type */ }
+func t4[T Numeric4](s[]T){if( /* ERROR non-boolean */ 0){*s /* ERROR cannot indirect */ [0]}}
+
+// crash 7
+type foo7 interface { bar() }
+type x7[A any] struct{ foo7 }
+func main7() { var _ foo7 = x7[int]{} }
+
+// crash 8
+type foo8[A any] interface { type A }
+func bar8[A foo8[A]](a A) {}
+func main8() {}
+
+// crash 9
+type foo9[A any] interface { type foo9 /* ERROR interface contains type constraints */ [A] }
+func _() { var _ = new(foo9 /* ERROR interface contains type constraints */ [int]) }
+
+// crash 12
+var u /* ERROR cycle */ , i [func /* ERROR used as value */ /* ERROR used as value */ (u, c /* ERROR undeclared */ /* ERROR undeclared */ ) {}(0, len)]c /* ERROR undeclared */ /* ERROR undeclared */
+
+// crash 15
+func y15() { var a /* ERROR declared but not used */ interface{ p() } = G15[string]{} }
+type G15[X any] s /* ERROR undeclared name */
+func (G15 /* ERROR generic type .* without instantiation */ ) p()
+
+// crash 16
+type Foo16[T any] r16 /* ERROR not a type */
+func r16[T any]() Foo16[Foo16[T]]
+
+// crash 17
+type Y17 interface{ c() }
+type Z17 interface {
+	c() Y17
+	Y17 /* ERROR duplicate method */
+}
+func F17[T Z17](T)
+
+// crash 18
+type o18[T any] []func(_ o18[[]_ /* ERROR cannot use _ */ ])
+
+// crash 19
+type Z19 [][[]Z19{}[0][0]]c19 /* ERROR undeclared */
+
+// crash 20
+type Z20 /* ERROR illegal cycle */ interface{ Z20 }
+func F20[t Z20]() { F20(t /* ERROR invalid composite literal type */ {}) }
+
+// crash 21
+type Z21 /* ERROR illegal cycle */ interface{ Z21 }
+func F21[T Z21]() { ( /* ERROR not used */ F21[Z21]) }
+
+// crash 24
+type T24[P any] P
+func (r T24[P]) m() { T24 /* ERROR without instantiation */ .m() }
+
+// crash 25
+type T25[A any] int
+func (t T25[A]) m1() {}
+var x T25 /* ERROR without instantiation */ .m1
+
+// crash 26
+type T26 = interface{ F26[ /* ERROR cannot have type parameters */ Z any]() }
+func F26[Z any]() T26 { return F26 /* ERROR without instantiation */ /* ERROR missing method */ [] /* ERROR operand */ }
+
+// crash 27
+func e27[T any]() interface{ x27 /* ERROR not a type */ }
+func x27() { e27( /* ERROR cannot infer T */ ) }

src/cmd/compile/internal/types2/fixedbugs/issue39664.go2

@@ -0,0 +1,16 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[_ any] struct {}
+
+func (T /* ERROR instantiation */ ) m()
+
+func _() {
+	var x interface { m() }
+	x = T[int]{}
+	_ = x
+}

src/cmd/compile/internal/types2/fixedbugs/issue39680.go2

@@ -0,0 +1,28 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "fmt"
+
+// Minimal test case.
+func _[T interface{type T}](x T) T{
+	return x
+}
+
+// Test case from issue.
+type constr[T any] interface {
+	type T
+}
+
+func Print[T constr[T]](s []T) {
+	for _, v := range s {
+		fmt.Print(v)
+	}
+}
+
+func f() {
+	Print([]string{"Hello, ", "playground\n"})
+}

src/cmd/compile/internal/types2/fixedbugs/issue39693.go2

@@ -0,0 +1,15 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Number interface {
+	int     /* ERROR int is not an interface */
+	float64 /* ERROR float64 is not an interface */
+}
+
+func Add[T Number](a, b T) T {
+	return a /* ERROR not defined */ + b
+}

src/cmd/compile/internal/types2/fixedbugs/issue39699.go2

@@ -0,0 +1,30 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T0 interface{
+}
+
+type T1 interface{
+	type int
+}
+
+type T2 interface{
+	comparable
+}
+
+type T3 interface {
+	T0
+	T1
+	T2
+}
+
+func _() {
+	_ = T0(0)
+	_ = T1 /* ERROR cannot use interface T1 in conversion */ (1)
+	_ = T2 /* ERROR cannot use interface T2 in conversion */ (2)
+	_ = T3 /* ERROR cannot use interface T3 in conversion */ (3)
+}

src/cmd/compile/internal/types2/fixedbugs/issue39711.go2

@@ -0,0 +1,12 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Do not report a duplicate type error for this type list.
+// (Check types after interfaces have been completed.)
+type _ interface {
+	type interface{ Error() string }, interface{ String() string }
+}

src/cmd/compile/internal/types2/fixedbugs/issue39723.go2

@@ -0,0 +1,10 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// A constraint must be an interface; it cannot
+// be a type parameter, for instance.
+func _[A interface{ type interface{} }, B A /* ERROR not an interface */ ]()

src/cmd/compile/internal/types2/fixedbugs/issue39725.go2

@@ -0,0 +1,17 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f1[T1, T2 any](T1, T2, struct{a T1; b T2})
+func _() {
+	f1(42, string("foo"), struct /* ERROR does not match inferred type struct\{a int; b string\} */ {a, b int}{})
+}
+
+// simplified test case from issue
+func f2[T any](_ []T, _ func(T))
+func _() {
+	f2([]string{}, func /* ERROR does not match inferred type func\(string\) */ (f []byte) {})
+}

src/cmd/compile/internal/types2/fixedbugs/issue39754.go2

@@ -0,0 +1,21 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type Optional[T any] struct {}
+
+func (_ Optional[T]) Val() (T, bool)
+
+type Box[T any] interface {
+	Val() (T, bool)
+}
+
+func f[V interface{}, A, B Box[V]]() {}
+
+func _() {
+	f[int, Optional[int], Optional[int]]()
+	f[int, Optional[int], Optional /* ERROR does not satisfy Box */ [string]]()
+}

src/cmd/compile/internal/types2/fixedbugs/issue39755.go2

@@ -0,0 +1,24 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _[T interface{type map[string]int}](x T) {
+	_ = x == nil
+}
+
+// simplified test case from issue
+
+type PathParamsConstraint interface {
+        type map[string]string, []struct{key, value string}
+}
+
+type PathParams[T PathParamsConstraint] struct {
+	t T
+}
+
+func (pp *PathParams[T]) IsNil() bool {
+	return pp.t == nil // this must succeed
+}

src/cmd/compile/internal/types2/fixedbugs/issue39768.go2

@@ -0,0 +1,21 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[P any] P
+type A = T
+var x A[int]
+var _ A /* ERROR cannot use generic type */
+
+type B = T[int]
+var y B = x
+var _ B /* ERROR not a generic type */ [int]
+
+// test case from issue
+
+type Vector[T any] []T
+type VectorAlias = Vector
+var v Vector[int]

src/cmd/compile/internal/types2/fixedbugs/issue39938.go2

@@ -0,0 +1,51 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check "infinite expansion" cycle errors across instantiated types.
+
+package p
+
+type E0[P any] P
+type E1[P any] *P
+type E2[P any] struct{ P }
+type E3[P any] struct{ *P }
+
+type T0 /* ERROR illegal cycle */ struct {
+        _ E0[T0]
+}
+
+type T0_ /* ERROR illegal cycle */ struct {
+        E0[T0_]
+}
+
+type T1 struct {
+        _ E1[T1]
+}
+
+type T2 /* ERROR illegal cycle */ struct {
+        _ E2[T2]
+}
+
+type T3 struct {
+        _ E3[T3]
+}
+
+// some more complex cases
+
+type T4 /* ERROR illegal cycle */ struct {
+	_ E0[E2[T4]]
+}
+
+type T5 struct {
+	_ E0[E2[E0[E1[E2[[10]T5]]]]]
+}
+
+type T6 /* ERROR illegal cycle */ struct {
+	_ E0[[10]E2[E0[E2[E2[T6]]]]]
+}
+
+type T7 struct {
+	_ E0[[]E2[E0[E2[E2[T6]]]]]
+}

src/cmd/compile/internal/types2/fixedbugs/issue39948.go2

@@ -0,0 +1,10 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[P any] interface{
+	P // ERROR P is a type parameter, not an interface
+}

src/cmd/compile/internal/types2/fixedbugs/issue39976.go2

@@ -0,0 +1,17 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type policy[K, V any] interface{}
+type LRU[K, V any] struct{}
+
+func NewCache[K, V any](p policy[K, V])
+
+func _() {
+	var lru LRU[int, string]
+	NewCache[int, string](&lru)
+	NewCache(& /* ERROR does not match policy\[K, V\] \(cannot infer K and V\) */ lru)
+}

src/cmd/compile/internal/types2/fixedbugs/issue39982.go2

@@ -0,0 +1,37 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+	T[_ any] struct{}
+	S[_ any] struct {
+		data T[*T[int]]
+	}
+)
+
+func _() {
+	_ = S[int]{
+		data: T[*T[int]]{},
+	}
+}
+
+// full test case from issue
+
+type (
+	Element[TElem any] struct{}
+
+	entry[K comparable] struct{}
+
+	Cache[K comparable] struct {
+		data map[K]*Element[*entry[K]]
+	}
+)
+
+func _() {
+	_ = Cache[int]{
+		data: make(map[int](*Element[*entry[int]])),
+	}
+}

src/cmd/compile/internal/types2/fixedbugs/issue40038.go2

@@ -0,0 +1,16 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type A[T any] int
+
+func (A[T]) m(A[T])
+
+func f[P interface{m(P)}]()
+
+func _() {
+	_ = f[A[int]]
+}

src/cmd/compile/internal/types2/fixedbugs/issue40056.go2

@@ -0,0 +1,16 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	NewS( /* ERROR cannot infer T */ ) .M()
+}
+
+type S struct {}
+
+func NewS[T any]() *S
+
+func (_ *S /* ERROR S is not a generic type */ [T]) M()

src/cmd/compile/internal/types2/fixedbugs/issue40057.go2

@@ -0,0 +1,18 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func _() {
+	var x interface{}
+	switch t := x.(type) {
+	case S /* ERROR cannot use generic type */ :
+		t.m()
+	}
+}
+
+type S[T any] struct {}
+
+func (_ S[T]) m()

src/cmd/compile/internal/types2/fixedbugs/issue40301.go2

@@ -0,0 +1,13 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+func _[T any](x T) {
+	_ = unsafe /* ERROR undefined */ .Alignof(x)
+	_ = unsafe /* ERROR undefined */ .Sizeof(x)
+}

src/cmd/compile/internal/types2/fixedbugs/issue40684.go2

@@ -0,0 +1,16 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T[_ any] int
+
+func f[_ any]()
+func g[_, _ any]()
+
+func _() {
+	_ = f[T /* ERROR without instantiation */ ]
+	_ = g[T /* ERROR without instantiation */ , T /* ERROR without instantiation */ ]
+}

src/cmd/compile/internal/types2/fixedbugs/issue41124.go2

@@ -0,0 +1,92 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Test case from issue.
+
+type Nat interface {
+	type Zero, Succ
+}
+
+type Zero struct{}
+type Succ struct{
+	Nat // ERROR interface contains type constraints
+}
+
+// Struct tests.
+
+type I1 interface {
+	comparable
+}
+
+type I2 interface {
+	type int
+}
+
+type I3 interface {
+	I1
+	I2
+}
+
+type _ struct {
+	f I1 // ERROR interface is .* comparable
+}
+
+type _ struct {
+	comparable // ERROR interface is .* comparable
+}
+
+type _ struct{
+	I1 // ERROR interface is .* comparable
+}
+
+type _ struct{
+	I2 // ERROR interface contains type constraints
+}
+
+type _ struct{
+	I3 // ERROR interface contains type constraints
+}
+
+// General composite types.
+
+type (
+	_ [10]I1 // ERROR interface is .* comparable
+	_ [10]I2 // ERROR interface contains type constraints
+
+	_ []I1 // ERROR interface is .* comparable
+	_ []I2 // ERROR interface contains type constraints
+
+	_ *I3 // ERROR interface contains type constraints
+	_ map[I1 /* ERROR interface is .* comparable */ ]I2 // ERROR interface contains type constraints
+	_ chan I3 // ERROR interface contains type constraints
+	_ func(I1 /* ERROR interface is .* comparable */ )
+	_ func() I2 // ERROR interface contains type constraints
+)
+
+// Other cases.
+
+var _ = [...]I3 /* ERROR interface contains type constraints */ {}
+
+func _(x interface{}) {
+	_ = x.(I3 /* ERROR interface contains type constraints */ )
+}
+
+type T1[_ any] struct{}
+type T3[_, _, _ any] struct{}
+var _ T1[I2 /* ERROR interface contains type constraints */ ]
+var _ T3[int, I2 /* ERROR interface contains type constraints */ , float32]
+
+func f1[_ any]() int
+var _ = f1[I2 /* ERROR interface contains type constraints */ ]()
+func f3[_, _, _ any]() int
+var _ = f3[int, I2 /* ERROR interface contains type constraints */ , float32]()
+
+func _(x interface{}) {
+	switch x.(type) {
+	case I2 /* ERROR interface contains type constraints */ :
+	}
+}

src/cmd/compile/internal/types2/gccgosizes.go

@@ -0,0 +1,41 @@
+// UNREVIEWED
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a copy of the file generated during the gccgo build process.
+// Last update 2019-01-22.
+
+package types2
+
+var gccgoArchSizes = map[string]*StdSizes{
+	"386":         {4, 4},
+	"alpha":       {8, 8},
+	"amd64":       {8, 8},
+	"amd64p32":    {4, 8},
+	"arm":         {4, 8},
+	"armbe":       {4, 8},
+	"arm64":       {8, 8},
+	"arm64be":     {8, 8},
+	"ia64":        {8, 8},
+	"m68k":        {4, 2},
+	"mips":        {4, 8},
+	"mipsle":      {4, 8},
+	"mips64":      {8, 8},
+	"mips64le":    {8, 8},
+	"mips64p32":   {4, 8},
+	"mips64p32le": {4, 8},
+	"nios2":       {4, 8},
+	"ppc":         {4, 8},
+	"ppc64":       {8, 8},
+	"ppc64le":     {8, 8},
+	"riscv":       {4, 8},
+	"riscv64":     {8, 8},
+	"s390":        {4, 8},
+	"s390x":       {8, 8},
+	"sh":          {4, 8},
+	"shbe":        {4, 8},
+	"sparc":       {4, 8},
+	"sparc64":     {8, 8},
+	"wasm":        {8, 8},
+}

src/cmd/compile/internal/types2/hilbert_test.go

@@ -0,0 +1,220 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2_test
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"testing"
+
+	. "cmd/compile/internal/types2"
+)
+
+var (
+	H   = flag.Int("H", 5, "Hilbert matrix size")
+	out = flag.String("out", "", "write generated program to out")
+)
+
+func TestHilbert(t *testing.T) {
+	// generate source
+	src := program(*H, *out)
+	if *out != "" {
+		ioutil.WriteFile(*out, src, 0666)
+		return
+	}
+
+	// parse source
+	// TODO(gri) get rid of []bytes to string conversion below
+	f, err := parseSrc("hilbert.go", string(src))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// type-check file
+	DefPredeclaredTestFuncs() // define assert built-in
+	conf := Config{Importer: defaultImporter()}
+	_, err = conf.Check(f.PkgName.Value, []*syntax.File{f}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func program(n int, out string) []byte {
+	var g gen
+
+	g.p(`// Code generated by: go test -run=Hilbert -H=%d -out=%q. DO NOT EDIT.
+
+// +`+`build ignore
+
+// This program tests arbitrary precision constant arithmetic
+// by generating the constant elements of a Hilbert matrix H,
+// its inverse I, and the product P = H*I. The product should
+// be the identity matrix.
+package main
+
+func main() {
+	if !ok {
+		printProduct()
+		return
+	}
+	println("PASS")
+}
+
+`, n, out)
+	g.hilbert(n)
+	g.inverse(n)
+	g.product(n)
+	g.verify(n)
+	g.printProduct(n)
+	g.binomials(2*n - 1)
+	g.factorials(2*n - 1)
+
+	return g.Bytes()
+}
+
+type gen struct {
+	bytes.Buffer
+}
+
+func (g *gen) p(format string, args ...interface{}) {
+	fmt.Fprintf(&g.Buffer, format, args...)
+}
+
+func (g *gen) hilbert(n int) {
+	g.p(`// Hilbert matrix, n = %d
+const (
+`, n)
+	for i := 0; i < n; i++ {
+		g.p("\t")
+		for j := 0; j < n; j++ {
+			if j > 0 {
+				g.p(", ")
+			}
+			g.p("h%d_%d", i, j)
+		}
+		if i == 0 {
+			g.p(" = ")
+			for j := 0; j < n; j++ {
+				if j > 0 {
+					g.p(", ")
+				}
+				g.p("1.0/(iota + %d)", j+1)
+			}
+		}
+		g.p("\n")
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) inverse(n int) {
+	g.p(`// Inverse Hilbert matrix
+const (
+`)
+	for i := 0; i < n; i++ {
+		for j := 0; j < n; j++ {
+			s := "+"
+			if (i+j)&1 != 0 {
+				s = "-"
+			}
+			g.p("\ti%d_%d = %s%d * b%d_%d * b%d_%d * b%d_%d * b%d_%d\n",
+				i, j, s, i+j+1, n+i, n-j-1, n+j, n-i-1, i+j, i, i+j, i)
+		}
+		g.p("\n")
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) product(n int) {
+	g.p(`// Product matrix
+const (
+`)
+	for i := 0; i < n; i++ {
+		for j := 0; j < n; j++ {
+			g.p("\tp%d_%d = ", i, j)
+			for k := 0; k < n; k++ {
+				if k > 0 {
+					g.p(" + ")
+				}
+				g.p("h%d_%d*i%d_%d", i, k, k, j)
+			}
+			g.p("\n")
+		}
+		g.p("\n")
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) verify(n int) {
+	g.p(`// Verify that product is the identity matrix
+const ok =
+`)
+	for i := 0; i < n; i++ {
+		for j := 0; j < n; j++ {
+			if j == 0 {
+				g.p("\t")
+			} else {
+				g.p(" && ")
+			}
+			v := 0
+			if i == j {
+				v = 1
+			}
+			g.p("p%d_%d == %d", i, j, v)
+		}
+		g.p(" &&\n")
+	}
+	g.p("\ttrue\n\n")
+
+	// verify ok at type-check time
+	if *out == "" {
+		g.p("const _ = assert(ok)\n\n")
+	}
+}
+
+func (g *gen) printProduct(n int) {
+	g.p("func printProduct() {\n")
+	for i := 0; i < n; i++ {
+		g.p("\tprintln(")
+		for j := 0; j < n; j++ {
+			if j > 0 {
+				g.p(", ")
+			}
+			g.p("p%d_%d", i, j)
+		}
+		g.p(")\n")
+	}
+	g.p("}\n\n")
+}
+
+func (g *gen) binomials(n int) {
+	g.p(`// Binomials
+const (
+`)
+	for j := 0; j <= n; j++ {
+		if j > 0 {
+			g.p("\n")
+		}
+		for k := 0; k <= j; k++ {
+			g.p("\tb%d_%d = f%d / (f%d*f%d)\n", j, k, j, k, j-k)
+		}
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) factorials(n int) {
+	g.p(`// Factorials
+const (
+	f0 = 1
+	f1 = 1
+`)
+	for i := 2; i <= n; i++ {
+		g.p("\tf%d = f%d * %d\n", i, i-1, i)
+	}
+	g.p(")\n\n")
+}

src/cmd/compile/internal/types2/importer_test.go

@@ -0,0 +1,36 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements the (temporary) plumbing to get importing to work.
+
+package types2_test
+
+import (
+	gcimporter "cmd/compile/internal/importer"
+	"cmd/compile/internal/types2"
+	"io"
+)
+
+func defaultImporter() types2.Importer {
+	return &gcimports{
+		packages: make(map[string]*types2.Package),
+	}
+}
+
+type gcimports struct {
+	packages map[string]*types2.Package
+	lookup   func(path string) (io.ReadCloser, error)
+}
+
+func (m *gcimports) Import(path string) (*types2.Package, error) {
+	return m.ImportFrom(path, "" /* no vendoring */, 0)
+}
+
+func (m *gcimports) ImportFrom(path, srcDir string, mode types2.ImportMode) (*types2.Package, error) {
+	if mode != 0 {
+		panic("mode must be 0")
+	}
+	return gcimporter.Import(m.packages, path, srcDir, m.lookup)
+}

src/cmd/compile/internal/types2/infer.go

@@ -0,0 +1,359 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements type parameter inference given
+// a list of concrete arguments and a parameter list.
+
+package types2
+
+import "strings"
+
+// infer returns the list of actual type arguments for the given list of type parameters tparams
+// by inferring them from the actual arguments args for the parameters params. If type inference
+// is impossible because unification fails, an error is reported and the resulting types list is
+// nil, and index is 0. Otherwise, types is the list of inferred type arguments, and index is
+// the index of the first type argument in that list that couldn't be inferred (and thus is nil).
+// If all type arguments where inferred successfully, index is < 0.
+func (check *Checker) infer(tparams []*TypeName, params *Tuple, args []*operand) (types []Type, index int) {
+	assert(params.Len() == len(args))
+
+	u := newUnifier(check, false)
+	u.x.init(tparams)
+
+	errorf := func(kind string, tpar, targ Type, arg *operand) {
+		// provide a better error message if we can
+		targs, failed := u.x.types()
+		if failed == 0 {
+			// The first type parameter couldn't be inferred.
+			// If none of them could be inferred, don't try
+			// to provide the inferred type in the error msg.
+			allFailed := true
+			for _, targ := range targs {
+				if targ != nil {
+					allFailed = false
+					break
+				}
+			}
+			if allFailed {
+				check.errorf(arg, "%s %s of %s does not match %s (cannot infer %s)", kind, targ, arg.expr, tpar, typeNamesString(tparams))
+				return
+			}
+		}
+		smap := makeSubstMap(tparams, targs)
+		inferred := check.subst(arg.Pos(), tpar, smap)
+		if inferred != tpar {
+			check.errorf(arg, "%s %s of %s does not match inferred type %s for %s", kind, targ, arg.expr, inferred, tpar)
+		} else {
+			check.errorf(arg, "%s %s of %s does not match %s", kind, targ, arg.expr, tpar)
+		}
+	}
+
+	// Terminology: generic parameter = function parameter with a type-parameterized type
+
+	// 1st pass: Unify parameter and argument types for generic parameters with typed arguments
+	//           and collect the indices of generic parameters with untyped arguments.
+	var indices []int
+	for i, arg := range args {
+		par := params.At(i)
+		// If we permit bidirectional unification, this conditional code needs to be
+		// executed even if par.typ is not parameterized since the argument may be a
+		// generic function (for which we want to infer // its type arguments).
+		if isParameterized(tparams, par.typ) {
+			if arg.mode == invalid {
+				// An error was reported earlier. Ignore this targ
+				// and continue, we may still be able to infer all
+				// targs resulting in fewer follon-on errors.
+				continue
+			}
+			if targ := arg.typ; isTyped(targ) {
+				// If we permit bidirectional unification, and targ is
+				// a generic function, we need to initialize u.y with
+				// the respectice type parameters of targ.
+				if !u.unify(par.typ, targ) {
+					errorf("type", par.typ, targ, arg)
+					return nil, 0
+				}
+			} else {
+				indices = append(indices, i)
+			}
+		}
+	}
+
+	// Some generic parameters with untyped arguments may have been given a type
+	// indirectly through another generic parameter with a typed argument; we can
+	// ignore those now. (This only means that we know the types for those generic
+	// parameters; it doesn't mean untyped arguments can be passed safely. We still
+	// need to verify that assignment of those arguments is valid when we check
+	// function parameter passing external to infer.)
+	j := 0
+	for _, i := range indices {
+		par := params.At(i)
+		// Since untyped types are all basic (i.e., non-composite) types, an
+		// untyped argument will never match a composite parameter type; the
+		// only parameter type it can possibly match against is a *TypeParam.
+		// Thus, only keep the indices of generic parameters that are not of
+		// composite types and which don't have a type inferred yet.
+		if tpar, _ := par.typ.(*TypeParam); tpar != nil && u.x.at(tpar.index) == nil {
+			indices[j] = i
+			j++
+		}
+	}
+	indices = indices[:j]
+
+	// 2nd pass: Unify parameter and default argument types for remaining generic parameters.
+	for _, i := range indices {
+		par := params.At(i)
+		arg := args[i]
+		targ := Default(arg.typ)
+		// The default type for an untyped nil is untyped nil. We must not
+		// infer an untyped nil type as type parameter type. Ignore untyped
+		// nil by making sure all default argument types are typed.
+		if isTyped(targ) && !u.unify(par.typ, targ) {
+			errorf("default type", par.typ, targ, arg)
+			return nil, 0
+		}
+	}
+
+	return u.x.types()
+}
+
+// typeNamesString produces a string containing all the
+// type names in list suitable for human consumption.
+func typeNamesString(list []*TypeName) string {
+	// common cases
+	n := len(list)
+	switch n {
+	case 0:
+		return ""
+	case 1:
+		return list[0].name
+	case 2:
+		return list[0].name + " and " + list[1].name
+	}
+
+	// general case (n > 2)
+	var b strings.Builder
+	for i, tname := range list[:n-1] {
+		if i > 0 {
+			b.WriteString(", ")
+		}
+		b.WriteString(tname.name)
+	}
+	b.WriteString(", and ")
+	b.WriteString(list[n-1].name)
+	return b.String()
+}
+
+// IsParameterized reports whether typ contains any of the type parameters of tparams.
+func isParameterized(tparams []*TypeName, typ Type) bool {
+	w := tpWalker{
+		seen:    make(map[Type]bool),
+		tparams: tparams,
+	}
+	return w.isParameterized(typ)
+}
+
+type tpWalker struct {
+	seen    map[Type]bool
+	tparams []*TypeName
+}
+
+func (w *tpWalker) isParameterized(typ Type) (res bool) {
+	// detect cycles
+	if x, ok := w.seen[typ]; ok {
+		return x
+	}
+	w.seen[typ] = false
+	defer func() {
+		w.seen[typ] = res
+	}()
+
+	switch t := typ.(type) {
+	case nil, *Basic: // TODO(gri) should nil be handled here?
+		break
+
+	case *Array:
+		return w.isParameterized(t.elem)
+
+	case *Slice:
+		return w.isParameterized(t.elem)
+
+	case *Struct:
+		for _, fld := range t.fields {
+			if w.isParameterized(fld.typ) {
+				return true
+			}
+		}
+
+	case *Pointer:
+		return w.isParameterized(t.base)
+
+	case *Tuple:
+		n := t.Len()
+		for i := 0; i < n; i++ {
+			if w.isParameterized(t.At(i).typ) {
+				return true
+			}
+		}
+
+	case *Sum:
+		return w.isParameterizedList(t.types)
+
+	case *Signature:
+		// t.tparams may not be nil if we are looking at a signature
+		// of a generic function type (or an interface method) that is
+		// part of the type we're testing. We don't care about these type
+		// parameters.
+		// Similarly, the receiver of a method may declare (rather then
+		// use) type parameters, we don't care about those either.
+		// Thus, we only need to look at the input and result parameters.
+		return w.isParameterized(t.params) || w.isParameterized(t.results)
+
+	case *Interface:
+		if t.allMethods != nil {
+			// interface is complete - quick test
+			for _, m := range t.allMethods {
+				if w.isParameterized(m.typ) {
+					return true
+				}
+			}
+			return w.isParameterizedList(unpack(t.allTypes))
+		}
+
+		return t.iterate(func(t *Interface) bool {
+			for _, m := range t.methods {
+				if w.isParameterized(m.typ) {
+					return true
+				}
+			}
+			return w.isParameterizedList(unpack(t.types))
+		}, nil)
+
+	case *Map:
+		return w.isParameterized(t.key) || w.isParameterized(t.elem)
+
+	case *Chan:
+		return w.isParameterized(t.elem)
+
+	case *Named:
+		return w.isParameterizedList(t.targs)
+
+	case *TypeParam:
+		// t must be one of w.tparams
+		return t.index < len(w.tparams) && w.tparams[t.index].typ == t
+
+	case *instance:
+		return w.isParameterizedList(t.targs)
+
+	default:
+		unreachable()
+	}
+
+	return false
+}
+
+func (w *tpWalker) isParameterizedList(list []Type) bool {
+	for _, t := range list {
+		if w.isParameterized(t) {
+			return true
+		}
+	}
+	return false
+}
+
+// inferB returns the list of actual type arguments inferred from the type parameters'
+// bounds and an initial set of type arguments. If type inference is impossible because
+// unification fails, an error is reported, the resulting types list is nil, and index is 0.
+// Otherwise, types is the list of inferred type arguments, and index is the index of the
+// first type argument in that list that couldn't be inferred (and thus is nil). If all
+// type arguments where inferred successfully, index is < 0. The number of type arguments
+// provided may be less than the number of type parameters, but there must be at least one.
+func (check *Checker) inferB(tparams []*TypeName, targs []Type) (types []Type, index int) {
+	assert(len(tparams) >= len(targs) && len(targs) > 0)
+
+	// Setup bidirectional unification between those structural bounds
+	// and the corresponding type arguments (which may be nil!).
+	u := newUnifier(check, false)
+	u.x.init(tparams)
+	u.y = u.x // type parameters between LHS and RHS of unification are identical
+
+	// Set the type arguments which we know already.
+	for i, targ := range targs {
+		if targ != nil {
+			u.x.set(i, targ)
+		}
+	}
+
+	// Unify type parameters with their structural constraints, if any.
+	for _, tpar := range tparams {
+		typ := tpar.typ.(*TypeParam)
+		sbound := check.structuralType(typ.bound.Under())
+		if sbound != nil {
+			//check.dump(">>> unify(%s, %s)", tpar, sbound)
+			if !u.unify(typ, sbound) {
+				check.errorf(tpar.pos, "%s does not match %s", tpar, sbound)
+				return nil, 0
+			}
+			//check.dump(">>> => indices = %v, types = %s", u.x.indices, u.types)
+		}
+	}
+
+	// u.x.types() now contains the incoming type arguments plus any additional type
+	// arguments for which there were structural constraints. The newly inferred non-
+	// nil entries may still contain references to other type parameters. For instance,
+	// for [type A interface{}, B interface{type []C}, C interface{type *A}], if A == int
+	// was given, unification produced the type list [int, []C, *A]. We eliminate the
+	// remaining type parameters by substituting the type parameters in this type list
+	// until nothing changes anymore.
+	types, index = u.x.types()
+	if debug {
+		for i, targ := range targs {
+			assert(targ == nil || types[i] == targ)
+		}
+	}
+
+	// dirty tracks the indices of all types that may still contain type parameters.
+	// We know that nil types entries and entries corresponding to provided (non-nil)
+	// type arguments are clean, so exclude them from the start.
+	var dirty []int
+	for i, typ := range types {
+		if typ != nil && (i >= len(targs) || targs[i] == nil) {
+			dirty = append(dirty, i)
+		}
+	}
+
+	for len(dirty) > 0 {
+		// TODO(gri) Instead of creating a new smap for each iteration,
+		// provide an update operation for smaps and only change when
+		// needed. Optimization.
+		smap := makeSubstMap(tparams, types)
+		n := 0
+		for _, index := range dirty {
+			t0 := types[index]
+			if t1 := check.subst(nopos, t0, smap); t1 != t0 {
+				types[index] = t1
+				dirty[n] = index
+				n++
+			}
+		}
+		dirty = dirty[:n]
+	}
+	//check.dump(">>> inferred types = %s", types)
+
+	return
+}
+
+// structuralType returns the structural type of a constraint, if any.
+func (check *Checker) structuralType(constraint Type) Type {
+	if iface, _ := constraint.(*Interface); iface != nil {
+		check.completeInterface(nopos, iface)
+		types := unpack(iface.allTypes)
+		if len(types) == 1 {
+			return types[0]
+		}
+		return nil
+	}
+	return constraint
+}

src/cmd/compile/internal/types2/initorder.go

@@ -0,0 +1,298 @@
+// UNREVIEWED
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"container/heap"
+	"fmt"
+)
+
+// initOrder computes the Info.InitOrder for package variables.
+func (check *Checker) initOrder() {
+	// An InitOrder may already have been computed if a package is
+	// built from several calls to (*Checker).Files. Clear it.
+	check.Info.InitOrder = check.Info.InitOrder[:0]
+
+	// Compute the object dependency graph and initialize
+	// a priority queue with the list of graph nodes.
+	pq := nodeQueue(dependencyGraph(check.objMap))
+	heap.Init(&pq)
+
+	const debug = false
+	if debug {
+		fmt.Printf("Computing initialization order for %s\n\n", check.pkg)
+		fmt.Println("Object dependency graph:")
+		for obj, d := range check.objMap {
+			// only print objects that may appear in the dependency graph
+			if obj, _ := obj.(dependency); obj != nil {
+				if len(d.deps) > 0 {
+					fmt.Printf("\t%s depends on\n", obj.Name())
+					for dep := range d.deps {
+						fmt.Printf("\t\t%s\n", dep.Name())
+					}
+				} else {
+					fmt.Printf("\t%s has no dependencies\n", obj.Name())
+				}
+			}
+		}
+		fmt.Println()
+
+		fmt.Println("Transposed object dependency graph (functions eliminated):")
+		for _, n := range pq {
+			fmt.Printf("\t%s depends on %d nodes\n", n.obj.Name(), n.ndeps)
+			for p := range n.pred {
+				fmt.Printf("\t\t%s is dependent\n", p.obj.Name())
+			}
+		}
+		fmt.Println()
+
+		fmt.Println("Processing nodes:")
+	}
+
+	// Determine initialization order by removing the highest priority node
+	// (the one with the fewest dependencies) and its edges from the graph,
+	// repeatedly, until there are no nodes left.
+	// In a valid Go program, those nodes always have zero dependencies (after
+	// removing all incoming dependencies), otherwise there are initialization
+	// cycles.
+	emitted := make(map[*declInfo]bool)
+	for len(pq) > 0 {
+		// get the next node
+		n := heap.Pop(&pq).(*graphNode)
+
+		if debug {
+			fmt.Printf("\t%s (src pos %d) depends on %d nodes now\n",
+				n.obj.Name(), n.obj.order(), n.ndeps)
+		}
+
+		// if n still depends on other nodes, we have a cycle
+		if n.ndeps > 0 {
+			cycle := findPath(check.objMap, n.obj, n.obj, make(map[Object]bool))
+			// If n.obj is not part of the cycle (e.g., n.obj->b->c->d->c),
+			// cycle will be nil. Don't report anything in that case since
+			// the cycle is reported when the algorithm gets to an object
+			// in the cycle.
+			// Furthermore, once an object in the cycle is encountered,
+			// the cycle will be broken (dependency count will be reduced
+			// below), and so the remaining nodes in the cycle don't trigger
+			// another error (unless they are part of multiple cycles).
+			if cycle != nil {
+				check.reportCycle(cycle)
+			}
+			// Ok to continue, but the variable initialization order
+			// will be incorrect at this point since it assumes no
+			// cycle errors.
+		}
+
+		// reduce dependency count of all dependent nodes
+		// and update priority queue
+		for p := range n.pred {
+			p.ndeps--
+			heap.Fix(&pq, p.index)
+		}
+
+		// record the init order for variables with initializers only
+		v, _ := n.obj.(*Var)
+		info := check.objMap[v]
+		if v == nil || !info.hasInitializer() {
+			continue
+		}
+
+		// n:1 variable declarations such as: a, b = f()
+		// introduce a node for each lhs variable (here: a, b);
+		// but they all have the same initializer - emit only
+		// one, for the first variable seen
+		if emitted[info] {
+			continue // initializer already emitted, if any
+		}
+		emitted[info] = true
+
+		infoLhs := info.lhs // possibly nil (see declInfo.lhs field comment)
+		if infoLhs == nil {
+			infoLhs = []*Var{v}
+		}
+		init := &Initializer{infoLhs, info.init}
+		check.Info.InitOrder = append(check.Info.InitOrder, init)
+	}
+
+	if debug {
+		fmt.Println()
+		fmt.Println("Initialization order:")
+		for _, init := range check.Info.InitOrder {
+			fmt.Printf("\t%s\n", init)
+		}
+		fmt.Println()
+	}
+}
+
+// findPath returns the (reversed) list of objects []Object{to, ... from}
+// such that there is a path of object dependencies from 'from' to 'to'.
+// If there is no such path, the result is nil.
+func findPath(objMap map[Object]*declInfo, from, to Object, seen map[Object]bool) []Object {
+	if seen[from] {
+		return nil
+	}
+	seen[from] = true
+
+	for d := range objMap[from].deps {
+		if d == to {
+			return []Object{d}
+		}
+		if P := findPath(objMap, d, to, seen); P != nil {
+			return append(P, d)
+		}
+	}
+
+	return nil
+}
+
+// reportCycle reports an error for the given cycle.
+func (check *Checker) reportCycle(cycle []Object) {
+	obj := cycle[0]
+	check.errorf(obj, "initialization cycle for %s", obj.Name())
+	// subtle loop: print cycle[i] for i = 0, n-1, n-2, ... 1 for len(cycle) = n
+	for i := len(cycle) - 1; i >= 0; i-- {
+		check.errorf(obj, "\t%s refers to", obj.Name()) // secondary error, \t indented
+		obj = cycle[i]
+	}
+	// print cycle[0] again to close the cycle
+	check.errorf(obj, "\t%s", obj.Name())
+}
+
+// ----------------------------------------------------------------------------
+// Object dependency graph
+
+// A dependency is an object that may be a dependency in an initialization
+// expression. Only constants, variables, and functions can be dependencies.
+// Constants are here because constant expression cycles are reported during
+// initialization order computation.
+type dependency interface {
+	Object
+	isDependency()
+}
+
+// A graphNode represents a node in the object dependency graph.
+// Each node p in n.pred represents an edge p->n, and each node
+// s in n.succ represents an edge n->s; with a->b indicating that
+// a depends on b.
+type graphNode struct {
+	obj        dependency // object represented by this node
+	pred, succ nodeSet    // consumers and dependencies of this node (lazily initialized)
+	index      int        // node index in graph slice/priority queue
+	ndeps      int        // number of outstanding dependencies before this object can be initialized
+}
+
+type nodeSet map[*graphNode]bool
+
+func (s *nodeSet) add(p *graphNode) {
+	if *s == nil {
+		*s = make(nodeSet)
+	}
+	(*s)[p] = true
+}
+
+// dependencyGraph computes the object dependency graph from the given objMap,
+// with any function nodes removed. The resulting graph contains only constants
+// and variables.
+func dependencyGraph(objMap map[Object]*declInfo) []*graphNode {
+	// M is the dependency (Object) -> graphNode mapping
+	M := make(map[dependency]*graphNode)
+	for obj := range objMap {
+		// only consider nodes that may be an initialization dependency
+		if obj, _ := obj.(dependency); obj != nil {
+			M[obj] = &graphNode{obj: obj}
+		}
+	}
+
+	// compute edges for graph M
+	// (We need to include all nodes, even isolated ones, because they still need
+	// to be scheduled for initialization in correct order relative to other nodes.)
+	for obj, n := range M {
+		// for each dependency obj -> d (= deps[i]), create graph edges n->s and s->n
+		for d := range objMap[obj].deps {
+			// only consider nodes that may be an initialization dependency
+			if d, _ := d.(dependency); d != nil {
+				d := M[d]
+				n.succ.add(d)
+				d.pred.add(n)
+			}
+		}
+	}
+
+	// remove function nodes and collect remaining graph nodes in G
+	// (Mutually recursive functions may introduce cycles among themselves
+	// which are permitted. Yet such cycles may incorrectly inflate the dependency
+	// count for variables which in turn may not get scheduled for initialization
+	// in correct order.)
+	var G []*graphNode
+	for obj, n := range M {
+		if _, ok := obj.(*Func); ok {
+			// connect each predecessor p of n with each successor s
+			// and drop the function node (don't collect it in G)
+			for p := range n.pred {
+				// ignore self-cycles
+				if p != n {
+					// Each successor s of n becomes a successor of p, and
+					// each predecessor p of n becomes a predecessor of s.
+					for s := range n.succ {
+						// ignore self-cycles
+						if s != n {
+							p.succ.add(s)
+							s.pred.add(p)
+							delete(s.pred, n) // remove edge to n
+						}
+					}
+					delete(p.succ, n) // remove edge to n
+				}
+			}
+		} else {
+			// collect non-function nodes
+			G = append(G, n)
+		}
+	}
+
+	// fill in index and ndeps fields
+	for i, n := range G {
+		n.index = i
+		n.ndeps = len(n.succ)
+	}
+
+	return G
+}
+
+// ----------------------------------------------------------------------------
+// Priority queue
+
+// nodeQueue implements the container/heap interface;
+// a nodeQueue may be used as a priority queue.
+type nodeQueue []*graphNode
+
+func (a nodeQueue) Len() int { return len(a) }
+
+func (a nodeQueue) Swap(i, j int) {
+	x, y := a[i], a[j]
+	a[i], a[j] = y, x
+	x.index, y.index = j, i
+}
+
+func (a nodeQueue) Less(i, j int) bool {
+	x, y := a[i], a[j]
+	// nodes are prioritized by number of incoming dependencies (1st key)
+	// and source order (2nd key)
+	return x.ndeps < y.ndeps || x.ndeps == y.ndeps && x.obj.order() < y.obj.order()
+}
+
+func (a *nodeQueue) Push(x interface{}) {
+	panic("unreachable")
+}
+
+func (a *nodeQueue) Pop() interface{} {
+	n := len(*a)
+	x := (*a)[n-1]
+	x.index = -1 // for safety
+	*a = (*a)[:n-1]
+	return x
+}

src/cmd/compile/internal/types2/issues_test.go

@@ -0,0 +1,533 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements tests for various issues.
+
+package types2_test
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"internal/testenv"
+	"sort"
+	"strings"
+	"testing"
+
+	. "cmd/compile/internal/types2"
+)
+
+func mustParse(t *testing.T, src string) *syntax.File {
+	f, err := parseSrc("", src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return f
+}
+func TestIssue5770(t *testing.T) {
+	f := mustParse(t, `package p; type S struct{T}`)
+	var conf Config
+	// conf := Config{Importer: importer.Default()}
+	_, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, nil) // do not crash
+	want := "undeclared name: T"
+	if err == nil || !strings.Contains(err.Error(), want) {
+		t.Errorf("got: %v; want: %s", err, want)
+	}
+}
+
+func TestIssue5849(t *testing.T) {
+	src := `
+package p
+var (
+	s uint
+	_ = uint8(8)
+	_ = uint16(16) << s
+	_ = uint32(32 << s)
+	_ = uint64(64 << s + s)
+	_ = (interface{})("foo")
+	_ = (interface{})(nil)
+)`
+	f := mustParse(t, src)
+
+	var conf Config
+	types := make(map[syntax.Expr]TypeAndValue)
+	_, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, &Info{Types: types})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for x, tv := range types {
+		var want Type
+		switch x := x.(type) {
+		case *syntax.BasicLit:
+			switch x.Value {
+			case `8`:
+				want = Typ[Uint8]
+			case `16`:
+				want = Typ[Uint16]
+			case `32`:
+				want = Typ[Uint32]
+			case `64`:
+				want = Typ[Uint] // because of "+ s", s is of type uint
+			case `"foo"`:
+				want = Typ[String]
+			}
+		case *syntax.Name:
+			if x.Value == "nil" {
+				want = Typ[UntypedNil]
+			}
+		}
+		if want != nil && !Identical(tv.Type, want) {
+			t.Errorf("got %s; want %s", tv.Type, want)
+		}
+	}
+}
+
+func TestIssue6413(t *testing.T) {
+	src := `
+package p
+func f() int {
+	defer f()
+	go f()
+	return 0
+}
+`
+	f := mustParse(t, src)
+
+	var conf Config
+	types := make(map[syntax.Expr]TypeAndValue)
+	_, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, &Info{Types: types})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := Typ[Int]
+	n := 0
+	for x, tv := range types {
+		if _, ok := x.(*syntax.CallExpr); ok {
+			if tv.Type != want {
+				t.Errorf("%s: got %s; want %s", x.Pos(), tv.Type, want)
+			}
+			n++
+		}
+	}
+
+	if n != 2 {
+		t.Errorf("got %d CallExprs; want 2", n)
+	}
+}
+
+func TestIssue7245(t *testing.T) {
+	src := `
+package p
+func (T) m() (res bool) { return }
+type T struct{} // receiver type after method declaration
+`
+	f := mustParse(t, src)
+
+	var conf Config
+	defs := make(map[*syntax.Name]Object)
+	_, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, &Info{Defs: defs})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	m := f.DeclList[0].(*syntax.FuncDecl)
+	res1 := defs[m.Name].(*Func).Type().(*Signature).Results().At(0)
+	res2 := defs[m.Type.ResultList[0].Name].(*Var)
+
+	if res1 != res2 {
+		t.Errorf("got %s (%p) != %s (%p)", res1, res2, res1, res2)
+	}
+}
+
+// This tests that uses of existing vars on the LHS of an assignment
+// are Uses, not Defs; and also that the (illegal) use of a non-var on
+// the LHS of an assignment is a Use nonetheless.
+func TestIssue7827(t *testing.T) {
+	const src = `
+package p
+func _() {
+	const w = 1        // defs w
+        x, y := 2, 3       // defs x, y
+        w, x, z := 4, 5, 6 // uses w, x, defs z; error: cannot assign to w
+        _, _, _ = x, y, z  // uses x, y, z
+}
+`
+	f := mustParse(t, src)
+
+	const want = `L3 defs func p._()
+L4 defs const w untyped int
+L5 defs var x int
+L5 defs var y int
+L6 defs var z int
+L6 uses const w untyped int
+L6 uses var x int
+L7 uses var x int
+L7 uses var y int
+L7 uses var z int`
+
+	// don't abort at the first error
+	conf := Config{Error: func(err error) { t.Log(err) }}
+	defs := make(map[*syntax.Name]Object)
+	uses := make(map[*syntax.Name]Object)
+	_, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, &Info{Defs: defs, Uses: uses})
+	if s := fmt.Sprint(err); !strings.HasSuffix(s, "cannot assign to w") {
+		t.Errorf("Check: unexpected error: %s", s)
+	}
+
+	var facts []string
+	for id, obj := range defs {
+		if obj != nil {
+			fact := fmt.Sprintf("L%d defs %s", id.Pos().Line(), obj)
+			facts = append(facts, fact)
+		}
+	}
+	for id, obj := range uses {
+		fact := fmt.Sprintf("L%d uses %s", id.Pos().Line(), obj)
+		facts = append(facts, fact)
+	}
+	sort.Strings(facts)
+
+	got := strings.Join(facts, "\n")
+	if got != want {
+		t.Errorf("Unexpected defs/uses\ngot:\n%s\nwant:\n%s", got, want)
+	}
+}
+
+// This tests that the package associated with the types.Object.Pkg method
+// is the type's package independent of the order in which the imports are
+// listed in the sources src1, src2 below.
+// The actual issue is in go/internal/gcimporter which has a corresponding
+// test; we leave this test here to verify correct behavior at the go/types
+// level.
+func TestIssue13898(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	const src0 = `
+package main
+
+import "go/types"
+
+func main() {
+	var info types.Info
+	for _, obj := range info.Uses {
+		_ = obj.Pkg()
+	}
+}
+`
+	// like src0, but also imports go/importer
+	const src1 = `
+package main
+
+import (
+	"go/types"
+	_ "go/importer"
+)
+
+func main() {
+	var info types.Info
+	for _, obj := range info.Uses {
+		_ = obj.Pkg()
+	}
+}
+`
+	// like src1 but with different import order
+	// (used to fail with this issue)
+	const src2 = `
+package main
+
+import (
+	_ "go/importer"
+	"go/types"
+)
+
+func main() {
+	var info types.Info
+	for _, obj := range info.Uses {
+		_ = obj.Pkg()
+	}
+}
+`
+	f := func(test, src string) {
+		f := mustParse(t, src)
+		conf := Config{Importer: defaultImporter()}
+		info := Info{Uses: make(map[*syntax.Name]Object)}
+		_, err := conf.Check("main", []*syntax.File{f}, &info)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var pkg *Package
+		count := 0
+		for id, obj := range info.Uses {
+			if id.Value == "Pkg" {
+				pkg = obj.Pkg()
+				count++
+			}
+		}
+		if count != 1 {
+			t.Fatalf("%s: got %d entries named Pkg; want 1", test, count)
+		}
+		if pkg.Name() != "types" {
+			t.Fatalf("%s: got %v; want package types2", test, pkg)
+		}
+	}
+
+	f("src0", src0)
+	f("src1", src1)
+	f("src2", src2)
+}
+
+func TestIssue22525(t *testing.T) {
+	f := mustParse(t, `package p; func f() { var a, b, c, d, e int }`)
+
+	got := "\n"
+	conf := Config{Error: func(err error) { got += err.Error() + "\n" }}
+	conf.Check(f.PkgName.Value, []*syntax.File{f}, nil) // do not crash
+	want := `
+:1:27: a declared but not used
+:1:30: b declared but not used
+:1:33: c declared but not used
+:1:36: d declared but not used
+:1:39: e declared but not used
+`
+	if got != want {
+		t.Errorf("got: %swant: %s", got, want)
+	}
+}
+
+func TestIssue25627(t *testing.T) {
+	t.Skip("requires syntax tree inspection")
+
+	const prefix = `package p; import "unsafe"; type P *struct{}; type I interface{}; type T `
+	// The src strings (without prefix) are constructed such that the number of semicolons
+	// plus one corresponds to the number of fields expected in the respective struct.
+	for _, src := range []string{
+		`struct { x Missing }`,
+		`struct { Missing }`,
+		`struct { *Missing }`,
+		`struct { unsafe.Pointer }`,
+		`struct { P }`,
+		`struct { *I }`,
+		`struct { a int; b Missing; *Missing }`,
+	} {
+		f := mustParse(t, prefix+src)
+
+		conf := Config{Importer: defaultImporter(), Error: func(err error) {}}
+		info := &Info{Types: make(map[syntax.Expr]TypeAndValue)}
+		_, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, info)
+		if err != nil {
+			if _, ok := err.(Error); !ok {
+				t.Fatal(err)
+			}
+		}
+
+		unimplemented()
+		/*
+			ast.Inspect(f, func(n syntax.Node) bool {
+				if spec, _ := n.(*syntax.TypeDecl); spec != nil {
+					if tv, ok := info.Types[spec.Type]; ok && spec.Name.Value == "T" {
+						want := strings.Count(src, ";") + 1
+						if got := tv.Type.(*Struct).NumFields(); got != want {
+							t.Errorf("%s: got %d fields; want %d", src, got, want)
+						}
+					}
+				}
+				return true
+			})
+		*/
+	}
+}
+
+func TestIssue28005(t *testing.T) {
+	// method names must match defining interface name for this test
+	// (see last comment in this function)
+	sources := [...]string{
+		"package p; type A interface{ A() }",
+		"package p; type B interface{ B() }",
+		"package p; type X interface{ A; B }",
+	}
+
+	// compute original file ASTs
+	var orig [len(sources)]*syntax.File
+	for i, src := range sources {
+		orig[i] = mustParse(t, src)
+	}
+
+	// run the test for all order permutations of the incoming files
+	for _, perm := range [][len(sources)]int{
+		{0, 1, 2},
+		{0, 2, 1},
+		{1, 0, 2},
+		{1, 2, 0},
+		{2, 0, 1},
+		{2, 1, 0},
+	} {
+		// create file order permutation
+		files := make([]*syntax.File, len(sources))
+		for i := range perm {
+			files[i] = orig[perm[i]]
+		}
+
+		// type-check package with given file order permutation
+		var conf Config
+		info := &Info{Defs: make(map[*syntax.Name]Object)}
+		_, err := conf.Check("", files, info)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// look for interface object X
+		var obj Object
+		for name, def := range info.Defs {
+			if name.Value == "X" {
+				obj = def
+				break
+			}
+		}
+		if obj == nil {
+			t.Fatal("object X not found")
+		}
+		iface := obj.Type().Interface() // object X must be an interface
+		if iface == nil {
+			t.Fatalf("%s is not an interface", obj)
+		}
+
+		// Each iface method m is embedded; and m's receiver base type name
+		// must match the method's name per the choice in the source file.
+		for i := 0; i < iface.NumMethods(); i++ {
+			m := iface.Method(i)
+			recvName := m.Type().(*Signature).Recv().Type().(*Named).Obj().Name()
+			if recvName != m.Name() {
+				t.Errorf("perm %v: got recv %s; want %s", perm, recvName, m.Name())
+			}
+		}
+	}
+}
+
+func TestIssue28282(t *testing.T) {
+	// create type interface { error }
+	et := Universe.Lookup("error").Type()
+	it := NewInterfaceType(nil, []Type{et})
+	it.Complete()
+	// verify that after completing the interface, the embedded method remains unchanged
+	want := et.Interface().Method(0)
+	got := it.Method(0)
+	if got != want {
+		t.Fatalf("%s.Method(0): got %q (%p); want %q (%p)", it, got, got, want, want)
+	}
+	// verify that lookup finds the same method in both interfaces (redundant check)
+	obj, _, _ := LookupFieldOrMethod(et, false, nil, "Error")
+	if obj != want {
+		t.Fatalf("%s.Lookup: got %q (%p); want %q (%p)", et, obj, obj, want, want)
+	}
+	obj, _, _ = LookupFieldOrMethod(it, false, nil, "Error")
+	if obj != want {
+		t.Fatalf("%s.Lookup: got %q (%p); want %q (%p)", it, obj, obj, want, want)
+	}
+}
+
+func TestIssue29029(t *testing.T) {
+	f1 := mustParse(t, `package p; type A interface { M() }`)
+	f2 := mustParse(t, `package p; var B interface { A }`)
+
+	// printInfo prints the *Func definitions recorded in info, one *Func per line.
+	printInfo := func(info *Info) string {
+		var buf bytes.Buffer
+		for _, obj := range info.Defs {
+			if fn, ok := obj.(*Func); ok {
+				fmt.Fprintln(&buf, fn)
+			}
+		}
+		return buf.String()
+	}
+
+	// The *Func (method) definitions for package p must be the same
+	// independent on whether f1 and f2 are type-checked together, or
+	// incrementally.
+
+	// type-check together
+	var conf Config
+	info := &Info{Defs: make(map[*syntax.Name]Object)}
+	check := NewChecker(&conf, NewPackage("", "p"), info)
+	if err := check.Files([]*syntax.File{f1, f2}); err != nil {
+		t.Fatal(err)
+	}
+	want := printInfo(info)
+
+	// type-check incrementally
+	info = &Info{Defs: make(map[*syntax.Name]Object)}
+	check = NewChecker(&conf, NewPackage("", "p"), info)
+	if err := check.Files([]*syntax.File{f1}); err != nil {
+		t.Fatal(err)
+	}
+	if err := check.Files([]*syntax.File{f2}); err != nil {
+		t.Fatal(err)
+	}
+	got := printInfo(info)
+
+	if got != want {
+		t.Errorf("\ngot : %swant: %s", got, want)
+	}
+}
+
+func TestIssue34151(t *testing.T) {
+	const asrc = `package a; type I interface{ M() }; type T struct { F interface { I } }`
+	const bsrc = `package b; import "a"; type T struct { F interface { a.I } }; var _ = a.T(T{})`
+
+	a, err := pkgFor("a", asrc, nil)
+	if err != nil {
+		t.Fatalf("package %s failed to typecheck: %v", a.Name(), err)
+	}
+
+	bast := mustParse(t, bsrc)
+	conf := Config{Importer: importHelper{a}}
+	b, err := conf.Check(bast.PkgName.Value, []*syntax.File{bast}, nil)
+	if err != nil {
+		t.Errorf("package %s failed to typecheck: %v", b.Name(), err)
+	}
+}
+
+type importHelper struct {
+	pkg *Package
+}
+
+func (h importHelper) Import(path string) (*Package, error) {
+	if path != h.pkg.Path() {
+		return nil, fmt.Errorf("got package path %q; want %q", path, h.pkg.Path())
+	}
+	return h.pkg, nil
+}
+
+// TestIssue34921 verifies that we don't update an imported type's underlying
+// type when resolving an underlying type. Specifically, when determining the
+// underlying type of b.T (which is the underlying type of a.T, which is int)
+// we must not set the underlying type of a.T again since that would lead to
+// a race condition if package b is imported elsewhere, in a package that is
+// concurrently type-checked.
+func TestIssue34921(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Error(r)
+		}
+	}()
+
+	var sources = []string{
+		`package a; type T int`,
+		`package b; import "a"; type T a.T`,
+	}
+
+	var pkg *Package
+	for _, src := range sources {
+		f := mustParse(t, src)
+		conf := Config{Importer: importHelper{pkg}}
+		res, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, nil)
+		if err != nil {
+			t.Errorf("%q failed to typecheck: %v", src, err)
+		}
+		pkg = res // res is imported by the next package in this test
+	}
+}

src/cmd/compile/internal/types2/labels.go

@@ -0,0 +1,260 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+)
+
+// labels checks correct label use in body.
+func (check *Checker) labels(body *syntax.BlockStmt) {
+	// set of all labels in this body
+	all := NewScope(nil, body.Pos(), endPos(body), "label")
+
+	fwdJumps := check.blockBranches(all, nil, nil, body.List)
+
+	// If there are any forward jumps left, no label was found for
+	// the corresponding goto statements. Either those labels were
+	// never defined, or they are inside blocks and not reachable
+	// for the respective gotos.
+	for _, jmp := range fwdJumps {
+		var msg string
+		name := jmp.Label.Value
+		if alt := all.Lookup(name); alt != nil {
+			msg = "goto %s jumps into block"
+			alt.(*Label).used = true // avoid another error
+		} else {
+			msg = "label %s not declared"
+		}
+		check.errorf(jmp.Label, msg, name)
+	}
+
+	// spec: "It is illegal to define a label that is never used."
+	for _, obj := range all.elems {
+		if lbl := obj.(*Label); !lbl.used {
+			check.softErrorf(lbl.pos, "label %s declared but not used", lbl.name)
+		}
+	}
+}
+
+// A block tracks label declarations in a block and its enclosing blocks.
+type block struct {
+	parent *block                         // enclosing block
+	lstmt  *syntax.LabeledStmt            // labeled statement to which this block belongs, or nil
+	labels map[string]*syntax.LabeledStmt // allocated lazily
+}
+
+// insert records a new label declaration for the current block.
+// The label must not have been declared before in any block.
+func (b *block) insert(s *syntax.LabeledStmt) {
+	name := s.Label.Value
+	if debug {
+		assert(b.gotoTarget(name) == nil)
+	}
+	labels := b.labels
+	if labels == nil {
+		labels = make(map[string]*syntax.LabeledStmt)
+		b.labels = labels
+	}
+	labels[name] = s
+}
+
+// gotoTarget returns the labeled statement in the current
+// or an enclosing block with the given label name, or nil.
+func (b *block) gotoTarget(name string) *syntax.LabeledStmt {
+	for s := b; s != nil; s = s.parent {
+		if t := s.labels[name]; t != nil {
+			return t
+		}
+	}
+	return nil
+}
+
+// enclosingTarget returns the innermost enclosing labeled
+// statement with the given label name, or nil.
+func (b *block) enclosingTarget(name string) *syntax.LabeledStmt {
+	for s := b; s != nil; s = s.parent {
+		if t := s.lstmt; t != nil && t.Label.Value == name {
+			return t
+		}
+	}
+	return nil
+}
+
+// blockBranches processes a block's statement list and returns the set of outgoing forward jumps.
+// all is the scope of all declared labels, parent the set of labels declared in the immediately
+// enclosing block, and lstmt is the labeled statement this block is associated with (or nil).
+func (check *Checker) blockBranches(all *Scope, parent *block, lstmt *syntax.LabeledStmt, list []syntax.Stmt) []*syntax.BranchStmt {
+	b := &block{parent, lstmt, nil}
+
+	var (
+		varDeclPos         syntax.Pos
+		fwdJumps, badJumps []*syntax.BranchStmt
+	)
+
+	// All forward jumps jumping over a variable declaration are possibly
+	// invalid (they may still jump out of the block and be ok).
+	// recordVarDecl records them for the given position.
+	recordVarDecl := func(pos syntax.Pos) {
+		varDeclPos = pos
+		badJumps = append(badJumps[:0], fwdJumps...) // copy fwdJumps to badJumps
+	}
+
+	jumpsOverVarDecl := func(jmp *syntax.BranchStmt) bool {
+		if varDeclPos.IsKnown() {
+			for _, bad := range badJumps {
+				if jmp == bad {
+					return true
+				}
+			}
+		}
+		return false
+	}
+
+	var stmtBranches func(syntax.Stmt)
+	stmtBranches = func(s syntax.Stmt) {
+		switch s := s.(type) {
+		case *syntax.DeclStmt:
+			for _, d := range s.DeclList {
+				if d, _ := d.(*syntax.VarDecl); d != nil {
+					recordVarDecl(d.Pos())
+				}
+			}
+
+		case *syntax.LabeledStmt:
+			// declare non-blank label
+			if name := s.Label.Value; name != "_" {
+				lbl := NewLabel(s.Label.Pos(), check.pkg, name)
+				if alt := all.Insert(lbl); alt != nil {
+					check.softErrorf(lbl.pos, "label %s already declared", name)
+					check.reportAltDecl(alt)
+					// ok to continue
+				} else {
+					b.insert(s)
+					check.recordDef(s.Label, lbl)
+				}
+				// resolve matching forward jumps and remove them from fwdJumps
+				i := 0
+				for _, jmp := range fwdJumps {
+					if jmp.Label.Value == name {
+						// match
+						lbl.used = true
+						check.recordUse(jmp.Label, lbl)
+						if jumpsOverVarDecl(jmp) {
+							check.softErrorf(
+								jmp.Label,
+								"goto %s jumps over variable declaration at line %d",
+								name,
+								varDeclPos.Line(),
+							)
+							// ok to continue
+						}
+					} else {
+						// no match - record new forward jump
+						fwdJumps[i] = jmp
+						i++
+					}
+				}
+				fwdJumps = fwdJumps[:i]
+				lstmt = s
+			}
+			stmtBranches(s.Stmt)
+
+		case *syntax.BranchStmt:
+			if s.Label == nil {
+				return // checked in 1st pass (check.stmt)
+			}
+
+			// determine and validate target
+			name := s.Label.Value
+			switch s.Tok {
+			case syntax.Break:
+				// spec: "If there is a label, it must be that of an enclosing
+				// "for", "switch", or "select" statement, and that is the one
+				// whose execution terminates."
+				valid := false
+				if t := b.enclosingTarget(name); t != nil {
+					switch t.Stmt.(type) {
+					case *syntax.SwitchStmt, *syntax.SelectStmt, *syntax.ForStmt:
+						valid = true
+					}
+				}
+				if !valid {
+					check.errorf(s.Label, "invalid break label %s", name)
+					return
+				}
+
+			case syntax.Continue:
+				// spec: "If there is a label, it must be that of an enclosing
+				// "for" statement, and that is the one whose execution advances."
+				valid := false
+				if t := b.enclosingTarget(name); t != nil {
+					switch t.Stmt.(type) {
+					case *syntax.ForStmt:
+						valid = true
+					}
+				}
+				if !valid {
+					check.errorf(s.Label, "invalid continue label %s", name)
+					return
+				}
+
+			case syntax.Goto:
+				if b.gotoTarget(name) == nil {
+					// label may be declared later - add branch to forward jumps
+					fwdJumps = append(fwdJumps, s)
+					return
+				}
+
+			default:
+				check.invalidASTf(s, "branch statement: %s %s", s.Tok, name)
+				return
+			}
+
+			// record label use
+			obj := all.Lookup(name)
+			obj.(*Label).used = true
+			check.recordUse(s.Label, obj)
+
+		case *syntax.AssignStmt:
+			if s.Op == syntax.Def {
+				recordVarDecl(s.Pos())
+			}
+
+		case *syntax.BlockStmt:
+			// Unresolved forward jumps inside the nested block
+			// become forward jumps in the current block.
+			fwdJumps = append(fwdJumps, check.blockBranches(all, b, lstmt, s.List)...)
+
+		case *syntax.IfStmt:
+			stmtBranches(s.Then)
+			if s.Else != nil {
+				stmtBranches(s.Else)
+			}
+
+		case *syntax.SwitchStmt:
+			b := &block{b, lstmt, nil}
+			for _, s := range s.Body {
+				fwdJumps = append(fwdJumps, check.blockBranches(all, b, nil, s.Body)...)
+			}
+
+		case *syntax.SelectStmt:
+			b := &block{b, lstmt, nil}
+			for _, s := range s.Body {
+				fwdJumps = append(fwdJumps, check.blockBranches(all, b, nil, s.Body)...)
+			}
+
+		case *syntax.ForStmt:
+			stmtBranches(s.Body)
+		}
+	}
+
+	for _, s := range list {
+		stmtBranches(s)
+	}
+
+	return fwdJumps
+}

src/cmd/compile/internal/types2/lookup.go

@@ -0,0 +1,493 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements various field and method lookup functions.
+
+package types2
+
+// LookupFieldOrMethod looks up a field or method with given package and name
+// in T and returns the corresponding *Var or *Func, an index sequence, and a
+// bool indicating if there were any pointer indirections on the path to the
+// field or method. If addressable is set, T is the type of an addressable
+// variable (only matters for method lookups).
+//
+// The last index entry is the field or method index in the (possibly embedded)
+// type where the entry was found, either:
+//
+//	1) the list of declared methods of a named type; or
+//	2) the list of all methods (method set) of an interface type; or
+//	3) the list of fields of a struct type.
+//
+// The earlier index entries are the indices of the embedded struct fields
+// traversed to get to the found entry, starting at depth 0.
+//
+// If no entry is found, a nil object is returned. In this case, the returned
+// index and indirect values have the following meaning:
+//
+//	- If index != nil, the index sequence points to an ambiguous entry
+//	(the same name appeared more than once at the same embedding level).
+//
+//	- If indirect is set, a method with a pointer receiver type was found
+//      but there was no pointer on the path from the actual receiver type to
+//	the method's formal receiver base type, nor was the receiver addressable.
+//
+func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	return (*Checker)(nil).lookupFieldOrMethod(T, addressable, pkg, name)
+}
+
+// Internal use of Checker.lookupFieldOrMethod: If the obj result is a method
+// associated with a concrete (non-interface) type, the method's signature
+// may not be fully set up. Call Checker.objDecl(obj, nil) before accessing
+// the method's type.
+// TODO(gri) Now that we provide the *Checker, we can probably remove this
+// caveat by calling Checker.objDecl from lookupFieldOrMethod. Investigate.
+
+// lookupFieldOrMethod is like the external version but completes interfaces
+// as necessary.
+func (check *Checker) lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	// Methods cannot be associated to a named pointer type
+	// (spec: "The type denoted by T is called the receiver base type;
+	// it must not be a pointer or interface type and it must be declared
+	// in the same package as the method.").
+	// Thus, if we have a named pointer type, proceed with the underlying
+	// pointer type but discard the result if it is a method since we would
+	// not have found it for T (see also issue 8590).
+	if t := T.Named(); t != nil {
+		if p, _ := t.underlying.(*Pointer); p != nil {
+			obj, index, indirect = check.rawLookupFieldOrMethod(p, false, pkg, name)
+			if _, ok := obj.(*Func); ok {
+				return nil, nil, false
+			}
+			return
+		}
+	}
+
+	return check.rawLookupFieldOrMethod(T, addressable, pkg, name)
+}
+
+// TODO(gri) The named type consolidation and seen maps below must be
+//           indexed by unique keys for a given type. Verify that named
+//           types always have only one representation (even when imported
+//           indirectly via different packages.)
+
+// rawLookupFieldOrMethod should only be called by lookupFieldOrMethod and missingMethod.
+func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	// WARNING: The code in this function is extremely subtle - do not modify casually!
+	//          This function and NewMethodSet should be kept in sync.
+
+	if name == "_" {
+		return // blank fields/methods are never found
+	}
+
+	typ, isPtr := deref(T)
+
+	// *typ where typ is an interface has no methods.
+	// Be cautious: typ may be nil (issue 39634, crash #3).
+	if typ == nil || isPtr && IsInterface(typ) {
+		return
+	}
+
+	// Start with typ as single entry at shallowest depth.
+	current := []embeddedType{{typ, nil, isPtr, false}}
+
+	// Named types that we have seen already, allocated lazily.
+	// Used to avoid endless searches in case of recursive types.
+	// Since only Named types can be used for recursive types, we
+	// only need to track those.
+	// (If we ever allow type aliases to construct recursive types,
+	// we must use type identity rather than pointer equality for
+	// the map key comparison, as we do in consolidateMultiples.)
+	var seen map[*Named]bool
+
+	// search current depth
+	for len(current) > 0 {
+		var next []embeddedType // embedded types found at current depth
+
+		// look for (pkg, name) in all types at current depth
+		var tpar *TypeParam // set if obj receiver is a type parameter
+		for _, e := range current {
+			typ := e.typ
+
+			// If we have a named type, we may have associated methods.
+			// Look for those first.
+			if named := typ.Named(); named != nil {
+				if seen[named] {
+					// We have seen this type before, at a more shallow depth
+					// (note that multiples of this type at the current depth
+					// were consolidated before). The type at that depth shadows
+					// this same type at the current depth, so we can ignore
+					// this one.
+					continue
+				}
+				if seen == nil {
+					seen = make(map[*Named]bool)
+				}
+				seen[named] = true
+
+				// look for a matching attached method
+				if i, m := lookupMethod(named.methods, pkg, name); m != nil {
+					// potential match
+					// caution: method may not have a proper signature yet
+					index = concat(e.index, i)
+					if obj != nil || e.multiples {
+						return nil, index, false // collision
+					}
+					obj = m
+					indirect = e.indirect
+					continue // we can't have a matching field or interface method
+				}
+
+				// continue with underlying type, but only if it's not a type parameter
+				// TODO(gri) is this what we want to do for type parameters? (spec question)
+				typ = named.Under()
+				if typ.TypeParam() != nil {
+					continue
+				}
+			}
+
+			tpar = nil
+			switch t := typ.(type) {
+			case *Struct:
+				// look for a matching field and collect embedded types
+				for i, f := range t.fields {
+					if f.sameId(pkg, name) {
+						assert(f.typ != nil)
+						index = concat(e.index, i)
+						if obj != nil || e.multiples {
+							return nil, index, false // collision
+						}
+						obj = f
+						indirect = e.indirect
+						continue // we can't have a matching interface method
+					}
+					// Collect embedded struct fields for searching the next
+					// lower depth, but only if we have not seen a match yet
+					// (if we have a match it is either the desired field or
+					// we have a name collision on the same depth; in either
+					// case we don't need to look further).
+					// Embedded fields are always of the form T or *T where
+					// T is a type name. If e.typ appeared multiple times at
+					// this depth, f.typ appears multiple times at the next
+					// depth.
+					if obj == nil && f.embedded {
+						typ, isPtr := deref(f.typ)
+						// TODO(gri) optimization: ignore types that can't
+						// have fields or methods (only Named, Struct, and
+						// Interface types need to be considered).
+						next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
+					}
+				}
+
+			case *Interface:
+				// look for a matching method
+				// TODO(gri) t.allMethods is sorted - use binary search
+				check.completeInterface(nopos, t)
+				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
+					assert(m.typ != nil)
+					index = concat(e.index, i)
+					if obj != nil || e.multiples {
+						return nil, index, false // collision
+					}
+					obj = m
+					indirect = e.indirect
+				}
+
+			case *TypeParam:
+				if i, m := lookupMethod(t.Bound().allMethods, pkg, name); m != nil {
+					assert(m.typ != nil)
+					index = concat(e.index, i)
+					if obj != nil || e.multiples {
+						return nil, index, false // collision
+					}
+					tpar = t
+					obj = m
+					indirect = e.indirect
+				}
+				if obj == nil {
+					// At this point we're not (yet) looking into methods
+					// that any underlyng type of the types in the type list
+					// migth have.
+					// TODO(gri) Do we want to specify the language that way?
+				}
+			}
+		}
+
+		if obj != nil {
+			// found a potential match
+			// spec: "A method call x.m() is valid if the method set of (the type of) x
+			//        contains m and the argument list can be assigned to the parameter
+			//        list of m. If x is addressable and &x's method set contains m, x.m()
+			//        is shorthand for (&x).m()".
+			if f, _ := obj.(*Func); f != nil {
+				// determine if method has a pointer receiver
+				hasPtrRecv := tpar == nil && ptrRecv(f) || tpar != nil && tpar.ptr
+				if hasPtrRecv && !indirect && !addressable {
+					return nil, nil, true // pointer/addressable receiver required
+				}
+			}
+			return
+		}
+
+		current = check.consolidateMultiples(next)
+	}
+
+	return nil, nil, false // not found
+}
+
+// embeddedType represents an embedded type
+type embeddedType struct {
+	typ       Type
+	index     []int // embedded field indices, starting with index at depth 0
+	indirect  bool  // if set, there was a pointer indirection on the path to this field
+	multiples bool  // if set, typ appears multiple times at this depth
+}
+
+// consolidateMultiples collects multiple list entries with the same type
+// into a single entry marked as containing multiples. The result is the
+// consolidated list.
+func (check *Checker) consolidateMultiples(list []embeddedType) []embeddedType {
+	if len(list) <= 1 {
+		return list // at most one entry - nothing to do
+	}
+
+	n := 0                     // number of entries w/ unique type
+	prev := make(map[Type]int) // index at which type was previously seen
+	for _, e := range list {
+		if i, found := check.lookupType(prev, e.typ); found {
+			list[i].multiples = true
+			// ignore this entry
+		} else {
+			prev[e.typ] = n
+			list[n] = e
+			n++
+		}
+	}
+	return list[:n]
+}
+
+func (check *Checker) lookupType(m map[Type]int, typ Type) (int, bool) {
+	// fast path: maybe the types are equal
+	if i, found := m[typ]; found {
+		return i, true
+	}
+
+	for t, i := range m {
+		if check.identical(t, typ) {
+			return i, true
+		}
+	}
+
+	return 0, false
+}
+
+// MissingMethod returns (nil, false) if V implements T, otherwise it
+// returns a missing method required by T and whether it is missing or
+// just has the wrong type.
+//
+// For non-interface types V, or if static is set, V implements T if all
+// methods of T are present in V. Otherwise (V is an interface and static
+// is not set), MissingMethod only checks that methods of T which are also
+// present in V have matching types (e.g., for a type assertion x.(T) where
+// x is of interface type V).
+//
+func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
+	m, typ := (*Checker)(nil).missingMethod(V, T, static)
+	return m, typ != nil
+}
+
+// missingMethod is like MissingMethod but accepts a *Checker as
+// receiver and an addressable flag.
+// The receiver may be nil if missingMethod is invoked through
+// an exported API call (such as MissingMethod), i.e., when all
+// methods have been type-checked.
+// If the type has the correctly named method, but with the wrong
+// signature, the existing method is returned as well.
+// To improve error messages, also report the wrong signature
+// when the method exists on *V instead of V.
+func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
+	check.completeInterface(nopos, T)
+
+	// fast path for common case
+	if T.Empty() {
+		return
+	}
+
+	if ityp := V.Interface(); ityp != nil {
+		check.completeInterface(nopos, ityp)
+		// TODO(gri) allMethods is sorted - can do this more efficiently
+		for _, m := range T.allMethods {
+			_, f := lookupMethod(ityp.allMethods, m.pkg, m.name)
+
+			if f == nil {
+				// if m is the magic method == we're ok (interfaces are comparable)
+				if m.name == "==" || !static {
+					continue
+				}
+				return m, f
+			}
+
+			// both methods must have the same number of type parameters
+			ftyp := f.typ.(*Signature)
+			mtyp := m.typ.(*Signature)
+			if len(ftyp.tparams) != len(mtyp.tparams) {
+				return m, f
+			}
+
+			// If the methods have type parameters we don't care whether they
+			// are the same or not, as long as they match up. Use unification
+			// to see if they can be made to match.
+			// TODO(gri) is this always correct? what about type bounds?
+			// (Alternative is to rename/subst type parameters and compare.)
+			u := newUnifier(check, true)
+			u.x.init(ftyp.tparams)
+			if !u.unify(ftyp, mtyp) {
+				return m, f
+			}
+		}
+
+		return
+	}
+
+	// A concrete type implements T if it implements all methods of T.
+	Vd, _ := deref(V)
+	Vn := Vd.Named()
+	for _, m := range T.allMethods {
+		// TODO(gri) should this be calling lookupFieldOrMethod instead (and why not)?
+		obj, _, _ := check.rawLookupFieldOrMethod(V, false, m.pkg, m.name)
+
+		// Check if *V implements this method of T.
+		if obj == nil {
+			ptr := NewPointer(V)
+			obj, _, _ = check.rawLookupFieldOrMethod(ptr, false, m.pkg, m.name)
+			if obj != nil {
+				return m, obj.(*Func)
+			}
+		}
+
+		// we must have a method (not a field of matching function type)
+		f, _ := obj.(*Func)
+		if f == nil {
+			// if m is the magic method == and V is comparable, we're ok
+			if m.name == "==" && Comparable(V) {
+				continue
+			}
+			return m, nil
+		}
+
+		// methods may not have a fully set up signature yet
+		if check != nil {
+			check.objDecl(f, nil)
+		}
+
+		// both methods must have the same number of type parameters
+		ftyp := f.typ.(*Signature)
+		mtyp := m.typ.(*Signature)
+		if len(ftyp.tparams) != len(mtyp.tparams) {
+			return m, f
+		}
+
+		// If V is a (instantiated) generic type, its methods are still
+		// parameterized using the original (declaration) receiver type
+		// parameters (subst simply copies the existing method list, it
+		// does not instantiate the methods).
+		// In order to compare the signatures, substitute the receiver
+		// type parameters of ftyp with V's instantiation type arguments.
+		// This lazily instantiates the signature of method f.
+		if Vn != nil && len(Vn.tparams) > 0 {
+			// Be careful: The number of type arguments may not match
+			// the number of receiver parameters. If so, an error was
+			// reported earlier but the length discrepancy is still
+			// here. Exit early in this case to prevent an assertion
+			// failure in makeSubstMap.
+			// TODO(gri) Can we avoid this check by fixing the lengths?
+			if len(ftyp.rparams) != len(Vn.targs) {
+				return
+			}
+			ftyp = check.subst(nopos, ftyp, makeSubstMap(ftyp.rparams, Vn.targs)).(*Signature)
+		}
+
+		// If the methods have type parameters we don't care whether they
+		// are the same or not, as long as they match up. Use unification
+		// to see if they can be made to match.
+		// TODO(gri) is this always correct? what about type bounds?
+		// (Alternative is to rename/subst type parameters and compare.)
+		u := newUnifier(check, true)
+		u.x.init(ftyp.tparams)
+		if !u.unify(ftyp, mtyp) {
+			return m, f
+		}
+	}
+
+	return
+}
+
+// assertableTo reports whether a value of type V can be asserted to have type T.
+// It returns (nil, false) as affirmative answer. Otherwise it returns a missing
+// method required by V and whether it is missing or just has the wrong type.
+// The receiver may be nil if assertableTo is invoked through an exported API call
+// (such as AssertableTo), i.e., when all methods have been type-checked.
+// If strict (or the global constant forceStrict) is set, assertions that
+// are known to fail are not permitted.
+func (check *Checker) assertableTo(V *Interface, T Type, strict bool) (method, wrongType *Func) {
+	// no static check is required if T is an interface
+	// spec: "If T is an interface type, x.(T) asserts that the
+	//        dynamic type of x implements the interface T."
+	if T.Interface() != nil && !(strict || forceStrict) {
+		return
+	}
+	return check.missingMethod(T, V, false)
+}
+
+// deref dereferences typ if it is a *Pointer and returns its base and true.
+// Otherwise it returns (typ, false).
+func deref(typ Type) (Type, bool) {
+	if p, _ := typ.(*Pointer); p != nil {
+		return p.base, true
+	}
+	return typ, false
+}
+
+// derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
+// (named or unnamed) struct and returns its base. Otherwise it returns typ.
+func derefStructPtr(typ Type) Type {
+	if p := typ.Pointer(); p != nil {
+		if p.base.Struct() != nil {
+			return p.base
+		}
+	}
+	return typ
+}
+
+// concat returns the result of concatenating list and i.
+// The result does not share its underlying array with list.
+func concat(list []int, i int) []int {
+	var t []int
+	t = append(t, list...)
+	return append(t, i)
+}
+
+// fieldIndex returns the index for the field with matching package and name, or a value < 0.
+func fieldIndex(fields []*Var, pkg *Package, name string) int {
+	if name != "_" {
+		for i, f := range fields {
+			if f.sameId(pkg, name) {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
+// lookupMethod returns the index of and method with matching package and name, or (-1, nil).
+func lookupMethod(methods []*Func, pkg *Package, name string) (int, *Func) {
+	if name != "_" {
+		for i, m := range methods {
+			if m.sameId(pkg, name) {
+				return i, m
+			}
+		}
+	}
+	return -1, nil
+}

src/cmd/compile/internal/types2/methodset.go

@@ -0,0 +1,261 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements method sets.
+
+package types2
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// A MethodSet is an ordered set of concrete or abstract (interface) methods;
+// a method is a MethodVal selection, and they are ordered by ascending m.Obj().Id().
+// The zero value for a MethodSet is a ready-to-use empty method set.
+type MethodSet struct {
+	list []*Selection
+}
+
+func (s *MethodSet) String() string {
+	if s.Len() == 0 {
+		return "MethodSet {}"
+	}
+
+	var buf strings.Builder
+	fmt.Fprintln(&buf, "MethodSet {")
+	for _, f := range s.list {
+		fmt.Fprintf(&buf, "\t%s\n", f)
+	}
+	fmt.Fprintln(&buf, "}")
+	return buf.String()
+}
+
+// Len returns the number of methods in s.
+func (s *MethodSet) Len() int { return len(s.list) }
+
+// At returns the i'th method in s for 0 <= i < s.Len().
+func (s *MethodSet) At(i int) *Selection { return s.list[i] }
+
+// Lookup returns the method with matching package and name, or nil if not found.
+func (s *MethodSet) Lookup(pkg *Package, name string) *Selection {
+	if s.Len() == 0 {
+		return nil
+	}
+
+	key := Id(pkg, name)
+	i := sort.Search(len(s.list), func(i int) bool {
+		m := s.list[i]
+		return m.obj.Id() >= key
+	})
+	if i < len(s.list) {
+		m := s.list[i]
+		if m.obj.Id() == key {
+			return m
+		}
+	}
+	return nil
+}
+
+// Shared empty method set.
+var emptyMethodSet MethodSet
+
+// Note: NewMethodSet is intended for external use only as it
+//       requires interfaces to be complete. If may be used
+//       internally if LookupFieldOrMethod completed the same
+//       interfaces beforehand.
+
+// NewMethodSet returns the method set for the given type T.
+// It always returns a non-nil method set, even if it is empty.
+func NewMethodSet(T Type) *MethodSet {
+	// WARNING: The code in this function is extremely subtle - do not modify casually!
+	//          This function and lookupFieldOrMethod should be kept in sync.
+
+	// method set up to the current depth, allocated lazily
+	var base methodSet
+
+	typ, isPtr := deref(T)
+
+	// *typ where typ is an interface has no methods.
+	if isPtr && IsInterface(typ) {
+		return &emptyMethodSet
+	}
+
+	// Start with typ as single entry at shallowest depth.
+	current := []embeddedType{{typ, nil, isPtr, false}}
+
+	// Named types that we have seen already, allocated lazily.
+	// Used to avoid endless searches in case of recursive types.
+	// Since only Named types can be used for recursive types, we
+	// only need to track those.
+	// (If we ever allow type aliases to construct recursive types,
+	// we must use type identity rather than pointer equality for
+	// the map key comparison, as we do in consolidateMultiples.)
+	var seen map[*Named]bool
+
+	// collect methods at current depth
+	for len(current) > 0 {
+		var next []embeddedType // embedded types found at current depth
+
+		// field and method sets at current depth, indexed by names (Id's), and allocated lazily
+		var fset map[string]bool // we only care about the field names
+		var mset methodSet
+
+		for _, e := range current {
+			typ := e.typ
+
+			// If we have a named type, we may have associated methods.
+			// Look for those first.
+			if named := typ.Named(); named != nil {
+				if seen[named] {
+					// We have seen this type before, at a more shallow depth
+					// (note that multiples of this type at the current depth
+					// were consolidated before). The type at that depth shadows
+					// this same type at the current depth, so we can ignore
+					// this one.
+					continue
+				}
+				if seen == nil {
+					seen = make(map[*Named]bool)
+				}
+				seen[named] = true
+
+				mset = mset.add(named.methods, e.index, e.indirect, e.multiples)
+
+				// continue with underlying type
+				typ = named.underlying
+			}
+
+			switch t := typ.(type) {
+			case *Struct:
+				for i, f := range t.fields {
+					if fset == nil {
+						fset = make(map[string]bool)
+					}
+					fset[f.Id()] = true
+
+					// Embedded fields are always of the form T or *T where
+					// T is a type name. If typ appeared multiple times at
+					// this depth, f.Type appears multiple times at the next
+					// depth.
+					if f.embedded {
+						typ, isPtr := deref(f.typ)
+						// TODO(gri) optimization: ignore types that can't
+						// have fields or methods (only Named, Struct, and
+						// Interface types need to be considered).
+						next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
+					}
+				}
+
+			case *Interface:
+				mset = mset.add(t.allMethods, e.index, true, e.multiples)
+			}
+		}
+
+		// Add methods and collisions at this depth to base if no entries with matching
+		// names exist already.
+		for k, m := range mset {
+			if _, found := base[k]; !found {
+				// Fields collide with methods of the same name at this depth.
+				if fset[k] {
+					m = nil // collision
+				}
+				if base == nil {
+					base = make(methodSet)
+				}
+				base[k] = m
+			}
+		}
+
+		// Add all (remaining) fields at this depth as collisions (since they will
+		// hide any method further down) if no entries with matching names exist already.
+		for k := range fset {
+			if _, found := base[k]; !found {
+				if base == nil {
+					base = make(methodSet)
+				}
+				base[k] = nil // collision
+			}
+		}
+
+		// It's ok to call consolidateMultiples with a nil *Checker because
+		// MethodSets are not used internally (outside debug mode). When used
+		// externally, interfaces are expected to be completed and then we do
+		// not need a *Checker to complete them when (indirectly) calling
+		// Checker.identical via consolidateMultiples.
+		current = (*Checker)(nil).consolidateMultiples(next)
+	}
+
+	if len(base) == 0 {
+		return &emptyMethodSet
+	}
+
+	// collect methods
+	var list []*Selection
+	for _, m := range base {
+		if m != nil {
+			m.recv = T
+			list = append(list, m)
+		}
+	}
+	// sort by unique name
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].obj.Id() < list[j].obj.Id()
+	})
+	return &MethodSet{list}
+}
+
+// A methodSet is a set of methods and name collisions.
+// A collision indicates that multiple methods with the
+// same unique id, or a field with that id appeared.
+type methodSet map[string]*Selection // a nil entry indicates a name collision
+
+// Add adds all functions in list to the method set s.
+// If multiples is set, every function in list appears multiple times
+// and is treated as a collision.
+func (s methodSet) add(list []*Func, index []int, indirect bool, multiples bool) methodSet {
+	if len(list) == 0 {
+		return s
+	}
+	if s == nil {
+		s = make(methodSet)
+	}
+	for i, f := range list {
+		key := f.Id()
+		// if f is not in the set, add it
+		if !multiples {
+			// TODO(gri) A found method may not be added because it's not in the method set
+			// (!indirect && ptrRecv(f)). A 2nd method on the same level may be in the method
+			// set and may not collide with the first one, thus leading to a false positive.
+			// Is that possible? Investigate.
+			if _, found := s[key]; !found && (indirect || !ptrRecv(f)) {
+				s[key] = &Selection{MethodVal, nil, f, concat(index, i), indirect}
+				continue
+			}
+		}
+		s[key] = nil // collision
+	}
+	return s
+}
+
+// ptrRecv reports whether the receiver is of the form *T.
+func ptrRecv(f *Func) bool {
+	// If a method's receiver type is set, use that as the source of truth for the receiver.
+	// Caution: Checker.funcDecl (decl.go) marks a function by setting its type to an empty
+	// signature. We may reach here before the signature is fully set up: we must explicitly
+	// check if the receiver is set (we cannot just look for non-nil f.typ).
+	if sig, _ := f.typ.(*Signature); sig != nil && sig.recv != nil {
+		_, isPtr := deref(sig.recv.typ)
+		return isPtr
+	}
+
+	// If a method's type is not set it may be a method/function that is:
+	// 1) client-supplied (via NewFunc with no signature), or
+	// 2) internally created but not yet type-checked.
+	// For case 1) we can't do anything; the client must know what they are doing.
+	// For case 2) we can use the information gathered by the resolver.
+	return f.hasPtrRecv
+}

src/cmd/compile/internal/types2/object.go

@@ -0,0 +1,492 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+	"go/token"
+)
+
+// An Object describes a named language entity such as a package,
+// constant, type, variable, function (incl. methods), or label.
+// All objects implement the Object interface.
+//
+type Object interface {
+	Parent() *Scope  // scope in which this object is declared; nil for methods and struct fields
+	Pos() syntax.Pos // position of object identifier in declaration
+	Pkg() *Package   // package to which this object belongs; nil for labels and objects in the Universe scope
+	Name() string    // package local object name
+	Type() Type      // object type
+	Exported() bool  // reports whether the name starts with a capital letter
+	Id() string      // object name if exported, qualified name if not exported (see func Id)
+
+	// String returns a human-readable string of the object.
+	String() string
+
+	// order reflects a package-level object's source order: if object
+	// a is before object b in the source, then a.order() < b.order().
+	// order returns a value > 0 for package-level objects; it returns
+	// 0 for all other objects (including objects in file scopes).
+	order() uint32
+
+	// color returns the object's color.
+	color() color
+
+	// setType sets the type of the object.
+	setType(Type)
+
+	// setOrder sets the order number of the object. It must be > 0.
+	setOrder(uint32)
+
+	// setColor sets the object's color. It must not be white.
+	setColor(color color)
+
+	// setParent sets the parent scope of the object.
+	setParent(*Scope)
+
+	// sameId reports whether obj.Id() and Id(pkg, name) are the same.
+	sameId(pkg *Package, name string) bool
+
+	// scopePos returns the start position of the scope of this Object
+	scopePos() syntax.Pos
+
+	// setScopePos sets the start position of the scope for this Object.
+	setScopePos(pos syntax.Pos)
+}
+
+// Id returns name if it is exported, otherwise it
+// returns the name qualified with the package path.
+func Id(pkg *Package, name string) string {
+	if token.IsExported(name) {
+		return name
+	}
+	// unexported names need the package path for differentiation
+	// (if there's no package, make sure we don't start with '.'
+	// as that may change the order of methods between a setup
+	// inside a package and outside a package - which breaks some
+	// tests)
+	path := "_"
+	// pkg is nil for objects in Universe scope and possibly types
+	// introduced via Eval (see also comment in object.sameId)
+	if pkg != nil && pkg.path != "" {
+		path = pkg.path
+	}
+	return path + "." + name
+}
+
+// An object implements the common parts of an Object.
+type object struct {
+	parent    *Scope
+	pos       syntax.Pos
+	pkg       *Package
+	name      string
+	typ       Type
+	order_    uint32
+	color_    color
+	scopePos_ syntax.Pos
+}
+
+// color encodes the color of an object (see Checker.objDecl for details).
+type color uint32
+
+// An object may be painted in one of three colors.
+// Color values other than white or black are considered grey.
+const (
+	white color = iota
+	black
+	grey // must be > white and black
+)
+
+func (c color) String() string {
+	switch c {
+	case white:
+		return "white"
+	case black:
+		return "black"
+	default:
+		return "grey"
+	}
+}
+
+// colorFor returns the (initial) color for an object depending on
+// whether its type t is known or not.
+func colorFor(t Type) color {
+	if t != nil {
+		return black
+	}
+	return white
+}
+
+// Parent returns the scope in which the object is declared.
+// The result is nil for methods and struct fields.
+func (obj *object) Parent() *Scope { return obj.parent }
+
+// Pos returns the declaration position of the object's identifier.
+func (obj *object) Pos() syntax.Pos { return obj.pos }
+
+// Pkg returns the package to which the object belongs.
+// The result is nil for labels and objects in the Universe scope.
+func (obj *object) Pkg() *Package { return obj.pkg }
+
+// Name returns the object's (package-local, unqualified) name.
+func (obj *object) Name() string { return obj.name }
+
+// Type returns the object's type.
+func (obj *object) Type() Type { return obj.typ }
+
+// Exported reports whether the object is exported (starts with a capital letter).
+// It doesn't take into account whether the object is in a local (function) scope
+// or not.
+func (obj *object) Exported() bool { return token.IsExported(obj.name) }
+
+// Id is a wrapper for Id(obj.Pkg(), obj.Name()).
+func (obj *object) Id() string { return Id(obj.pkg, obj.name) }
+
+func (obj *object) String() string       { panic("abstract") }
+func (obj *object) order() uint32        { return obj.order_ }
+func (obj *object) color() color         { return obj.color_ }
+func (obj *object) scopePos() syntax.Pos { return obj.scopePos_ }
+
+func (obj *object) setParent(parent *Scope)    { obj.parent = parent }
+func (obj *object) setType(typ Type)           { obj.typ = typ }
+func (obj *object) setOrder(order uint32)      { assert(order > 0); obj.order_ = order }
+func (obj *object) setColor(color color)       { assert(color != white); obj.color_ = color }
+func (obj *object) setScopePos(pos syntax.Pos) { obj.scopePos_ = pos }
+
+func (obj *object) sameId(pkg *Package, name string) bool {
+	// spec:
+	// "Two identifiers are different if they are spelled differently,
+	// or if they appear in different packages and are not exported.
+	// Otherwise, they are the same."
+	if name != obj.name {
+		return false
+	}
+	// obj.Name == name
+	if obj.Exported() {
+		return true
+	}
+	// not exported, so packages must be the same (pkg == nil for
+	// fields in Universe scope; this can only happen for types
+	// introduced via Eval)
+	if pkg == nil || obj.pkg == nil {
+		return pkg == obj.pkg
+	}
+	// pkg != nil && obj.pkg != nil
+	return pkg.path == obj.pkg.path
+}
+
+// A PkgName represents an imported Go package.
+// PkgNames don't have a type.
+type PkgName struct {
+	object
+	imported *Package
+	used     bool // set if the package was used
+}
+
+// NewPkgName returns a new PkgName object representing an imported package.
+// The remaining arguments set the attributes found with all Objects.
+func NewPkgName(pos syntax.Pos, pkg *Package, name string, imported *Package) *PkgName {
+	return &PkgName{object{nil, pos, pkg, name, Typ[Invalid], 0, black, nopos}, imported, false}
+}
+
+// Imported returns the package that was imported.
+// It is distinct from Pkg(), which is the package containing the import statement.
+func (obj *PkgName) Imported() *Package { return obj.imported }
+
+// A Const represents a declared constant.
+type Const struct {
+	object
+	val constant.Value
+}
+
+// NewConst returns a new constant with value val.
+// The remaining arguments set the attributes found with all Objects.
+func NewConst(pos syntax.Pos, pkg *Package, name string, typ Type, val constant.Value) *Const {
+	return &Const{object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}, val}
+}
+
+// Val returns the constant's value.
+func (obj *Const) Val() constant.Value { return obj.val }
+
+func (*Const) isDependency() {} // a constant may be a dependency of an initialization expression
+
+// A TypeName represents a name for a (defined or alias) type.
+type TypeName struct {
+	object
+}
+
+// NewTypeName returns a new type name denoting the given typ.
+// The remaining arguments set the attributes found with all Objects.
+//
+// The typ argument may be a defined (Named) type or an alias type.
+// It may also be nil such that the returned TypeName can be used as
+// argument for NewNamed, which will set the TypeName's type as a side-
+// effect.
+func NewTypeName(pos syntax.Pos, pkg *Package, name string, typ Type) *TypeName {
+	return &TypeName{object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}}
+}
+
+// IsAlias reports whether obj is an alias name for a type.
+func (obj *TypeName) IsAlias() bool {
+	switch t := obj.typ.(type) {
+	case nil:
+		return false
+	case *Basic:
+		// unsafe.Pointer is not an alias.
+		if obj.pkg == Unsafe {
+			return false
+		}
+		// Any user-defined type name for a basic type is an alias for a
+		// basic type (because basic types are pre-declared in the Universe
+		// scope, outside any package scope), and so is any type name with
+		// a different name than the name of the basic type it refers to.
+		// Additionally, we need to look for "byte" and "rune" because they
+		// are aliases but have the same names (for better error messages).
+		return obj.pkg != nil || t.name != obj.name || t == universeByte || t == universeRune
+	case *Named:
+		return obj != t.obj
+	default:
+		return true
+	}
+}
+
+// A Variable represents a declared variable (including function parameters and results, and struct fields).
+type Var struct {
+	object
+	embedded bool // if set, the variable is an embedded struct field, and name is the type name
+	isField  bool // var is struct field
+	used     bool // set if the variable was used
+}
+
+// NewVar returns a new variable.
+// The arguments set the attributes found with all Objects.
+func NewVar(pos syntax.Pos, pkg *Package, name string, typ Type) *Var {
+	return &Var{object: object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}}
+}
+
+// NewParam returns a new variable representing a function parameter.
+func NewParam(pos syntax.Pos, pkg *Package, name string, typ Type) *Var {
+	return &Var{object: object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}, used: true} // parameters are always 'used'
+}
+
+// NewField returns a new variable representing a struct field.
+// For embedded fields, the name is the unqualified type name
+/// under which the field is accessible.
+func NewField(pos syntax.Pos, pkg *Package, name string, typ Type, embedded bool) *Var {
+	return &Var{object: object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}, embedded: embedded, isField: true}
+}
+
+// Anonymous reports whether the variable is an embedded field.
+// Same as Embedded; only present for backward-compatibility.
+func (obj *Var) Anonymous() bool { return obj.embedded }
+
+// Embedded reports whether the variable is an embedded field.
+func (obj *Var) Embedded() bool { return obj.embedded }
+
+// IsField reports whether the variable is a struct field.
+func (obj *Var) IsField() bool { return obj.isField }
+
+func (*Var) isDependency() {} // a variable may be a dependency of an initialization expression
+
+// A Func represents a declared function, concrete method, or abstract
+// (interface) method. Its Type() is always a *Signature.
+// An abstract method may belong to many interfaces due to embedding.
+type Func struct {
+	object
+	hasPtrRecv bool // only valid for methods that don't have a type yet
+}
+
+// NewFunc returns a new function with the given signature, representing
+// the function's type.
+func NewFunc(pos syntax.Pos, pkg *Package, name string, sig *Signature) *Func {
+	// don't store a (typed) nil signature
+	var typ Type
+	if sig != nil {
+		typ = sig
+	}
+	return &Func{object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}, false}
+}
+
+// FullName returns the package- or receiver-type-qualified name of
+// function or method obj.
+func (obj *Func) FullName() string {
+	var buf bytes.Buffer
+	writeFuncName(&buf, obj, nil)
+	return buf.String()
+}
+
+// Scope returns the scope of the function's body block.
+func (obj *Func) Scope() *Scope { return obj.typ.(*Signature).scope }
+
+func (*Func) isDependency() {} // a function may be a dependency of an initialization expression
+
+// A Label represents a declared label.
+// Labels don't have a type.
+type Label struct {
+	object
+	used bool // set if the label was used
+}
+
+// NewLabel returns a new label.
+func NewLabel(pos syntax.Pos, pkg *Package, name string) *Label {
+	return &Label{object{pos: pos, pkg: pkg, name: name, typ: Typ[Invalid], color_: black}, false}
+}
+
+// A Builtin represents a built-in function.
+// Builtins don't have a valid type.
+type Builtin struct {
+	object
+	id builtinId
+}
+
+func newBuiltin(id builtinId) *Builtin {
+	return &Builtin{object{name: predeclaredFuncs[id].name, typ: Typ[Invalid], color_: black}, id}
+}
+
+// Nil represents the predeclared value nil.
+type Nil struct {
+	object
+}
+
+func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
+	var tname *TypeName
+	typ := obj.Type()
+
+	switch obj := obj.(type) {
+	case *PkgName:
+		fmt.Fprintf(buf, "package %s", obj.Name())
+		if path := obj.imported.path; path != "" && path != obj.name {
+			fmt.Fprintf(buf, " (%q)", path)
+		}
+		return
+
+	case *Const:
+		buf.WriteString("const")
+
+	case *TypeName:
+		tname = obj
+		buf.WriteString("type")
+
+	case *Var:
+		if obj.isField {
+			buf.WriteString("field")
+		} else {
+			buf.WriteString("var")
+		}
+
+	case *Func:
+		buf.WriteString("func ")
+		writeFuncName(buf, obj, qf)
+		if typ != nil {
+			WriteSignature(buf, typ.(*Signature), qf)
+		}
+		return
+
+	case *Label:
+		buf.WriteString("label")
+		typ = nil
+
+	case *Builtin:
+		buf.WriteString("builtin")
+		typ = nil
+
+	case *Nil:
+		buf.WriteString("nil")
+		return
+
+	default:
+		panic(fmt.Sprintf("writeObject(%T)", obj))
+	}
+
+	buf.WriteByte(' ')
+
+	// For package-level objects, qualify the name.
+	if obj.Pkg() != nil && obj.Pkg().scope.Lookup(obj.Name()) == obj {
+		writePackage(buf, obj.Pkg(), qf)
+	}
+	buf.WriteString(obj.Name())
+
+	if typ == nil {
+		return
+	}
+
+	if tname != nil {
+		// We have a type object: Don't print anything more for
+		// basic types since there's no more information (names
+		// are the same; see also comment in TypeName.IsAlias).
+		if _, ok := typ.(*Basic); ok {
+			return
+		}
+		if tname.IsAlias() {
+			buf.WriteString(" =")
+		} else {
+			typ = typ.Under()
+		}
+	}
+
+	buf.WriteByte(' ')
+	WriteType(buf, typ, qf)
+}
+
+func writePackage(buf *bytes.Buffer, pkg *Package, qf Qualifier) {
+	if pkg == nil {
+		return
+	}
+	var s string
+	if qf != nil {
+		s = qf(pkg)
+	} else {
+		s = pkg.Path()
+	}
+	if s != "" {
+		buf.WriteString(s)
+		buf.WriteByte('.')
+	}
+}
+
+// ObjectString returns the string form of obj.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func ObjectString(obj Object, qf Qualifier) string {
+	var buf bytes.Buffer
+	writeObject(&buf, obj, qf)
+	return buf.String()
+}
+
+func (obj *PkgName) String() string  { return ObjectString(obj, nil) }
+func (obj *Const) String() string    { return ObjectString(obj, nil) }
+func (obj *TypeName) String() string { return ObjectString(obj, nil) }
+func (obj *Var) String() string      { return ObjectString(obj, nil) }
+func (obj *Func) String() string     { return ObjectString(obj, nil) }
+func (obj *Label) String() string    { return ObjectString(obj, nil) }
+func (obj *Builtin) String() string  { return ObjectString(obj, nil) }
+func (obj *Nil) String() string      { return ObjectString(obj, nil) }
+
+func writeFuncName(buf *bytes.Buffer, f *Func, qf Qualifier) {
+	if f.typ != nil {
+		sig := f.typ.(*Signature)
+		if recv := sig.Recv(); recv != nil {
+			buf.WriteByte('(')
+			if _, ok := recv.Type().(*Interface); ok {
+				// gcimporter creates abstract methods of
+				// named interfaces using the interface type
+				// (not the named type) as the receiver.
+				// Don't print it in full.
+				buf.WriteString("interface")
+			} else {
+				WriteType(buf, recv.Type(), qf)
+			}
+			buf.WriteByte(')')
+			buf.WriteByte('.')
+		} else if f.pkg != nil {
+			writePackage(buf, f.pkg, qf)
+		}
+	}
+	buf.WriteString(f.name)
+}

src/cmd/compile/internal/types2/object_test.go

@@ -0,0 +1,89 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"strings"
+	"testing"
+)
+
+func parseSrc(path, src string) (*syntax.File, error) {
+	return syntax.Parse(syntax.NewFileBase(path), strings.NewReader(src), nil, nil, 0)
+}
+
+func TestIsAlias(t *testing.T) {
+	check := func(obj *TypeName, want bool) {
+		if got := obj.IsAlias(); got != want {
+			t.Errorf("%v: got IsAlias = %v; want %v", obj, got, want)
+		}
+	}
+
+	// predeclared types
+	check(Unsafe.Scope().Lookup("Pointer").(*TypeName), false)
+	for _, name := range Universe.Names() {
+		if obj, _ := Universe.Lookup(name).(*TypeName); obj != nil {
+			check(obj, name == "byte" || name == "rune")
+		}
+	}
+
+	// various other types
+	pkg := NewPackage("p", "p")
+	t1 := NewTypeName(nopos, pkg, "t1", nil)
+	n1 := NewNamed(t1, new(Struct), nil)
+	for _, test := range []struct {
+		name  *TypeName
+		alias bool
+	}{
+		{NewTypeName(nopos, nil, "t0", nil), false}, // no type yet
+		{NewTypeName(nopos, pkg, "t0", nil), false}, // no type yet
+		{t1, false}, // type name refers to named type and vice versa
+		{NewTypeName(nopos, nil, "t2", &emptyInterface), true}, // type name refers to unnamed type
+		{NewTypeName(nopos, pkg, "t3", n1), true},              // type name refers to named type with different type name
+		{NewTypeName(nopos, nil, "t4", Typ[Int32]), true},      // type name refers to basic type with different name
+		{NewTypeName(nopos, nil, "int32", Typ[Int32]), false},  // type name refers to basic type with same name
+		{NewTypeName(nopos, pkg, "int32", Typ[Int32]), true},   // type name is declared in user-defined package (outside Universe)
+		{NewTypeName(nopos, nil, "rune", Typ[Rune]), true},     // type name refers to basic type rune which is an alias already
+	} {
+		check(test.name, test.alias)
+	}
+}
+
+// TestEmbeddedMethod checks that an embedded method is represented by
+// the same Func Object as the original method. See also issue #34421.
+func TestEmbeddedMethod(t *testing.T) {
+	const src = `package p; type I interface { error }`
+
+	// type-check src
+	f, err := parseSrc("", src)
+	if err != nil {
+		t.Fatalf("parse failed: %s", err)
+	}
+	var conf Config
+	pkg, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, nil)
+	if err != nil {
+		t.Fatalf("typecheck failed: %s", err)
+	}
+
+	// get original error.Error method
+	eface := Universe.Lookup("error")
+	orig, _, _ := LookupFieldOrMethod(eface.Type(), false, nil, "Error")
+	if orig == nil {
+		t.Fatalf("original error.Error not found")
+	}
+
+	// get embedded error.Error method
+	iface := pkg.Scope().Lookup("I")
+	embed, _, _ := LookupFieldOrMethod(iface.Type(), false, nil, "Error")
+	if embed == nil {
+		t.Fatalf("embedded error.Error not found")
+	}
+
+	// original and embedded Error object should be identical
+	if orig != embed {
+		t.Fatalf("%s (%p) != %s (%p)", orig, orig, embed, embed)
+	}
+}

src/cmd/compile/internal/types2/objset.go

@@ -0,0 +1,32 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements objsets.
+//
+// An objset is similar to a Scope but objset elements
+// are identified by their unique id, instead of their
+// object name.
+
+package types2
+
+// An objset is a set of objects identified by their unique id.
+// The zero value for objset is a ready-to-use empty objset.
+type objset map[string]Object // initialized lazily
+
+// insert attempts to insert an object obj into objset s.
+// If s already contains an alternative object alt with
+// the same name, insert leaves s unchanged and returns alt.
+// Otherwise it inserts obj and returns nil.
+func (s *objset) insert(obj Object) Object {
+	id := obj.Id()
+	if alt := (*s)[id]; alt != nil {
+		return alt
+	}
+	if *s == nil {
+		*s = make(map[string]Object)
+	}
+	(*s)[id] = obj
+	return nil
+}

src/cmd/compile/internal/types2/operand.go

@@ -0,0 +1,315 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file defines operands and associated operations.
+
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+	"go/token"
+)
+
+// An operandMode specifies the (addressing) mode of an operand.
+type operandMode byte
+
+const (
+	invalid   operandMode = iota // operand is invalid
+	novalue                      // operand represents no value (result of a function call w/o result)
+	builtin                      // operand is a built-in function
+	typexpr                      // operand is a type
+	constant_                    // operand is a constant; the operand's typ is a Basic type
+	variable                     // operand is an addressable variable
+	mapindex                     // operand is a map index expression (acts like a variable on lhs, commaok on rhs of an assignment)
+	value                        // operand is a computed value
+	commaok                      // like value, but operand may be used in a comma,ok expression
+	commaerr                     // like commaok, but second value is error, not boolean
+	cgofunc                      // operand is a cgo function
+)
+
+var operandModeString = [...]string{
+	invalid:   "invalid operand",
+	novalue:   "no value",
+	builtin:   "built-in",
+	typexpr:   "type",
+	constant_: "constant",
+	variable:  "variable",
+	mapindex:  "map index expression",
+	value:     "value",
+	commaok:   "comma, ok expression",
+	commaerr:  "comma, error expression",
+	cgofunc:   "cgo function",
+}
+
+// An operand represents an intermediate value during type checking.
+// Operands have an (addressing) mode, the expression evaluating to
+// the operand, the operand's type, a value for constants, and an id
+// for built-in functions.
+// The zero value of operand is a ready to use invalid operand.
+//
+type operand struct {
+	mode operandMode
+	expr syntax.Expr
+	typ  Type
+	val  constant.Value
+	id   builtinId
+}
+
+// Pos returns the position of the expression corresponding to x.
+// If x is invalid the position is nopos.
+//
+func (x *operand) Pos() syntax.Pos {
+	// x.expr may not be set if x is invalid
+	if x.expr == nil {
+		return nopos
+	}
+	return x.expr.Pos()
+}
+
+// Operand string formats
+// (not all "untyped" cases can appear due to the type system,
+// but they fall out naturally here)
+//
+// mode       format
+//
+// invalid    <expr> (               <mode>                    )
+// novalue    <expr> (               <mode>                    )
+// builtin    <expr> (               <mode>                    )
+// typexpr    <expr> (               <mode>                    )
+//
+// constant   <expr> (<untyped kind> <mode>                    )
+// constant   <expr> (               <mode>       of type <typ>)
+// constant   <expr> (<untyped kind> <mode> <val>              )
+// constant   <expr> (               <mode> <val> of type <typ>)
+//
+// variable   <expr> (<untyped kind> <mode>                    )
+// variable   <expr> (               <mode>       of type <typ>)
+//
+// mapindex   <expr> (<untyped kind> <mode>                    )
+// mapindex   <expr> (               <mode>       of type <typ>)
+//
+// value      <expr> (<untyped kind> <mode>                    )
+// value      <expr> (               <mode>       of type <typ>)
+//
+// commaok    <expr> (<untyped kind> <mode>                    )
+// commaok    <expr> (               <mode>       of type <typ>)
+//
+// commaerr   <expr> (<untyped kind> <mode>                    )
+// commaerr   <expr> (               <mode>       of type <typ>)
+//
+// cgofunc    <expr> (<untyped kind> <mode>                    )
+// cgofunc    <expr> (               <mode>       of type <typ>)
+//
+func operandString(x *operand, qf Qualifier) string {
+	var buf bytes.Buffer
+
+	var expr string
+	if x.expr != nil {
+		expr = ExprString(x.expr)
+	} else {
+		switch x.mode {
+		case builtin:
+			expr = predeclaredFuncs[x.id].name
+		case typexpr:
+			expr = TypeString(x.typ, qf)
+		case constant_:
+			expr = x.val.String()
+		}
+	}
+
+	// <expr> (
+	if expr != "" {
+		buf.WriteString(expr)
+		buf.WriteString(" (")
+	}
+
+	// <untyped kind>
+	hasType := false
+	switch x.mode {
+	case invalid, novalue, builtin, typexpr:
+		// no type
+	default:
+		// should have a type, but be cautious (don't crash during printing)
+		if x.typ != nil {
+			if isUntyped(x.typ) {
+				buf.WriteString(x.typ.(*Basic).name)
+				buf.WriteByte(' ')
+				break
+			}
+			hasType = true
+		}
+	}
+
+	// <mode>
+	buf.WriteString(operandModeString[x.mode])
+
+	// <val>
+	if x.mode == constant_ {
+		if s := x.val.String(); s != expr {
+			buf.WriteByte(' ')
+			buf.WriteString(s)
+		}
+	}
+
+	// <typ>
+	if hasType {
+		if x.typ != Typ[Invalid] {
+			var intro string
+			switch {
+			case isGeneric(x.typ):
+				intro = " of generic type "
+			case x.typ.TypeParam() != nil:
+				intro = " of type parameter type "
+			default:
+				intro = " of type "
+			}
+			buf.WriteString(intro)
+			WriteType(&buf, x.typ, qf)
+		} else {
+			buf.WriteString(" with invalid type")
+		}
+	}
+
+	// )
+	if expr != "" {
+		buf.WriteByte(')')
+	}
+
+	return buf.String()
+}
+
+func (x *operand) String() string {
+	return operandString(x, nil)
+}
+
+// setConst sets x to the untyped constant for literal lit.
+func (x *operand) setConst(k syntax.LitKind, lit string) {
+	var tok token.Token
+	var kind BasicKind
+	switch k {
+	case syntax.IntLit:
+		tok = token.INT
+		kind = UntypedInt
+	case syntax.FloatLit:
+		tok = token.FLOAT
+		kind = UntypedFloat
+	case syntax.ImagLit:
+		tok = token.IMAG
+		kind = UntypedComplex
+	case syntax.RuneLit:
+		tok = token.CHAR
+		kind = UntypedRune
+	case syntax.StringLit:
+		tok = token.STRING
+		kind = UntypedString
+	default:
+		unreachable()
+	}
+
+	x.mode = constant_
+	x.typ = Typ[kind]
+	x.val = constant.MakeFromLiteral(lit, tok, 0)
+}
+
+// isNil reports whether x is the nil value.
+func (x *operand) isNil() bool {
+	return x.mode == value && x.typ == Typ[UntypedNil]
+}
+
+// TODO(gri) The functions operand.assignableTo, checker.convertUntyped,
+//           checker.representable, and checker.assignment are
+//           overlapping in functionality. Need to simplify and clean up.
+
+// assignableTo reports whether x is assignable to a variable of type T.
+// If the result is false and a non-nil reason is provided, it may be set
+// to a more detailed explanation of the failure (result != "").
+// The check parameter may be nil if assignableTo is invoked through
+// an exported API call, i.e., when all methods have been type-checked.
+func (x *operand) assignableTo(check *Checker, T Type, reason *string) bool {
+	if x.mode == invalid || T == Typ[Invalid] {
+		return true // avoid spurious errors
+	}
+
+	V := x.typ
+
+	// x's type is identical to T
+	if check.identical(V, T) {
+		return true
+	}
+
+	Vu := optype(V.Under())
+	Tu := optype(T.Under())
+
+	// x is an untyped value representable by a value of type T
+	// TODO(gri) This is borrowing from checker.convertUntyped and
+	//           checker.representable. Need to clean up.
+	if isUntyped(Vu) {
+		switch t := Tu.(type) {
+		case *Basic:
+			if x.isNil() && t.kind == UnsafePointer {
+				return true
+			}
+			if x.mode == constant_ {
+				return representableConst(x.val, check, t, nil)
+			}
+			// The result of a comparison is an untyped boolean,
+			// but may not be a constant.
+			if Vb, _ := Vu.(*Basic); Vb != nil {
+				return Vb.kind == UntypedBool && isBoolean(Tu)
+			}
+		case *Sum:
+			return t.is(func(t Type) bool {
+				// TODO(gri) this could probably be more efficient
+				return x.assignableTo(check, t, reason)
+			})
+		case *Interface:
+			check.completeInterface(nopos, t)
+			return x.isNil() || t.Empty()
+		case *Pointer, *Signature, *Slice, *Map, *Chan:
+			return x.isNil()
+		}
+	}
+	// Vu is typed
+
+	// x's type V and T have identical underlying types
+	// and at least one of V or T is not a named type
+	if check.identical(Vu, Tu) && (!isNamed(V) || !isNamed(T)) {
+		return true
+	}
+
+	// T is an interface type and x implements T
+	if Ti, ok := Tu.(*Interface); ok {
+		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
+			if reason != nil {
+				if wrongType != nil {
+					if check.identical(m.typ, wrongType.typ) {
+						*reason = fmt.Sprintf("missing method %s (%s has pointer receiver)", m.name, m.name)
+					} else {
+						*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
+					}
+
+				} else {
+					*reason = "missing method " + m.Name()
+				}
+			}
+			return false
+		}
+		return true
+	}
+
+	// x is a bidirectional channel value, T is a channel
+	// type, x's type V and T have identical element types,
+	// and at least one of V or T is not a named type
+	if Vc, ok := Vu.(*Chan); ok && Vc.dir == SendRecv {
+		if Tc, ok := Tu.(*Chan); ok && check.identical(Vc.elem, Tc.elem) {
+			return !isNamed(V) || !isNamed(T)
+		}
+	}
+
+	return false
+}

src/cmd/compile/internal/types2/package.go

@@ -0,0 +1,65 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"fmt"
+)
+
+// A Package describes a Go package.
+type Package struct {
+	path     string
+	name     string
+	scope    *Scope
+	complete bool
+	imports  []*Package
+	fake     bool // scope lookup errors are silently dropped if package is fake (internal use only)
+	cgo      bool // uses of this package will be rewritten into uses of declarations from _cgo_gotypes.go
+}
+
+// NewPackage returns a new Package for the given package path and name.
+// The package is not complete and contains no explicit imports.
+func NewPackage(path, name string) *Package {
+	scope := NewScope(Universe, nopos, nopos, fmt.Sprintf("package %q", path))
+	return &Package{path: path, name: name, scope: scope}
+}
+
+// Path returns the package path.
+func (pkg *Package) Path() string { return pkg.path }
+
+// Name returns the package name.
+func (pkg *Package) Name() string { return pkg.name }
+
+// SetName sets the package name.
+func (pkg *Package) SetName(name string) { pkg.name = name }
+
+// Scope returns the (complete or incomplete) package scope
+// holding the objects declared at package level (TypeNames,
+// Consts, Vars, and Funcs).
+func (pkg *Package) Scope() *Scope { return pkg.scope }
+
+// A package is complete if its scope contains (at least) all
+// exported objects; otherwise it is incomplete.
+func (pkg *Package) Complete() bool { return pkg.complete }
+
+// MarkComplete marks a package as complete.
+func (pkg *Package) MarkComplete() { pkg.complete = true }
+
+// Imports returns the list of packages directly imported by
+// pkg; the list is in source order.
+//
+// If pkg was loaded from export data, Imports includes packages that
+// provide package-level objects referenced by pkg. This may be more or
+// less than the set of packages directly imported by pkg's source code.
+func (pkg *Package) Imports() []*Package { return pkg.imports }
+
+// SetImports sets the list of explicitly imported packages to list.
+// It is the caller's responsibility to make sure list elements are unique.
+func (pkg *Package) SetImports(list []*Package) { pkg.imports = list }
+
+func (pkg *Package) String() string {
+	return fmt.Sprintf("package %s (%q)", pkg.name, pkg.path)
+}

src/cmd/compile/internal/types2/pos.go

@@ -0,0 +1,364 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements helper functions for scope position computations.
+
+package types2
+
+import "cmd/compile/internal/syntax"
+
+// startPos returns the start position of n.
+func startPos(n syntax.Node) syntax.Pos {
+	// Cases for nodes which don't need a correction are commented out.
+	for m := n; ; {
+		switch n := m.(type) {
+		case nil:
+			panic("internal error: nil")
+
+		// packages
+		case *syntax.File:
+			// file block starts at the beginning of the file
+			return syntax.MakePos(n.Pos().Base(), 1, 1)
+
+		// declarations
+		// case *syntax.ImportDecl:
+		// case *syntax.ConstDecl:
+		// case *syntax.TypeDecl:
+		// case *syntax.VarDecl:
+		// case *syntax.FuncDecl:
+
+		// expressions
+		// case *syntax.BadExpr:
+		// case *syntax.Name:
+		// case *syntax.BasicLit:
+		case *syntax.CompositeLit:
+			if n.Type != nil {
+				m = n.Type
+				continue
+			}
+			return n.Pos()
+		// case *syntax.KeyValueExpr:
+		// case *syntax.FuncLit:
+		// case *syntax.ParenExpr:
+		case *syntax.SelectorExpr:
+			m = n.X
+		case *syntax.IndexExpr:
+			m = n.X
+		// case *syntax.SliceExpr:
+		case *syntax.AssertExpr:
+			m = n.X
+		case *syntax.TypeSwitchGuard:
+			if n.Lhs != nil {
+				m = n.Lhs
+				continue
+			}
+			m = n.X
+		case *syntax.Operation:
+			if n.Y != nil {
+				m = n.X
+				continue
+			}
+			return n.Pos()
+		case *syntax.CallExpr:
+			m = n.Fun
+		case *syntax.ListExpr:
+			if len(n.ElemList) > 0 {
+				m = n.ElemList[0]
+				continue
+			}
+			return n.Pos()
+		// types
+		// case *syntax.ArrayType:
+		// case *syntax.SliceType:
+		// case *syntax.DotsType:
+		// case *syntax.StructType:
+		// case *syntax.Field:
+		// case *syntax.InterfaceType:
+		// case *syntax.FuncType:
+		// case *syntax.MapType:
+		// case *syntax.ChanType:
+
+		// statements
+		// case *syntax.EmptyStmt:
+		// case *syntax.LabeledStmt:
+		// case *syntax.BlockStmt:
+		// case *syntax.ExprStmt:
+		case *syntax.SendStmt:
+			m = n.Chan
+		// case *syntax.DeclStmt:
+		case *syntax.AssignStmt:
+			m = n.Lhs
+		// case *syntax.BranchStmt:
+		// case *syntax.CallStmt:
+		// case *syntax.ReturnStmt:
+		// case *syntax.IfStmt:
+		// case *syntax.ForStmt:
+		// case *syntax.SwitchStmt:
+		// case *syntax.SelectStmt:
+
+		// helper nodes
+		case *syntax.RangeClause:
+			if n.Lhs != nil {
+				m = n.Lhs
+				continue
+			}
+			m = n.X
+		// case *syntax.CaseClause:
+		// case *syntax.CommClause:
+
+		default:
+			return n.Pos()
+		}
+	}
+}
+
+// endPos returns the approximate end position of n in the source.
+// For some nodes (*syntax.Name, *syntax.BasicLit) it returns
+// the position immediately following the node; for others
+// (*syntax.BlockStmt, *syntax.SwitchStmt, etc.) it returns
+// the position of the closing '}'; and for some (*syntax.ParenExpr)
+// the returned position is the end position of the last enclosed
+// expression.
+// Thus, endPos should not be used for exact demarcation of the
+// end of a node in the source; it is mostly useful to determine
+// scope ranges where there is some leeway.
+func endPos(n syntax.Node) syntax.Pos {
+	for m := n; ; {
+		switch n := m.(type) {
+		case nil:
+			panic("internal error: nil")
+
+		// packages
+		case *syntax.File:
+			return n.EOF
+
+		// declarations
+		case *syntax.ImportDecl:
+			m = n.Path
+		case *syntax.ConstDecl:
+			if n.Values != nil {
+				m = n.Values
+				continue
+			}
+			if n.Type != nil {
+				m = n.Type
+				continue
+			}
+			if l := len(n.NameList); l > 0 {
+				m = n.NameList[l-1]
+				continue
+			}
+			return n.Pos()
+		case *syntax.TypeDecl:
+			m = n.Type
+		case *syntax.VarDecl:
+			if n.Values != nil {
+				m = n.Values
+				continue
+			}
+			if n.Type != nil {
+				m = n.Type
+				continue
+			}
+			if l := len(n.NameList); l > 0 {
+				m = n.NameList[l-1]
+				continue
+			}
+			return n.Pos()
+		case *syntax.FuncDecl:
+			if n.Body != nil {
+				m = n.Body
+				continue
+			}
+			m = n.Type
+
+		// expressions
+		case *syntax.BadExpr:
+			return n.Pos()
+		case *syntax.Name:
+			p := n.Pos()
+			return syntax.MakePos(p.Base(), p.Line(), p.Col()+uint(len(n.Value)))
+		case *syntax.BasicLit:
+			p := n.Pos()
+			return syntax.MakePos(p.Base(), p.Line(), p.Col()+uint(len(n.Value)))
+		case *syntax.CompositeLit:
+			return n.Rbrace
+		case *syntax.KeyValueExpr:
+			m = n.Value
+		case *syntax.FuncLit:
+			m = n.Body
+		case *syntax.ParenExpr:
+			m = n.X
+		case *syntax.SelectorExpr:
+			m = n.Sel
+		case *syntax.IndexExpr:
+			m = n.Index
+		case *syntax.SliceExpr:
+			for i := len(n.Index) - 1; i >= 0; i-- {
+				if x := n.Index[i]; x != nil {
+					m = x
+					continue
+				}
+			}
+			m = n.X
+		case *syntax.AssertExpr:
+			m = n.Type
+		case *syntax.TypeSwitchGuard:
+			m = n.X
+		case *syntax.Operation:
+			if n.Y != nil {
+				m = n.Y
+				continue
+			}
+			m = n.X
+		case *syntax.CallExpr:
+			if l := lastExpr(n.ArgList); l != nil {
+				m = l
+				continue
+			}
+			m = n.Fun
+		case *syntax.ListExpr:
+			if l := lastExpr(n.ElemList); l != nil {
+				m = l
+				continue
+			}
+			return n.Pos()
+
+		// types
+		case *syntax.ArrayType:
+			m = n.Elem
+		case *syntax.SliceType:
+			m = n.Elem
+		case *syntax.DotsType:
+			m = n.Elem
+		case *syntax.StructType:
+			if l := lastField(n.FieldList); l != nil {
+				m = l
+				continue
+			}
+			return n.Pos()
+			// TODO(gri) need to take TagList into account
+		case *syntax.Field:
+			if n.Type != nil {
+				m = n.Type
+				continue
+			}
+			m = n.Name
+		case *syntax.InterfaceType:
+			if l := lastField(n.MethodList); l != nil {
+				m = l
+				continue
+			}
+			return n.Pos()
+		case *syntax.FuncType:
+			if l := lastField(n.ResultList); l != nil {
+				m = l
+				continue
+			}
+			if l := lastField(n.ParamList); l != nil {
+				m = l
+				continue
+			}
+			return n.Pos()
+		case *syntax.MapType:
+			m = n.Value
+		case *syntax.ChanType:
+			m = n.Elem
+
+		// statements
+		case *syntax.EmptyStmt:
+			return n.Pos()
+		case *syntax.LabeledStmt:
+			m = n.Stmt
+		case *syntax.BlockStmt:
+			return n.Rbrace
+		case *syntax.ExprStmt:
+			m = n.X
+		case *syntax.SendStmt:
+			m = n.Value
+		case *syntax.DeclStmt:
+			if l := lastDecl(n.DeclList); l != nil {
+				m = l
+				continue
+			}
+			return n.Pos()
+		case *syntax.AssignStmt:
+			m = n.Rhs
+		case *syntax.BranchStmt:
+			if n.Label != nil {
+				m = n.Label
+				continue
+			}
+			return n.Pos()
+		case *syntax.CallStmt:
+			m = n.Call
+		case *syntax.ReturnStmt:
+			if n.Results != nil {
+				m = n.Results
+				continue
+			}
+			return n.Pos()
+		case *syntax.IfStmt:
+			if n.Else != nil {
+				m = n.Else
+				continue
+			}
+			m = n.Then
+		case *syntax.ForStmt:
+			m = n.Body
+		case *syntax.SwitchStmt:
+			return n.Rbrace
+		case *syntax.SelectStmt:
+			return n.Rbrace
+
+		// helper nodes
+		case *syntax.RangeClause:
+			m = n.X
+		case *syntax.CaseClause:
+			if l := lastStmt(n.Body); l != nil {
+				m = l
+				continue
+			}
+			return n.Colon
+		case *syntax.CommClause:
+			if l := lastStmt(n.Body); l != nil {
+				m = l
+				continue
+			}
+			return n.Colon
+
+		default:
+			return n.Pos()
+		}
+	}
+}
+
+func lastDecl(list []syntax.Decl) syntax.Decl {
+	if l := len(list); l > 0 {
+		return list[l-1]
+	}
+	return nil
+}
+
+func lastExpr(list []syntax.Expr) syntax.Expr {
+	if l := len(list); l > 0 {
+		return list[l-1]
+	}
+	return nil
+}
+
+func lastStmt(list []syntax.Stmt) syntax.Stmt {
+	if l := len(list); l > 0 {
+		return list[l-1]
+	}
+	return nil
+}
+
+func lastField(list []*syntax.Field) *syntax.Field {
+	if l := len(list); l > 0 {
+		return list[l-1]
+	}
+	return nil
+}

src/cmd/compile/internal/types2/predicates.go

@@ -0,0 +1,413 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements commonly used type predicates.
+
+package types2
+
+import (
+	"sort"
+)
+
+// isNamed reports whether typ has a name.
+// isNamed may be called with types that are not fully set up.
+func isNamed(typ Type) bool {
+	switch typ.(type) {
+	case *Basic, *Named, *TypeParam, *instance:
+		return true
+	}
+	return false
+}
+
+// isGeneric reports whether a type is a generic, uninstantiated type (generic signatures are not included).
+func isGeneric(typ Type) bool {
+	// A parameterized type is only instantiated if it doesn't have an instantiation already.
+	named, _ := typ.(*Named)
+	return named != nil && named.obj != nil && named.tparams != nil && named.targs == nil
+}
+
+func is(typ Type, what BasicInfo) bool {
+	switch t := optype(typ.Under()).(type) {
+	case *Basic:
+		return t.info&what != 0
+	case *Sum:
+		return t.is(func(typ Type) bool { return is(typ, what) })
+	}
+	return false
+}
+
+func isBoolean(typ Type) bool  { return is(typ, IsBoolean) }
+func isInteger(typ Type) bool  { return is(typ, IsInteger) }
+func isUnsigned(typ Type) bool { return is(typ, IsUnsigned) }
+func isFloat(typ Type) bool    { return is(typ, IsFloat) }
+func isComplex(typ Type) bool  { return is(typ, IsComplex) }
+func isNumeric(typ Type) bool  { return is(typ, IsNumeric) }
+func isString(typ Type) bool   { return is(typ, IsString) }
+
+// Note that if typ is a type parameter, isInteger(typ) || isFloat(typ) does not
+// produce the expected result because a type list that contains both an integer
+// and a floating-point type is neither (all) integers, nor (all) floats.
+// Use isIntegerOrFloat instead.
+func isIntegerOrFloat(typ Type) bool { return is(typ, IsInteger|IsFloat) }
+
+// isNumericOrString is the equivalent of isIntegerOrFloat for isNumeric(typ) || isString(typ).
+func isNumericOrString(typ Type) bool { return is(typ, IsNumeric|IsString) }
+
+// isTyped reports whether typ is typed; i.e., not an untyped
+// constant or boolean. isTyped may be called with types that
+// are not fully set up.
+func isTyped(typ Type) bool {
+	// isTyped is called with types that are not fully
+	// set up. Must not call Basic()!
+	// A *Named or *instance type is always typed, so
+	// we only need to check if we have a true *Basic
+	// type.
+	t, _ := typ.(*Basic)
+	return t == nil || t.info&IsUntyped == 0
+}
+
+// isUntyped(typ) is the same as !isTyped(typ).
+func isUntyped(typ Type) bool {
+	return !isTyped(typ)
+}
+
+func isOrdered(typ Type) bool { return is(typ, IsOrdered) }
+
+func isConstType(typ Type) bool {
+	t := typ.Basic()
+	return t != nil && t.info&IsConstType != 0
+}
+
+// IsInterface reports whether typ is an interface type.
+func IsInterface(typ Type) bool {
+	return typ.Interface() != nil
+}
+
+// Comparable reports whether values of type T are comparable.
+func Comparable(T Type) bool {
+	// If T is a type parameter not constraint by any type
+	// list (i.e., it's underlying type is the top type),
+	// T is comparable if it has the == method. Otherwise,
+	// the underlying type "wins". For instance
+	//
+	//     interface{ comparable; type []byte }
+	//
+	// is not comparable because []byte is not comparable.
+	if t := T.TypeParam(); t != nil && optype(t) == theTop {
+		return t.Bound().IsComparable()
+	}
+
+	switch t := optype(T.Under()).(type) {
+	case *Basic:
+		// assume invalid types to be comparable
+		// to avoid follow-up errors
+		return t.kind != UntypedNil
+	case *Pointer, *Interface, *Chan:
+		return true
+	case *Struct:
+		for _, f := range t.fields {
+			if !Comparable(f.typ) {
+				return false
+			}
+		}
+		return true
+	case *Array:
+		return Comparable(t.elem)
+	case *Sum:
+		return t.is(Comparable)
+	case *TypeParam:
+		return t.Bound().IsComparable()
+	}
+	return false
+}
+
+// hasNil reports whether a type includes the nil value.
+func hasNil(typ Type) bool {
+	switch t := optype(typ.Under()).(type) {
+	case *Basic:
+		return t.kind == UnsafePointer
+	case *Slice, *Pointer, *Signature, *Interface, *Map, *Chan:
+		return true
+	case *Sum:
+		return t.is(hasNil)
+	}
+	return false
+}
+
+// identical reports whether x and y are identical types.
+// Receivers of Signature types are ignored.
+func (check *Checker) identical(x, y Type) bool {
+	return check.identical0(x, y, true, nil)
+}
+
+// identicalIgnoreTags reports whether x and y are identical types if tags are ignored.
+// Receivers of Signature types are ignored.
+func (check *Checker) identicalIgnoreTags(x, y Type) bool {
+	return check.identical0(x, y, false, nil)
+}
+
+// An ifacePair is a node in a stack of interface type pairs compared for identity.
+type ifacePair struct {
+	x, y *Interface
+	prev *ifacePair
+}
+
+func (p *ifacePair) identical(q *ifacePair) bool {
+	return p.x == q.x && p.y == q.y || p.x == q.y && p.y == q.x
+}
+
+// For changes to this code the corresponding changes should be made to unifier.nify.
+func (check *Checker) identical0(x, y Type, cmpTags bool, p *ifacePair) bool {
+	// types must be expanded for comparison
+	x = expandf(x)
+	y = expandf(y)
+
+	if x == y {
+		return true
+	}
+
+	switch x := x.(type) {
+	case *Basic:
+		// Basic types are singletons except for the rune and byte
+		// aliases, thus we cannot solely rely on the x == y check
+		// above. See also comment in TypeName.IsAlias.
+		if y, ok := y.(*Basic); ok {
+			return x.kind == y.kind
+		}
+
+	case *Array:
+		// Two array types are identical if they have identical element types
+		// and the same array length.
+		if y, ok := y.(*Array); ok {
+			// If one or both array lengths are unknown (< 0) due to some error,
+			// assume they are the same to avoid spurious follow-on errors.
+			return (x.len < 0 || y.len < 0 || x.len == y.len) && check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Slice:
+		// Two slice types are identical if they have identical element types.
+		if y, ok := y.(*Slice); ok {
+			return check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Struct:
+		// Two struct types are identical if they have the same sequence of fields,
+		// and if corresponding fields have the same names, and identical types,
+		// and identical tags. Two embedded fields are considered to have the same
+		// name. Lower-case field names from different packages are always different.
+		if y, ok := y.(*Struct); ok {
+			if x.NumFields() == y.NumFields() {
+				for i, f := range x.fields {
+					g := y.fields[i]
+					if f.embedded != g.embedded ||
+						cmpTags && x.Tag(i) != y.Tag(i) ||
+						!f.sameId(g.pkg, g.name) ||
+						!check.identical0(f.typ, g.typ, cmpTags, p) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Pointer:
+		// Two pointer types are identical if they have identical base types.
+		if y, ok := y.(*Pointer); ok {
+			return check.identical0(x.base, y.base, cmpTags, p)
+		}
+
+	case *Tuple:
+		// Two tuples types are identical if they have the same number of elements
+		// and corresponding elements have identical types.
+		if y, ok := y.(*Tuple); ok {
+			if x.Len() == y.Len() {
+				if x != nil {
+					for i, v := range x.vars {
+						w := y.vars[i]
+						if !check.identical0(v.typ, w.typ, cmpTags, p) {
+							return false
+						}
+					}
+				}
+				return true
+			}
+		}
+
+	case *Signature:
+		// Two function types are identical if they have the same number of parameters
+		// and result values, corresponding parameter and result types are identical,
+		// and either both functions are variadic or neither is. Parameter and result
+		// names are not required to match.
+		// Generic functions must also have matching type parameter lists, but for the
+		// parameter names.
+		if y, ok := y.(*Signature); ok {
+			return x.variadic == y.variadic &&
+				check.identicalTParams(x.tparams, y.tparams, cmpTags, p) &&
+				check.identical0(x.params, y.params, cmpTags, p) &&
+				check.identical0(x.results, y.results, cmpTags, p)
+		}
+
+	case *Sum:
+		// Two sum types are identical if they contain the same types.
+		// (Sum types always consist of at least two types. Also, the
+		// the set (list) of types in a sum type consists of unique
+		// types - each type appears exactly once. Thus, two sum types
+		// must contain the same number of types to have chance of
+		// being equal.
+		if y, ok := y.(*Sum); ok && len(x.types) == len(y.types) {
+			// Every type in x.types must be in y.types.
+			// Quadratic algorithm, but probably good enough for now.
+			// TODO(gri) we need a fast quick type ID/hash for all types.
+		L:
+			for _, x := range x.types {
+				for _, y := range y.types {
+					if Identical(x, y) {
+						continue L // x is in y.types
+					}
+				}
+				return false // x is not in y.types
+			}
+			return true
+		}
+
+	case *Interface:
+		// Two interface types are identical if they have the same set of methods with
+		// the same names and identical function types. Lower-case method names from
+		// different packages are always different. The order of the methods is irrelevant.
+		if y, ok := y.(*Interface); ok {
+			// If identical0 is called (indirectly) via an external API entry point
+			// (such as Identical, IdenticalIgnoreTags, etc.), check is nil. But in
+			// that case, interfaces are expected to be complete and lazy completion
+			// here is not needed.
+			if check != nil {
+				check.completeInterface(nopos, x)
+				check.completeInterface(nopos, y)
+			}
+			a := x.allMethods
+			b := y.allMethods
+			if len(a) == len(b) {
+				// Interface types are the only types where cycles can occur
+				// that are not "terminated" via named types; and such cycles
+				// can only be created via method parameter types that are
+				// anonymous interfaces (directly or indirectly) embedding
+				// the current interface. Example:
+				//
+				//    type T interface {
+				//        m() interface{T}
+				//    }
+				//
+				// If two such (differently named) interfaces are compared,
+				// endless recursion occurs if the cycle is not detected.
+				//
+				// If x and y were compared before, they must be equal
+				// (if they were not, the recursion would have stopped);
+				// search the ifacePair stack for the same pair.
+				//
+				// This is a quadratic algorithm, but in practice these stacks
+				// are extremely short (bounded by the nesting depth of interface
+				// type declarations that recur via parameter types, an extremely
+				// rare occurrence). An alternative implementation might use a
+				// "visited" map, but that is probably less efficient overall.
+				q := &ifacePair{x, y, p}
+				for p != nil {
+					if p.identical(q) {
+						return true // same pair was compared before
+					}
+					p = p.prev
+				}
+				if debug {
+					assert(sort.IsSorted(byUniqueMethodName(a)))
+					assert(sort.IsSorted(byUniqueMethodName(b)))
+				}
+				for i, f := range a {
+					g := b[i]
+					if f.Id() != g.Id() || !check.identical0(f.typ, g.typ, cmpTags, q) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Map:
+		// Two map types are identical if they have identical key and value types.
+		if y, ok := y.(*Map); ok {
+			return check.identical0(x.key, y.key, cmpTags, p) && check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Chan:
+		// Two channel types are identical if they have identical value types
+		// and the same direction.
+		if y, ok := y.(*Chan); ok {
+			return x.dir == y.dir && check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Named:
+		// Two named types are identical if their type names originate
+		// in the same type declaration.
+		if y, ok := y.(*Named); ok {
+			// TODO(gri) Why is x == y not sufficient? And if it is,
+			//           we can just return false here because x == y
+			//           is caught in the very beginning of this function.
+			return x.obj == y.obj
+		}
+
+	case *TypeParam:
+		// nothing to do (x and y being equal is caught in the very beginning of this function)
+
+	// case *instance:
+	//	unreachable since types are expanded
+
+	case *bottom, *top:
+		// Either both types are theBottom, or both are theTop in which
+		// case the initial x == y check will have caught them. Otherwise
+		// they are not identical.
+
+	case nil:
+		// avoid a crash in case of nil type
+
+	default:
+		unreachable()
+	}
+
+	return false
+}
+
+func (check *Checker) identicalTParams(x, y []*TypeName, cmpTags bool, p *ifacePair) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i, x := range x {
+		y := y[i]
+		if !check.identical0(x.typ.(*TypeParam).bound, y.typ.(*TypeParam).bound, cmpTags, p) {
+			return false
+		}
+	}
+	return true
+}
+
+// Default returns the default "typed" type for an "untyped" type;
+// it returns the incoming type for all other types. The default type
+// for untyped nil is untyped nil.
+//
+func Default(typ Type) Type {
+	if t, ok := typ.(*Basic); ok {
+		switch t.kind {
+		case UntypedBool:
+			return Typ[Bool]
+		case UntypedInt:
+			return Typ[Int]
+		case UntypedRune:
+			return universeRune // use 'rune' name
+		case UntypedFloat:
+			return Typ[Float64]
+		case UntypedComplex:
+			return Typ[Complex128]
+		case UntypedString:
+			return Typ[String]
+		}
+	}
+	return typ
+}

src/cmd/compile/internal/types2/resolver.go

@@ -0,0 +1,714 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/constant"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// A declInfo describes a package-level const, type, var, or func declaration.
+type declInfo struct {
+	file  *Scope           // scope of file containing this declaration
+	lhs   []*Var           // lhs of n:1 variable declarations, or nil
+	vtyp  syntax.Expr      // type, or nil (for const and var declarations only)
+	init  syntax.Expr      // init/orig expression, or nil (for const and var declarations only)
+	tdecl *syntax.TypeDecl // type declaration, or nil
+	fdecl *syntax.FuncDecl // func declaration, or nil
+
+	// The deps field tracks initialization expression dependencies.
+	deps map[Object]bool // lazily initialized
+}
+
+// hasInitializer reports whether the declared object has an initialization
+// expression or function body.
+func (d *declInfo) hasInitializer() bool {
+	return d.init != nil || d.fdecl != nil && d.fdecl.Body != nil
+}
+
+// addDep adds obj to the set of objects d's init expression depends on.
+func (d *declInfo) addDep(obj Object) {
+	m := d.deps
+	if m == nil {
+		m = make(map[Object]bool)
+		d.deps = m
+	}
+	m[obj] = true
+}
+
+// arity checks that the lhs and rhs of a const or var decl
+// have a matching number of names and initialization values.
+// If inherited is set, the initialization values are from
+// another (constant) declaration.
+func (check *Checker) arity(pos syntax.Pos, names []*syntax.Name, inits []syntax.Expr, constDecl, inherited bool) {
+	l := len(names)
+	r := len(inits)
+
+	switch {
+	case l < r:
+		n := inits[l]
+		if inherited {
+			check.errorf(pos, "extra init expr at %s", n.Pos())
+		} else {
+			check.errorf(n, "extra init expr %s", n)
+		}
+	case l > r && (constDecl || r != 1): // if r == 1 it may be a multi-valued function and we can't say anything yet
+		n := names[r]
+		check.errorf(n, "missing init expr for %s", n.Value)
+	}
+}
+
+func validatedImportPath(path string) (string, error) {
+	s, err := strconv.Unquote(path)
+	if err != nil {
+		return "", err
+	}
+	if s == "" {
+		return "", fmt.Errorf("empty string")
+	}
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return s, fmt.Errorf("invalid character %#U", r)
+		}
+	}
+	return s, nil
+}
+
+// declarePkgObj declares obj in the package scope, records its ident -> obj mapping,
+// and updates check.objMap. The object must not be a function or method.
+func (check *Checker) declarePkgObj(ident *syntax.Name, obj Object, d *declInfo) {
+	assert(ident.Value == obj.Name())
+
+	// spec: "A package-scope or file-scope identifier with name init
+	// may only be declared to be a function with this (func()) signature."
+	if ident.Value == "init" {
+		check.errorf(ident, "cannot declare init - must be func")
+		return
+	}
+
+	// spec: "The main package must have package name main and declare
+	// a function main that takes no arguments and returns no value."
+	if ident.Value == "main" && check.pkg.name == "main" {
+		check.errorf(ident, "cannot declare main - must be func")
+		return
+	}
+
+	check.declare(check.pkg.scope, ident, obj, nopos)
+	check.objMap[obj] = d
+	obj.setOrder(uint32(len(check.objMap)))
+}
+
+// filename returns a filename suitable for debugging output.
+func (check *Checker) filename(fileNo int) string {
+	file := check.files[fileNo]
+	if pos := file.Pos(); pos.IsKnown() {
+		// return check.fset.File(pos).Name()
+		// TODO(gri) do we need the actual file name here?
+		return pos.RelFilename()
+	}
+	return fmt.Sprintf("file[%d]", fileNo)
+}
+
+func (check *Checker) importPackage(pos syntax.Pos, path, dir string) *Package {
+	// If we already have a package for the given (path, dir)
+	// pair, use it instead of doing a full import.
+	// Checker.impMap only caches packages that are marked Complete
+	// or fake (dummy packages for failed imports). Incomplete but
+	// non-fake packages do require an import to complete them.
+	key := importKey{path, dir}
+	imp := check.impMap[key]
+	if imp != nil {
+		return imp
+	}
+
+	// no package yet => import it
+	if path == "C" && (check.conf.FakeImportC || check.conf.go115UsesCgo) {
+		imp = NewPackage("C", "C")
+		imp.fake = true // package scope is not populated
+		imp.cgo = check.conf.go115UsesCgo
+	} else {
+		// ordinary import
+		var err error
+		if importer := check.conf.Importer; importer == nil {
+			err = fmt.Errorf("Config.Importer not installed")
+		} else if importerFrom, ok := importer.(ImporterFrom); ok {
+			imp, err = importerFrom.ImportFrom(path, dir, 0)
+			if imp == nil && err == nil {
+				err = fmt.Errorf("Config.Importer.ImportFrom(%s, %s, 0) returned nil but no error", path, dir)
+			}
+		} else {
+			imp, err = importer.Import(path)
+			if imp == nil && err == nil {
+				err = fmt.Errorf("Config.Importer.Import(%s) returned nil but no error", path)
+			}
+		}
+		// make sure we have a valid package name
+		// (errors here can only happen through manipulation of packages after creation)
+		if err == nil && imp != nil && (imp.name == "_" || imp.name == "") {
+			err = fmt.Errorf("invalid package name: %q", imp.name)
+			imp = nil // create fake package below
+		}
+		if err != nil {
+			check.errorf(pos, "could not import %s (%s)", path, err)
+			if imp == nil {
+				// create a new fake package
+				// come up with a sensible package name (heuristic)
+				name := path
+				if i := len(name); i > 0 && name[i-1] == '/' {
+					name = name[:i-1]
+				}
+				if i := strings.LastIndex(name, "/"); i >= 0 {
+					name = name[i+1:]
+				}
+				imp = NewPackage(path, name)
+			}
+			// continue to use the package as best as we can
+			imp.fake = true // avoid follow-up lookup failures
+		}
+	}
+
+	// package should be complete or marked fake, but be cautious
+	if imp.complete || imp.fake {
+		check.impMap[key] = imp
+		check.pkgCnt[imp.name]++
+		return imp
+	}
+
+	// something went wrong (importer may have returned incomplete package without error)
+	return nil
+}
+
+// collectObjects collects all file and package objects and inserts them
+// into their respective scopes. It also performs imports and associates
+// methods with receiver base type names.
+func (check *Checker) collectObjects() {
+	pkg := check.pkg
+
+	// pkgImports is the set of packages already imported by any package file seen
+	// so far. Used to avoid duplicate entries in pkg.imports. Allocate and populate
+	// it (pkg.imports may not be empty if we are checking test files incrementally).
+	// Note that pkgImports is keyed by package (and thus package path), not by an
+	// importKey value. Two different importKey values may map to the same package
+	// which is why we cannot use the check.impMap here.
+	var pkgImports = make(map[*Package]bool)
+	for _, imp := range pkg.imports {
+		pkgImports[imp] = true
+	}
+
+	type methodInfo struct {
+		obj  *Func        // method
+		ptr  bool         // true if pointer receiver
+		recv *syntax.Name // receiver type name
+	}
+	var methods []methodInfo // collected methods with valid receivers and non-blank _ names
+	var fileScopes []*Scope
+	for fileNo, file := range check.files {
+		// The package identifier denotes the current package,
+		// but there is no corresponding package object.
+		check.recordDef(file.PkgName, nil)
+
+		fileScope := NewScope(check.pkg.scope, startPos(file), endPos(file), check.filename(fileNo))
+		fileScopes = append(fileScopes, fileScope)
+		check.recordScope(file, fileScope)
+
+		// determine file directory, necessary to resolve imports
+		// FileName may be "" (typically for tests) in which case
+		// we get "." as the directory which is what we would want.
+		fileDir := dir(file.PkgName.Pos().RelFilename()) // TODO(gri) should this be filename?
+
+		first := -1                // index of first ConstDecl in the current group, or -1
+		var last *syntax.ConstDecl // last ConstDecl with init expressions, or nil
+		for index, decl := range file.DeclList {
+			if _, ok := decl.(*syntax.ConstDecl); !ok {
+				first = -1 // we're not in a constant declaration
+			}
+
+			switch s := decl.(type) {
+			case *syntax.ImportDecl:
+				// import package
+				path, err := validatedImportPath(s.Path.Value)
+				if err != nil {
+					check.errorf(s.Path, "invalid import path (%s)", err)
+					continue
+				}
+
+				imp := check.importPackage(s.Path.Pos(), path, fileDir)
+				if imp == nil {
+					continue
+				}
+
+				// add package to list of explicit imports
+				// (this functionality is provided as a convenience
+				// for clients; it is not needed for type-checking)
+				if !pkgImports[imp] {
+					pkgImports[imp] = true
+					pkg.imports = append(pkg.imports, imp)
+				}
+
+				// local name overrides imported package name
+				name := imp.name
+				if s.LocalPkgName != nil {
+					name = s.LocalPkgName.Value
+					if path == "C" {
+						// match cmd/compile (not prescribed by spec)
+						check.errorf(s.LocalPkgName, `cannot rename import "C"`)
+						continue
+					}
+					if name == "init" {
+						check.errorf(s.LocalPkgName, "cannot declare init - must be func")
+						continue
+					}
+				}
+
+				obj := NewPkgName(s.Pos(), pkg, name, imp)
+				if s.LocalPkgName != nil {
+					// in a dot-import, the dot represents the package
+					check.recordDef(s.LocalPkgName, obj)
+				} else {
+					check.recordImplicit(s, obj)
+				}
+
+				if path == "C" {
+					// match cmd/compile (not prescribed by spec)
+					obj.used = true
+				}
+
+				// add import to file scope
+				if name == "." {
+					// merge imported scope with file scope
+					for _, obj := range imp.scope.elems {
+						// A package scope may contain non-exported objects,
+						// do not import them!
+						if obj.Exported() {
+							// declare dot-imported object
+							// (Do not use check.declare because it modifies the object
+							// via Object.setScopePos, which leads to a race condition;
+							// the object may be imported into more than one file scope
+							// concurrently. See issue #32154.)
+							if alt := fileScope.Insert(obj); alt != nil {
+								check.errorf(s.LocalPkgName, "%s redeclared in this block", obj.Name())
+								check.reportAltDecl(alt)
+							}
+						}
+					}
+					// add position to set of dot-import positions for this file
+					// (this is only needed for "imported but not used" errors)
+					check.addUnusedDotImport(fileScope, imp, s.Pos())
+				} else {
+					// declare imported package object in file scope
+					// (no need to provide s.LocalPkgName since we called check.recordDef earlier)
+					check.declare(fileScope, nil, obj, nopos)
+				}
+
+			case *syntax.ConstDecl:
+				// iota is the index of the current constDecl within the group
+				if first < 0 || file.DeclList[index-1].(*syntax.ConstDecl).Group != s.Group {
+					first = index
+					last = nil
+				}
+				iota := constant.MakeInt64(int64(index - first))
+
+				// determine which initialization expressions to use
+				inherited := true
+				switch {
+				case s.Type != nil || s.Values != nil:
+					last = s
+					inherited = false
+				case last == nil:
+					last = new(syntax.ConstDecl) // make sure last exists
+					inherited = false
+				}
+
+				// declare all constants
+				values := unpackExpr(last.Values)
+				for i, name := range s.NameList {
+					obj := NewConst(name.Pos(), pkg, name.Value, nil, iota)
+
+					var init syntax.Expr
+					if i < len(values) {
+						init = values[i]
+					}
+
+					d := &declInfo{file: fileScope, vtyp: last.Type, init: init}
+					check.declarePkgObj(name, obj, d)
+				}
+
+				// Constants must always have init values.
+				check.arity(s.Pos(), s.NameList, values, true, inherited)
+
+			case *syntax.VarDecl:
+				lhs := make([]*Var, len(s.NameList))
+				// If there's exactly one rhs initializer, use
+				// the same declInfo d1 for all lhs variables
+				// so that each lhs variable depends on the same
+				// rhs initializer (n:1 var declaration).
+				var d1 *declInfo
+				if _, ok := s.Values.(*syntax.ListExpr); !ok {
+					// The lhs elements are only set up after the for loop below,
+					// but that's ok because declarePkgObj only collects the declInfo
+					// for a later phase.
+					d1 = &declInfo{file: fileScope, lhs: lhs, vtyp: s.Type, init: s.Values}
+				}
+
+				// declare all variables
+				values := unpackExpr(s.Values)
+				for i, name := range s.NameList {
+					obj := NewVar(name.Pos(), pkg, name.Value, nil)
+					lhs[i] = obj
+
+					d := d1
+					if d == nil {
+						// individual assignments
+						var init syntax.Expr
+						if i < len(values) {
+							init = values[i]
+						}
+						d = &declInfo{file: fileScope, vtyp: s.Type, init: init}
+					}
+
+					check.declarePkgObj(name, obj, d)
+				}
+
+				// If we have no type, we must have values.
+				if s.Type == nil || values != nil {
+					check.arity(s.Pos(), s.NameList, values, false, false)
+				}
+
+			case *syntax.TypeDecl:
+				obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Value, nil)
+				check.declarePkgObj(s.Name, obj, &declInfo{file: fileScope, tdecl: s})
+
+			case *syntax.FuncDecl:
+				d := s // TODO(gri) get rid of this
+				name := d.Name.Value
+				obj := NewFunc(d.Name.Pos(), pkg, name, nil)
+				if d.Recv == nil {
+					// regular function
+					if name == "init" {
+						if d.TParamList != nil {
+							//check.softErrorf(d.TParamList.Pos(), "func init must have no type parameters")
+							check.softErrorf(d.Name, "func init must have no type parameters")
+						}
+						if t := d.Type; len(t.ParamList) != 0 || len(t.ResultList) != 0 {
+							check.softErrorf(d, "func init must have no arguments and no return values")
+						}
+						// don't declare init functions in the package scope - they are invisible
+						obj.parent = pkg.scope
+						check.recordDef(d.Name, obj)
+						// init functions must have a body
+						if d.Body == nil {
+							// TODO(gri) make this error message consistent with the others above
+							check.softErrorf(obj.pos, "missing function body")
+						}
+					} else {
+						check.declare(pkg.scope, d.Name, obj, nopos)
+					}
+				} else {
+					// method
+					// d.Recv != nil
+					if !methodTypeParamsOk && len(d.TParamList) != 0 {
+						//check.invalidASTf(d.TParamList.Pos(), "method must have no type parameters")
+						check.invalidASTf(d, "method must have no type parameters")
+					}
+					ptr, recv, _ := check.unpackRecv(d.Recv.Type, false)
+					// (Methods with invalid receiver cannot be associated to a type, and
+					// methods with blank _ names are never found; no need to collect any
+					// of them. They will still be type-checked with all the other functions.)
+					if recv != nil && name != "_" {
+						methods = append(methods, methodInfo{obj, ptr, recv})
+					}
+					check.recordDef(d.Name, obj)
+				}
+				info := &declInfo{file: fileScope, fdecl: d}
+				// Methods are not package-level objects but we still track them in the
+				// object map so that we can handle them like regular functions (if the
+				// receiver is invalid); also we need their fdecl info when associating
+				// them with their receiver base type, below.
+				check.objMap[obj] = info
+				obj.setOrder(uint32(len(check.objMap)))
+
+			default:
+				check.invalidASTf(s, "unknown syntax.Decl node %T", s)
+			}
+		}
+	}
+
+	// verify that objects in package and file scopes have different names
+	for _, scope := range fileScopes {
+		for _, obj := range scope.elems {
+			if alt := pkg.scope.Lookup(obj.Name()); alt != nil {
+				if pkg, ok := obj.(*PkgName); ok {
+					check.errorf(alt, "%s already declared through import of %s", alt.Name(), pkg.Imported())
+					check.reportAltDecl(pkg)
+				} else {
+					check.errorf(alt, "%s already declared through dot-import of %s", alt.Name(), obj.Pkg())
+					// TODO(gri) dot-imported objects don't have a position; reportAltDecl won't print anything
+					check.reportAltDecl(obj)
+				}
+			}
+		}
+	}
+
+	// Now that we have all package scope objects and all methods,
+	// associate methods with receiver base type name where possible.
+	// Ignore methods that have an invalid receiver. They will be
+	// type-checked later, with regular functions.
+	if methods != nil {
+		check.methods = make(map[*TypeName][]*Func)
+		for i := range methods {
+			m := &methods[i]
+			// Determine the receiver base type and associate m with it.
+			ptr, base := check.resolveBaseTypeName(m.ptr, m.recv)
+			if base != nil {
+				m.obj.hasPtrRecv = ptr
+				check.methods[base] = append(check.methods[base], m.obj)
+			}
+		}
+	}
+}
+
+// unpackRecv unpacks a receiver type and returns its components: ptr indicates whether
+// rtyp is a pointer receiver, rname is the receiver type name, and tparams are its
+// type parameters, if any. The type parameters are only unpacked if unpackParams is
+// set. If rname is nil, the receiver is unusable (i.e., the source has a bug which we
+// cannot easily work around).
+func (check *Checker) unpackRecv(rtyp syntax.Expr, unpackParams bool) (ptr bool, rname *syntax.Name, tparams []*syntax.Name) {
+L: // unpack receiver type
+	// This accepts invalid receivers such as ***T and does not
+	// work for other invalid receivers, but we don't care. The
+	// validity of receiver expressions is checked elsewhere.
+	for {
+		switch t := rtyp.(type) {
+		case *syntax.ParenExpr:
+			rtyp = t.X
+		// case *ast.StarExpr:
+		// 	rtyp = t.X
+		case *syntax.Operation:
+			if t.Op != syntax.Mul || t.Y != nil {
+				break
+			}
+			rtyp = t.X
+		default:
+			break L
+		}
+	}
+
+	// unpack type parameters, if any
+	if ptyp, _ := rtyp.(*syntax.IndexExpr); ptyp != nil {
+		rtyp = ptyp.X
+		if unpackParams {
+			for _, arg := range unpackExpr(ptyp.Index) {
+				var par *syntax.Name
+				switch arg := arg.(type) {
+				case *syntax.Name:
+					par = arg
+				case *syntax.BadExpr:
+					// ignore - error already reported by parser
+				case nil:
+					check.invalidASTf(ptyp, "parameterized receiver contains nil parameters")
+				default:
+					check.errorf(arg, "receiver type parameter %s must be an identifier", arg)
+				}
+				if par == nil {
+					par = newName(arg.Pos(), "_")
+				}
+				tparams = append(tparams, par)
+			}
+
+		}
+	}
+
+	// unpack receiver name
+	if name, _ := rtyp.(*syntax.Name); name != nil {
+		rname = name
+	}
+
+	return
+}
+
+// resolveBaseTypeName returns the non-alias base type name for typ, and whether
+// there was a pointer indirection to get to it. The base type name must be declared
+// in package scope, and there can be at most one pointer indirection. If no such type
+// name exists, the returned base is nil.
+func (check *Checker) resolveBaseTypeName(seenPtr bool, typ syntax.Expr) (ptr bool, base *TypeName) {
+	// Algorithm: Starting from a type expression, which may be a name,
+	// we follow that type through alias declarations until we reach a
+	// non-alias type name. If we encounter anything but pointer types or
+	// parentheses we're done. If we encounter more than one pointer type
+	// we're done.
+	ptr = seenPtr
+	var seen map[*TypeName]bool
+	for {
+		typ = unparen(typ)
+
+		// check if we have a pointer type
+		// if pexpr, _ := typ.(*ast.StarExpr); pexpr != nil {
+		if pexpr, _ := typ.(*syntax.Operation); pexpr != nil && pexpr.Op == syntax.Mul && pexpr.Y == nil {
+			// if we've already seen a pointer, we're done
+			if ptr {
+				return false, nil
+			}
+			ptr = true
+			typ = unparen(pexpr.X) // continue with pointer base type
+		}
+
+		// typ must be a name
+		name, _ := typ.(*syntax.Name)
+		if name == nil {
+			return false, nil
+		}
+
+		// name must denote an object found in the current package scope
+		// (note that dot-imported objects are not in the package scope!)
+		obj := check.pkg.scope.Lookup(name.Value)
+		if obj == nil {
+			return false, nil
+		}
+
+		// the object must be a type name...
+		tname, _ := obj.(*TypeName)
+		if tname == nil {
+			return false, nil
+		}
+
+		// ... which we have not seen before
+		if seen[tname] {
+			return false, nil
+		}
+
+		// we're done if tdecl defined tname as a new type
+		// (rather than an alias)
+		tdecl := check.objMap[tname].tdecl // must exist for objects in package scope
+		if !tdecl.Alias {
+			return ptr, tname
+		}
+
+		// otherwise, continue resolving
+		typ = tdecl.Type
+		if seen == nil {
+			seen = make(map[*TypeName]bool)
+		}
+		seen[tname] = true
+	}
+}
+
+// packageObjects typechecks all package objects, but not function bodies.
+func (check *Checker) packageObjects() {
+	// process package objects in source order for reproducible results
+	objList := make([]Object, len(check.objMap))
+	i := 0
+	for obj := range check.objMap {
+		objList[i] = obj
+		i++
+	}
+	sort.Sort(inSourceOrder(objList))
+
+	// add new methods to already type-checked types (from a prior Checker.Files call)
+	for _, obj := range objList {
+		if obj, _ := obj.(*TypeName); obj != nil && obj.typ != nil {
+			check.collectMethods(obj)
+		}
+	}
+
+	// We process non-alias declarations first, in order to avoid situations where
+	// the type of an alias declaration is needed before it is available. In general
+	// this is still not enough, as it is possible to create sufficiently convoluted
+	// recursive type definitions that will cause a type alias to be needed before it
+	// is available (see issue #25838 for examples).
+	// As an aside, the cmd/compiler suffers from the same problem (#25838).
+	var aliasList []*TypeName
+	// phase 1
+	for _, obj := range objList {
+		// If we have a type alias, collect it for the 2nd phase.
+		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].tdecl.Alias {
+			aliasList = append(aliasList, tname)
+			continue
+		}
+
+		check.objDecl(obj, nil)
+	}
+	// phase 2
+	for _, obj := range aliasList {
+		check.objDecl(obj, nil)
+	}
+
+	// At this point we may have a non-empty check.methods map; this means that not all
+	// entries were deleted at the end of typeDecl because the respective receiver base
+	// types were not found. In that case, an error was reported when declaring those
+	// methods. We can now safely discard this map.
+	check.methods = nil
+}
+
+// inSourceOrder implements the sort.Sort interface.
+type inSourceOrder []Object
+
+func (a inSourceOrder) Len() int           { return len(a) }
+func (a inSourceOrder) Less(i, j int) bool { return a[i].order() < a[j].order() }
+func (a inSourceOrder) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+// unusedImports checks for unused imports.
+func (check *Checker) unusedImports() {
+	// if function bodies are not checked, packages' uses are likely missing - don't check
+	if check.conf.IgnoreFuncBodies {
+		return
+	}
+
+	// spec: "It is illegal (...) to directly import a package without referring to
+	// any of its exported identifiers. To import a package solely for its side-effects
+	// (initialization), use the blank identifier as explicit package name."
+
+	// check use of regular imported packages
+	for _, scope := range check.pkg.scope.children /* file scopes */ {
+		for _, obj := range scope.elems {
+			if obj, ok := obj.(*PkgName); ok {
+				// Unused "blank imports" are automatically ignored
+				// since _ identifiers are not entered into scopes.
+				if !obj.used {
+					path := obj.imported.path
+					base := pkgName(path)
+					if obj.name == base {
+						check.softErrorf(obj.pos, "%q imported but not used", path)
+					} else {
+						check.softErrorf(obj.pos, "%q imported but not used as %s", path, obj.name)
+					}
+				}
+			}
+		}
+	}
+
+	// check use of dot-imported packages
+	for _, unusedDotImports := range check.unusedDotImports {
+		for pkg, pos := range unusedDotImports {
+			check.softErrorf(pos, "%q imported but not used", pkg.path)
+		}
+	}
+}
+
+// pkgName returns the package name (last element) of an import path.
+func pkgName(path string) string {
+	if i := strings.LastIndex(path, "/"); i >= 0 {
+		path = path[i+1:]
+	}
+	return path
+}
+
+// dir makes a good-faith attempt to return the directory
+// portion of path. If path is empty, the result is ".".
+// (Per the go/build package dependency tests, we cannot import
+// path/filepath and simply use filepath.Dir.)
+func dir(path string) string {
+	if i := strings.LastIndexAny(path, `/\`); i > 0 {
+		return path[:i]
+	}
+	// i <= 0
+	return "."
+}

src/cmd/compile/internal/types2/resolver_test.go

@@ -0,0 +1,223 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2_test
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"internal/testenv"
+	"sort"
+	"testing"
+
+	. "cmd/compile/internal/types2"
+)
+
+type resolveTestImporter struct {
+	importer ImporterFrom
+	imported map[string]bool
+}
+
+func (imp *resolveTestImporter) Import(string) (*Package, error) {
+	panic("should not be called")
+}
+
+func (imp *resolveTestImporter) ImportFrom(path, srcDir string, mode ImportMode) (*Package, error) {
+	if mode != 0 {
+		panic("mode must be 0")
+	}
+	if imp.importer == nil {
+		imp.importer = defaultImporter().(ImporterFrom)
+		imp.imported = make(map[string]bool)
+	}
+	pkg, err := imp.importer.ImportFrom(path, srcDir, mode)
+	if err != nil {
+		return nil, err
+	}
+	imp.imported[path] = true
+	return pkg, nil
+}
+
+func TestResolveIdents(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	sources := []string{
+		`
+		package p
+		import "fmt"
+		import "math"
+		const pi = math.Pi
+		func sin(x float64) float64 {
+			return math.Sin(x)
+		}
+		var Println = fmt.Println
+		`,
+		`
+		package p
+		import "fmt"
+		type errorStringer struct { fmt.Stringer; error }
+		func f() string {
+			_ = "foo"
+			return fmt.Sprintf("%d", g())
+		}
+		func g() (x int) { return }
+		`,
+		`
+		package p
+		import . "go/parser"
+		import "sync"
+		func h() Mode { return ImportsOnly }
+		var _, x int = 1, 2
+		func init() {}
+		type T struct{ *sync.Mutex; a, b, c int}
+		type I interface{ m() }
+		var _ = T{a: 1, b: 2, c: 3}
+		func (_ T) m() {}
+		func (T) _() {}
+		var i I
+		var _ = i.m
+		func _(s []int) { for i, x := range s { _, _ = i, x } }
+		func _(x interface{}) {
+			switch x := x.(type) {
+			case int:
+				_ = x
+			}
+			switch {} // implicit 'true' tag
+		}
+		`,
+		`
+		package p
+		type S struct{}
+		func (T) _() {}
+		func (T) _() {}
+		`,
+		`
+		package p
+		func _() {
+		L0:
+		L1:
+			goto L0
+			for {
+				goto L1
+			}
+			if true {
+				goto L2
+			}
+		L2:
+		}
+		`,
+	}
+
+	pkgnames := []string{
+		"fmt",
+		"math",
+	}
+
+	// parse package files
+	var files []*syntax.File
+	for i, src := range sources {
+		f, err := parseSrc(fmt.Sprintf("sources[%d]", i), src)
+		if err != nil {
+			t.Fatal(err)
+		}
+		files = append(files, f)
+	}
+
+	// resolve and type-check package AST
+	importer := new(resolveTestImporter)
+	conf := Config{Importer: importer}
+	uses := make(map[*syntax.Name]Object)
+	defs := make(map[*syntax.Name]Object)
+	_, err := conf.Check("testResolveIdents", files, &Info{Defs: defs, Uses: uses})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// check that all packages were imported
+	for _, name := range pkgnames {
+		if !importer.imported[name] {
+			t.Errorf("package %s not imported", name)
+		}
+	}
+
+	// check that qualified identifiers are resolved
+	for _, f := range files {
+		Walk(f, func(n syntax.Node) bool {
+			if s, ok := n.(*syntax.SelectorExpr); ok {
+				if x, ok := s.X.(*syntax.Name); ok {
+					obj := uses[x]
+					if obj == nil {
+						t.Errorf("%s: unresolved qualified identifier %s", x.Pos(), x.Value)
+						return true
+					}
+					if _, ok := obj.(*PkgName); ok && uses[s.Sel] == nil {
+						t.Errorf("%s: unresolved selector %s", s.Sel.Pos(), s.Sel.Value)
+						return true
+					}
+					return true
+				}
+				return true
+			}
+			return false
+		})
+	}
+
+	for id, obj := range uses {
+		if obj == nil {
+			t.Errorf("%s: Uses[%s] == nil", id.Pos(), id.Value)
+		}
+	}
+
+	// Check that each identifier in the source is found in uses or defs or both.
+	// We need the foundUses/Defs maps (rather then just deleting the found objects
+	// from the uses and defs maps) because Walk traverses shared nodes multiple
+	// times (e.g. types in field lists such as "a, b, c int").
+	foundUses := make(map[*syntax.Name]bool)
+	foundDefs := make(map[*syntax.Name]bool)
+	var both []string
+	for _, f := range files {
+		Walk(f, func(n syntax.Node) bool {
+			if x, ok := n.(*syntax.Name); ok {
+				var objects int
+				if _, found := uses[x]; found {
+					objects |= 1
+					foundUses[x] = true
+				}
+				if _, found := defs[x]; found {
+					objects |= 2
+					foundDefs[x] = true
+				}
+				switch objects {
+				case 0:
+					t.Errorf("%s: unresolved identifier %s", x.Pos(), x.Value)
+				case 3:
+					both = append(both, x.Value)
+				}
+				return true
+			}
+			return false
+		})
+	}
+
+	// check the expected set of idents that are simultaneously uses and defs
+	sort.Strings(both)
+	if got, want := fmt.Sprint(both), "[Mutex Stringer error]"; got != want {
+		t.Errorf("simultaneous uses/defs = %s, want %s", got, want)
+	}
+
+	// any left-over identifiers didn't exist in the source
+	for x := range uses {
+		if !foundUses[x] {
+			t.Errorf("%s: identifier %s not present in source", x.Pos(), x.Value)
+		}
+	}
+	for x := range defs {
+		if !foundDefs[x] {
+			t.Errorf("%s: identifier %s not present in source", x.Pos(), x.Value)
+		}
+	}
+
+	// TODO(gri) add tests to check ImplicitObj callbacks
+}

src/cmd/compile/internal/types2/return.go

@@ -0,0 +1,180 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements isTerminating.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+)
+
+// isTerminating reports if s is a terminating statement.
+// If s is labeled, label is the label name; otherwise s
+// is "".
+func (check *Checker) isTerminating(s syntax.Stmt, label string) bool {
+	switch s := s.(type) {
+	default:
+		unreachable()
+
+	case *syntax.DeclStmt, *syntax.EmptyStmt, *syntax.SendStmt,
+		*syntax.AssignStmt, *syntax.CallStmt:
+		// no chance
+
+	case *syntax.LabeledStmt:
+		return check.isTerminating(s.Stmt, s.Label.Value)
+
+	case *syntax.ExprStmt:
+		// calling the predeclared (possibly parenthesized) panic() function is terminating
+		if call, ok := unparen(s.X).(*syntax.CallExpr); ok && check.isPanic[call] {
+			return true
+		}
+
+	case *syntax.ReturnStmt:
+		return true
+
+	case *syntax.BranchStmt:
+		if s.Tok == syntax.Goto || s.Tok == syntax.Fallthrough {
+			return true
+		}
+
+	case *syntax.BlockStmt:
+		return check.isTerminatingList(s.List, "")
+
+	case *syntax.IfStmt:
+		if s.Else != nil &&
+			check.isTerminating(s.Then, "") &&
+			check.isTerminating(s.Else, "") {
+			return true
+		}
+
+	case *syntax.SwitchStmt:
+		return check.isTerminatingSwitch(s.Body, label)
+
+	case *syntax.SelectStmt:
+		for _, cc := range s.Body {
+			if !check.isTerminatingList(cc.Body, "") || hasBreakList(cc.Body, label, true) {
+				return false
+			}
+
+		}
+		return true
+
+	case *syntax.ForStmt:
+		if s.Cond == nil && !hasBreak(s.Body, label, true) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func (check *Checker) isTerminatingList(list []syntax.Stmt, label string) bool {
+	// trailing empty statements are permitted - skip them
+	for i := len(list) - 1; i >= 0; i-- {
+		if _, ok := list[i].(*syntax.EmptyStmt); !ok {
+			return check.isTerminating(list[i], label)
+		}
+	}
+	return false // all statements are empty
+}
+
+func (check *Checker) isTerminatingSwitch(body []*syntax.CaseClause, label string) bool {
+	hasDefault := false
+	for _, cc := range body {
+		if cc.Cases == nil {
+			hasDefault = true
+		}
+		if !check.isTerminatingList(cc.Body, "") || hasBreakList(cc.Body, label, true) {
+			return false
+		}
+	}
+	return hasDefault
+}
+
+// TODO(gri) For nested breakable statements, the current implementation of hasBreak
+//	     will traverse the same subtree repeatedly, once for each label. Replace
+//           with a single-pass label/break matching phase.
+
+// hasBreak reports if s is or contains a break statement
+// referring to the label-ed statement or implicit-ly the
+// closest outer breakable statement.
+func hasBreak(s syntax.Stmt, label string, implicit bool) bool {
+	switch s := s.(type) {
+	default:
+		unreachable()
+
+	case *syntax.DeclStmt, *syntax.EmptyStmt, *syntax.ExprStmt,
+		*syntax.SendStmt, *syntax.AssignStmt, *syntax.CallStmt,
+		*syntax.ReturnStmt:
+		// no chance
+
+	case *syntax.LabeledStmt:
+		return hasBreak(s.Stmt, label, implicit)
+
+	case *syntax.BranchStmt:
+		if s.Tok == syntax.Break {
+			if s.Label == nil {
+				return implicit
+			}
+			if s.Label.Value == label {
+				return true
+			}
+		}
+
+	case *syntax.BlockStmt:
+		return hasBreakList(s.List, label, implicit)
+
+	case *syntax.IfStmt:
+		if hasBreak(s.Then, label, implicit) ||
+			s.Else != nil && hasBreak(s.Else, label, implicit) {
+			return true
+		}
+
+	case *syntax.SwitchStmt:
+		if label != "" && hasBreakCaseList(s.Body, label, false) {
+			return true
+		}
+
+	case *syntax.SelectStmt:
+		if label != "" && hasBreakCommList(s.Body, label, false) {
+			return true
+		}
+
+	case *syntax.ForStmt:
+		if label != "" && hasBreak(s.Body, label, false) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func hasBreakList(list []syntax.Stmt, label string, implicit bool) bool {
+	for _, s := range list {
+		if hasBreak(s, label, implicit) {
+			return true
+		}
+	}
+	return false
+}
+
+func hasBreakCaseList(list []*syntax.CaseClause, label string, implicit bool) bool {
+	for _, s := range list {
+		if hasBreakList(s.Body, label, implicit) {
+			return true
+		}
+	}
+	return false
+}
+
+func hasBreakCommList(list []*syntax.CommClause, label string, implicit bool) bool {
+	for _, s := range list {
+		if hasBreakList(s.Body, label, implicit) {
+			return true
+		}
+	}
+	return false
+}

src/cmd/compile/internal/types2/sanitize.go

@@ -0,0 +1,149 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+func sanitizeInfo(info *Info) {
+	var s sanitizer = make(map[Type]Type)
+
+	// Note: Some map entries are not references.
+	// If modified, they must be assigned back.
+
+	for e, tv := range info.Types {
+		tv.Type = s.typ(tv.Type)
+		info.Types[e] = tv
+	}
+
+	for e, inf := range info.Inferred {
+		for i, targ := range inf.Targs {
+			inf.Targs[i] = s.typ(targ)
+		}
+		inf.Sig = s.typ(inf.Sig).(*Signature)
+		info.Inferred[e] = inf
+	}
+
+	for _, obj := range info.Defs {
+		if obj != nil {
+			obj.setType(s.typ(obj.Type()))
+		}
+	}
+
+	for _, obj := range info.Uses {
+		if obj != nil {
+			obj.setType(s.typ(obj.Type()))
+		}
+	}
+
+	// TODO(gri) sanitize as needed
+	// - info.Implicits
+	// - info.Selections
+	// - info.Scopes
+	// - info.InitOrder
+}
+
+type sanitizer map[Type]Type
+
+func (s sanitizer) typ(typ Type) Type {
+	if t, found := s[typ]; found {
+		return t
+	}
+	s[typ] = typ
+
+	switch t := typ.(type) {
+	case nil, *Basic, *bottom, *top:
+		// nothing to do
+
+	case *Array:
+		t.elem = s.typ(t.elem)
+
+	case *Slice:
+		t.elem = s.typ(t.elem)
+
+	case *Struct:
+		s.varList(t.fields)
+
+	case *Pointer:
+		t.base = s.typ(t.base)
+
+	case *Tuple:
+		s.tuple(t)
+
+	case *Signature:
+		s.var_(t.recv)
+		s.tuple(t.params)
+		s.tuple(t.results)
+
+	case *Sum:
+		s.typeList(t.types)
+
+	case *Interface:
+		s.funcList(t.methods)
+		s.typ(t.types)
+		s.typeList(t.embeddeds)
+		s.funcList(t.allMethods)
+		s.typ(t.allTypes)
+
+	case *Map:
+		t.key = s.typ(t.key)
+		t.elem = s.typ(t.elem)
+
+	case *Chan:
+		t.elem = s.typ(t.elem)
+
+	case *Named:
+		t.orig = s.typ(t.orig)
+		t.underlying = s.typ(t.underlying)
+		s.typeList(t.targs)
+		s.funcList(t.methods)
+
+	case *TypeParam:
+		t.bound = s.typ(t.bound)
+
+	case *instance:
+		typ = t.expand()
+		s[t] = typ
+
+	default:
+		unimplemented()
+	}
+
+	return typ
+}
+
+func (s sanitizer) var_(v *Var) {
+	if v != nil {
+		v.typ = s.typ(v.typ)
+	}
+}
+
+func (s sanitizer) varList(list []*Var) {
+	for _, v := range list {
+		s.var_(v)
+	}
+}
+
+func (s sanitizer) tuple(t *Tuple) {
+	if t != nil {
+		s.varList(t.vars)
+	}
+}
+
+func (s sanitizer) func_(f *Func) {
+	if f != nil {
+		f.typ = s.typ(f.typ)
+	}
+}
+
+func (s sanitizer) funcList(list []*Func) {
+	for _, f := range list {
+		s.func_(f)
+	}
+}
+
+func (s sanitizer) typeList(list []Type) {
+	for i, t := range list {
+		list[i] = s.typ(t)
+	}
+}

src/cmd/compile/internal/types2/scope.go

@@ -0,0 +1,217 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements Scopes.
+
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"io"
+	"sort"
+	"strings"
+)
+
+// A Scope maintains a set of objects and links to its containing
+// (parent) and contained (children) scopes. Objects may be inserted
+// and looked up by name. The zero value for Scope is a ready-to-use
+// empty scope.
+type Scope struct {
+	parent   *Scope
+	children []*Scope
+	elems    map[string]Object // lazily allocated
+	pos, end syntax.Pos        // scope extent; may be invalid
+	comment  string            // for debugging only
+	isFunc   bool              // set if this is a function scope (internal use only)
+}
+
+// NewScope returns a new, empty scope contained in the given parent
+// scope, if any. The comment is for debugging only.
+func NewScope(parent *Scope, pos, end syntax.Pos, comment string) *Scope {
+	s := &Scope{parent, nil, nil, pos, end, comment, false}
+	// don't add children to Universe scope!
+	if parent != nil && parent != Universe {
+		parent.children = append(parent.children, s)
+	}
+	return s
+}
+
+// Parent returns the scope's containing (parent) scope.
+func (s *Scope) Parent() *Scope { return s.parent }
+
+// Len returns the number of scope elements.
+func (s *Scope) Len() int { return len(s.elems) }
+
+// Names returns the scope's element names in sorted order.
+func (s *Scope) Names() []string {
+	names := make([]string, len(s.elems))
+	i := 0
+	for name := range s.elems {
+		names[i] = name
+		i++
+	}
+	sort.Strings(names)
+	return names
+}
+
+// NumChildren returns the number of scopes nested in s.
+func (s *Scope) NumChildren() int { return len(s.children) }
+
+// Child returns the i'th child scope for 0 <= i < NumChildren().
+func (s *Scope) Child(i int) *Scope { return s.children[i] }
+
+// Lookup returns the object in scope s with the given name if such an
+// object exists; otherwise the result is nil.
+func (s *Scope) Lookup(name string) Object {
+	return s.elems[name]
+}
+
+// LookupParent follows the parent chain of scopes starting with s until
+// it finds a scope where Lookup(name) returns a non-nil object, and then
+// returns that scope and object. If a valid position pos is provided,
+// only objects that were declared at or before pos are considered.
+// If no such scope and object exists, the result is (nil, nil).
+//
+// Note that obj.Parent() may be different from the returned scope if the
+// object was inserted into the scope and already had a parent at that
+// time (see Insert). This can only happen for dot-imported objects
+// whose scope is the scope of the package that exported them.
+func (s *Scope) LookupParent(name string, pos syntax.Pos) (*Scope, Object) {
+	for ; s != nil; s = s.parent {
+		if obj := s.elems[name]; obj != nil && (!pos.IsKnown() || cmpPos(obj.scopePos(), pos) <= 0) {
+			return s, obj
+		}
+	}
+	return nil, nil
+}
+
+// Insert attempts to insert an object obj into scope s.
+// If s already contains an alternative object alt with
+// the same name, Insert leaves s unchanged and returns alt.
+// Otherwise it inserts obj, sets the object's parent scope
+// if not already set, and returns nil.
+func (s *Scope) Insert(obj Object) Object {
+	name := obj.Name()
+	if alt := s.elems[name]; alt != nil {
+		return alt
+	}
+	if s.elems == nil {
+		s.elems = make(map[string]Object)
+	}
+	s.elems[name] = obj
+	if obj.Parent() == nil {
+		obj.setParent(s)
+	}
+	return nil
+}
+
+// Squash merges s with its parent scope p by adding all
+// objects of s to p, adding all children of s to the
+// children of p, and removing s from p's children.
+// The function f is called for each object obj in s which
+// has an object alt in p. s should be discarded after
+// having been squashed.
+func (s *Scope) Squash(err func(obj, alt Object)) {
+	p := s.parent
+	assert(p != nil)
+	for _, obj := range s.elems {
+		obj.setParent(nil)
+		if alt := p.Insert(obj); alt != nil {
+			err(obj, alt)
+		}
+	}
+
+	j := -1 // index of s in p.children
+	for i, ch := range p.children {
+		if ch == s {
+			j = i
+			break
+		}
+	}
+	assert(j >= 0)
+	k := len(p.children) - 1
+	p.children[j] = p.children[k]
+	p.children = p.children[:k]
+
+	p.children = append(p.children, s.children...)
+
+	s.children = nil
+	s.elems = nil
+}
+
+// Pos and End describe the scope's source code extent [pos, end).
+// The results are guaranteed to be valid only if the type-checked
+// AST has complete position information. The extent is undefined
+// for Universe and package scopes.
+func (s *Scope) Pos() syntax.Pos { return s.pos }
+func (s *Scope) End() syntax.Pos { return s.end }
+
+// Contains reports whether pos is within the scope's extent.
+// The result is guaranteed to be valid only if the type-checked
+// AST has complete position information.
+func (s *Scope) Contains(pos syntax.Pos) bool {
+	return cmpPos(s.pos, pos) <= 0 && cmpPos(pos, s.end) < 0
+}
+
+// Innermost returns the innermost (child) scope containing
+// pos. If pos is not within any scope, the result is nil.
+// The result is also nil for the Universe scope.
+// The result is guaranteed to be valid only if the type-checked
+// AST has complete position information.
+func (s *Scope) Innermost(pos syntax.Pos) *Scope {
+	// Package scopes do not have extents since they may be
+	// discontiguous, so iterate over the package's files.
+	if s.parent == Universe {
+		for _, s := range s.children {
+			if inner := s.Innermost(pos); inner != nil {
+				return inner
+			}
+		}
+	}
+
+	if s.Contains(pos) {
+		for _, s := range s.children {
+			if s.Contains(pos) {
+				return s.Innermost(pos)
+			}
+		}
+		return s
+	}
+	return nil
+}
+
+// WriteTo writes a string representation of the scope to w,
+// with the scope elements sorted by name.
+// The level of indentation is controlled by n >= 0, with
+// n == 0 for no indentation.
+// If recurse is set, it also writes nested (children) scopes.
+func (s *Scope) WriteTo(w io.Writer, n int, recurse bool) {
+	const ind = ".  "
+	indn := strings.Repeat(ind, n)
+
+	fmt.Fprintf(w, "%s%s scope %p {\n", indn, s.comment, s)
+
+	indn1 := indn + ind
+	for _, name := range s.Names() {
+		fmt.Fprintf(w, "%s%s\n", indn1, s.elems[name])
+	}
+
+	if recurse {
+		for _, s := range s.children {
+			s.WriteTo(w, n+1, recurse)
+		}
+	}
+
+	fmt.Fprintf(w, "%s}\n", indn)
+}
+
+// String returns a string representation of the scope, for debugging.
+func (s *Scope) String() string {
+	var buf bytes.Buffer
+	s.WriteTo(&buf, 0, false)
+	return buf.String()
+}

src/cmd/compile/internal/types2/selection.go

@@ -0,0 +1,144 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements Selections.
+
+package types2
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// SelectionKind describes the kind of a selector expression x.f
+// (excluding qualified identifiers).
+type SelectionKind int
+
+const (
+	FieldVal   SelectionKind = iota // x.f is a struct field selector
+	MethodVal                       // x.f is a method selector
+	MethodExpr                      // x.f is a method expression
+)
+
+// A Selection describes a selector expression x.f.
+// For the declarations:
+//
+//	type T struct{ x int; E }
+//	type E struct{}
+//	func (e E) m() {}
+//	var p *T
+//
+// the following relations exist:
+//
+//	Selector    Kind          Recv    Obj    Type       Index     Indirect
+//
+//	p.x         FieldVal      T       x      int        {0}       true
+//	p.m         MethodVal     *T      m      func()     {1, 0}    true
+//	T.m         MethodExpr    T       m      func(T)    {1, 0}    false
+//
+type Selection struct {
+	kind     SelectionKind
+	recv     Type   // type of x
+	obj      Object // object denoted by x.f
+	index    []int  // path from x to x.f
+	indirect bool   // set if there was any pointer indirection on the path
+}
+
+// Kind returns the selection kind.
+func (s *Selection) Kind() SelectionKind { return s.kind }
+
+// Recv returns the type of x in x.f.
+func (s *Selection) Recv() Type { return s.recv }
+
+// Obj returns the object denoted by x.f; a *Var for
+// a field selection, and a *Func in all other cases.
+func (s *Selection) Obj() Object { return s.obj }
+
+// Type returns the type of x.f, which may be different from the type of f.
+// See Selection for more information.
+func (s *Selection) Type() Type {
+	switch s.kind {
+	case MethodVal:
+		// The type of x.f is a method with its receiver type set
+		// to the type of x.
+		sig := *s.obj.(*Func).typ.(*Signature)
+		recv := *sig.recv
+		recv.typ = s.recv
+		sig.recv = &recv
+		return &sig
+
+	case MethodExpr:
+		// The type of x.f is a function (without receiver)
+		// and an additional first argument with the same type as x.
+		// TODO(gri) Similar code is already in call.go - factor!
+		// TODO(gri) Compute this eagerly to avoid allocations.
+		sig := *s.obj.(*Func).typ.(*Signature)
+		arg0 := *sig.recv
+		sig.recv = nil
+		arg0.typ = s.recv
+		var params []*Var
+		if sig.params != nil {
+			params = sig.params.vars
+		}
+		sig.params = NewTuple(append([]*Var{&arg0}, params...)...)
+		return &sig
+	}
+
+	// In all other cases, the type of x.f is the type of x.
+	return s.obj.Type()
+}
+
+// Index describes the path from x to f in x.f.
+// The last index entry is the field or method index of the type declaring f;
+// either:
+//
+//	1) the list of declared methods of a named type; or
+//	2) the list of methods of an interface type; or
+//	3) the list of fields of a struct type.
+//
+// The earlier index entries are the indices of the embedded fields implicitly
+// traversed to get from (the type of) x to f, starting at embedding depth 0.
+func (s *Selection) Index() []int { return s.index }
+
+// Indirect reports whether any pointer indirection was required to get from
+// x to f in x.f.
+func (s *Selection) Indirect() bool { return s.indirect }
+
+func (s *Selection) String() string { return SelectionString(s, nil) }
+
+// SelectionString returns the string form of s.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+//
+// Examples:
+//	"field (T) f int"
+//	"method (T) f(X) Y"
+//	"method expr (T) f(X) Y"
+//
+func SelectionString(s *Selection, qf Qualifier) string {
+	var k string
+	switch s.kind {
+	case FieldVal:
+		k = "field "
+	case MethodVal:
+		k = "method "
+	case MethodExpr:
+		k = "method expr "
+	default:
+		unreachable()
+	}
+	var buf bytes.Buffer
+	buf.WriteString(k)
+	buf.WriteByte('(')
+	WriteType(&buf, s.Recv(), qf)
+	fmt.Fprintf(&buf, ") %s", s.obj.Name())
+	if T := s.Type(); s.kind == FieldVal {
+		buf.WriteByte(' ')
+		WriteType(&buf, T, qf)
+	} else {
+		WriteSignature(&buf, T.(*Signature), qf)
+	}
+	return buf.String()
+}

src/cmd/compile/internal/types2/self_test.go

@@ -0,0 +1,97 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2_test
+
+import (
+	"cmd/compile/internal/syntax"
+	"flag"
+	"fmt"
+	"path/filepath"
+	"testing"
+	"time"
+
+	. "cmd/compile/internal/types2"
+)
+
+var benchmark = flag.Bool("b", false, "run benchmarks")
+
+func TestSelf(t *testing.T) {
+	files, err := pkgFiles(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	conf := Config{Importer: defaultImporter()}
+	_, err = conf.Check("go/types", files, nil)
+	if err != nil {
+		// Importing go/constant doesn't work in the
+		// build dashboard environment. Don't report an error
+		// for now so that the build remains green.
+		// TODO(gri) fix this
+		t.Log(err) // replace w/ t.Fatal eventually
+		return
+	}
+}
+
+func TestBenchmark(t *testing.T) {
+	if !*benchmark {
+		return
+	}
+
+	// We're not using testing's benchmarking mechanism directly
+	// because we want custom output.
+
+	for _, p := range []string{"types", "constant", filepath.Join("internal", "gcimporter")} {
+		path := filepath.Join("..", p)
+		runbench(t, path, false)
+		runbench(t, path, true)
+		fmt.Println()
+	}
+}
+
+func runbench(t *testing.T, path string, ignoreFuncBodies bool) {
+	files, err := pkgFiles(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	b := testing.Benchmark(func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			conf := Config{IgnoreFuncBodies: ignoreFuncBodies}
+			conf.Check(path, files, nil)
+		}
+	})
+
+	// determine line count
+	var lines uint
+	for _, f := range files {
+		lines += f.EOF.Line()
+	}
+
+	d := time.Duration(b.NsPerOp())
+	fmt.Printf(
+		"%s: %s for %d lines (%d lines/s), ignoreFuncBodies = %v\n",
+		filepath.Base(path), d, lines, int64(float64(lines)/d.Seconds()), ignoreFuncBodies,
+	)
+}
+
+func pkgFiles(path string) ([]*syntax.File, error) {
+	filenames, err := pkgFilenames(path) // from stdlib_test.go
+	if err != nil {
+		return nil, err
+	}
+
+	var files []*syntax.File
+	for _, filename := range filenames {
+		file, err := syntax.ParseFile(filename, nil, nil, 0)
+		if err != nil {
+			return nil, err
+		}
+		files = append(files, file)
+	}
+
+	return files, nil
+}

src/cmd/compile/internal/types2/sizes.go

@@ -0,0 +1,266 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements Sizes.
+
+package types2
+
+// Sizes defines the sizing functions for package unsafe.
+type Sizes interface {
+	// Alignof returns the alignment of a variable of type T.
+	// Alignof must implement the alignment guarantees required by the spec.
+	Alignof(T Type) int64
+
+	// Offsetsof returns the offsets of the given struct fields, in bytes.
+	// Offsetsof must implement the offset guarantees required by the spec.
+	Offsetsof(fields []*Var) []int64
+
+	// Sizeof returns the size of a variable of type T.
+	// Sizeof must implement the size guarantees required by the spec.
+	Sizeof(T Type) int64
+}
+
+// StdSizes is a convenience type for creating commonly used Sizes.
+// It makes the following simplifying assumptions:
+//
+//	- The size of explicitly sized basic types (int16, etc.) is the
+//	  specified size.
+//	- The size of strings and interfaces is 2*WordSize.
+//	- The size of slices is 3*WordSize.
+//	- The size of an array of n elements corresponds to the size of
+//	  a struct of n consecutive fields of the array's element type.
+//      - The size of a struct is the offset of the last field plus that
+//	  field's size. As with all element types, if the struct is used
+//	  in an array its size must first be aligned to a multiple of the
+//	  struct's alignment.
+//	- All other types have size WordSize.
+//	- Arrays and structs are aligned per spec definition; all other
+//	  types are naturally aligned with a maximum alignment MaxAlign.
+//
+// *StdSizes implements Sizes.
+//
+type StdSizes struct {
+	WordSize int64 // word size in bytes - must be >= 4 (32bits)
+	MaxAlign int64 // maximum alignment in bytes - must be >= 1
+}
+
+func (s *StdSizes) Alignof(T Type) int64 {
+	// For arrays and structs, alignment is defined in terms
+	// of alignment of the elements and fields, respectively.
+	switch t := optype(T.Under()).(type) {
+	case *Array:
+		// spec: "For a variable x of array type: unsafe.Alignof(x)
+		// is the same as unsafe.Alignof(x[0]), but at least 1."
+		return s.Alignof(t.elem)
+	case *Struct:
+		// spec: "For a variable x of struct type: unsafe.Alignof(x)
+		// is the largest of the values unsafe.Alignof(x.f) for each
+		// field f of x, but at least 1."
+		max := int64(1)
+		for _, f := range t.fields {
+			if a := s.Alignof(f.typ); a > max {
+				max = a
+			}
+		}
+		return max
+	case *Slice, *Interface:
+		// Multiword data structures are effectively structs
+		// in which each element has size WordSize.
+		return s.WordSize
+	case *Basic:
+		// Strings are like slices and interfaces.
+		if t.Info()&IsString != 0 {
+			return s.WordSize
+		}
+	}
+	a := s.Sizeof(T) // may be 0
+	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."
+	if a < 1 {
+		return 1
+	}
+	// complex{64,128} are aligned like [2]float{32,64}.
+	if isComplex(T) {
+		a /= 2
+	}
+	if a > s.MaxAlign {
+		return s.MaxAlign
+	}
+	return a
+}
+
+func (s *StdSizes) Offsetsof(fields []*Var) []int64 {
+	offsets := make([]int64, len(fields))
+	var o int64
+	for i, f := range fields {
+		a := s.Alignof(f.typ)
+		o = align(o, a)
+		offsets[i] = o
+		o += s.Sizeof(f.typ)
+	}
+	return offsets
+}
+
+var basicSizes = [...]byte{
+	Bool:       1,
+	Int8:       1,
+	Int16:      2,
+	Int32:      4,
+	Int64:      8,
+	Uint8:      1,
+	Uint16:     2,
+	Uint32:     4,
+	Uint64:     8,
+	Float32:    4,
+	Float64:    8,
+	Complex64:  8,
+	Complex128: 16,
+}
+
+func (s *StdSizes) Sizeof(T Type) int64 {
+	switch t := optype(T.Under()).(type) {
+	case *Basic:
+		assert(isTyped(T))
+		k := t.kind
+		if int(k) < len(basicSizes) {
+			if s := basicSizes[k]; s > 0 {
+				return int64(s)
+			}
+		}
+		if k == String {
+			return s.WordSize * 2
+		}
+	case *Array:
+		n := t.len
+		if n <= 0 {
+			return 0
+		}
+		// n > 0
+		a := s.Alignof(t.elem)
+		z := s.Sizeof(t.elem)
+		return align(z, a)*(n-1) + z
+	case *Slice:
+		return s.WordSize * 3
+	case *Struct:
+		n := t.NumFields()
+		if n == 0 {
+			return 0
+		}
+		offsets := s.Offsetsof(t.fields)
+		return offsets[n-1] + s.Sizeof(t.fields[n-1].typ)
+	case *Sum:
+		panic("Sizeof unimplemented for type sum")
+	case *Interface:
+		return s.WordSize * 2
+	}
+	return s.WordSize // catch-all
+}
+
+// common architecture word sizes and alignments
+var gcArchSizes = map[string]*StdSizes{
+	"386":      {4, 4},
+	"arm":      {4, 4},
+	"arm64":    {8, 8},
+	"amd64":    {8, 8},
+	"amd64p32": {4, 8},
+	"mips":     {4, 4},
+	"mipsle":   {4, 4},
+	"mips64":   {8, 8},
+	"mips64le": {8, 8},
+	"ppc64":    {8, 8},
+	"ppc64le":  {8, 8},
+	"riscv64":  {8, 8},
+	"s390x":    {8, 8},
+	"sparc64":  {8, 8},
+	"wasm":     {8, 8},
+	// When adding more architectures here,
+	// update the doc string of SizesFor below.
+}
+
+// SizesFor returns the Sizes used by a compiler for an architecture.
+// The result is nil if a compiler/architecture pair is not known.
+//
+// Supported architectures for compiler "gc":
+// "386", "arm", "arm64", "amd64", "amd64p32", "mips", "mipsle",
+// "mips64", "mips64le", "ppc64", "ppc64le", "riscv64", "s390x", "sparc64", "wasm".
+func SizesFor(compiler, arch string) Sizes {
+	var m map[string]*StdSizes
+	switch compiler {
+	case "gc":
+		m = gcArchSizes
+	case "gccgo":
+		m = gccgoArchSizes
+	default:
+		return nil
+	}
+	s, ok := m[arch]
+	if !ok {
+		return nil
+	}
+	return s
+}
+
+// stdSizes is used if Config.Sizes == nil.
+var stdSizes = SizesFor("gc", "amd64")
+
+func (conf *Config) alignof(T Type) int64 {
+	if s := conf.Sizes; s != nil {
+		if a := s.Alignof(T); a >= 1 {
+			return a
+		}
+		panic("Config.Sizes.Alignof returned an alignment < 1")
+	}
+	return stdSizes.Alignof(T)
+}
+
+func (conf *Config) offsetsof(T *Struct) []int64 {
+	var offsets []int64
+	if T.NumFields() > 0 {
+		// compute offsets on demand
+		if s := conf.Sizes; s != nil {
+			offsets = s.Offsetsof(T.fields)
+			// sanity checks
+			if len(offsets) != T.NumFields() {
+				panic("Config.Sizes.Offsetsof returned the wrong number of offsets")
+			}
+			for _, o := range offsets {
+				if o < 0 {
+					panic("Config.Sizes.Offsetsof returned an offset < 0")
+				}
+			}
+		} else {
+			offsets = stdSizes.Offsetsof(T.fields)
+		}
+	}
+	return offsets
+}
+
+// offsetof returns the offset of the field specified via
+// the index sequence relative to typ. All embedded fields
+// must be structs (rather than pointer to structs).
+func (conf *Config) offsetof(typ Type, index []int) int64 {
+	var o int64
+	for _, i := range index {
+		s := typ.Struct()
+		o += conf.offsetsof(s)[i]
+		typ = s.fields[i].typ
+	}
+	return o
+}
+
+func (conf *Config) sizeof(T Type) int64 {
+	if s := conf.Sizes; s != nil {
+		if z := s.Sizeof(T); z >= 0 {
+			return z
+		}
+		panic("Config.Sizes.Sizeof returned a size < 0")
+	}
+	return stdSizes.Sizeof(T)
+}
+
+// align returns the smallest y >= x such that y % a == 0.
+func align(x, a int64) int64 {
+	y := x + a - 1
+	return y - y%a
+}

src/cmd/compile/internal/types2/sizes_test.go

@@ -0,0 +1,108 @@
+// UNREVIEWED
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains tests for sizes.
+
+package types2_test
+
+import (
+	"cmd/compile/internal/syntax"
+	"cmd/compile/internal/types2"
+	"testing"
+)
+
+// findStructType typechecks src and returns the first struct type encountered.
+func findStructType(t *testing.T, src string) *types2.Struct {
+	f, err := parseSrc("x.go", src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	info := types2.Info{Types: make(map[syntax.Expr]types2.TypeAndValue)}
+	var conf types2.Config
+	_, err = conf.Check("x", []*syntax.File{f}, &info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, tv := range info.Types {
+		if ts, ok := tv.Type.(*types2.Struct); ok {
+			return ts
+		}
+	}
+	t.Fatalf("failed to find a struct type in src:\n%s\n", src)
+	return nil
+}
+
+// Issue 16316
+func TestMultipleSizeUse(t *testing.T) {
+	const src = `
+package main
+
+type S struct {
+    i int
+    b bool
+    s string
+    n int
+}
+`
+	ts := findStructType(t, src)
+	sizes := types2.StdSizes{WordSize: 4, MaxAlign: 4}
+	if got := sizes.Sizeof(ts); got != 20 {
+		t.Errorf("Sizeof(%v) with WordSize 4 = %d want 20", ts, got)
+	}
+	sizes = types2.StdSizes{WordSize: 8, MaxAlign: 8}
+	if got := sizes.Sizeof(ts); got != 40 {
+		t.Errorf("Sizeof(%v) with WordSize 8 = %d want 40", ts, got)
+	}
+}
+
+// Issue 16464
+func TestAlignofNaclSlice(t *testing.T) {
+	const src = `
+package main
+
+var s struct {
+	x *int
+	y []byte
+}
+`
+	ts := findStructType(t, src)
+	sizes := &types2.StdSizes{WordSize: 4, MaxAlign: 8}
+	var fields []*types2.Var
+	// Make a copy manually :(
+	for i := 0; i < ts.NumFields(); i++ {
+		fields = append(fields, ts.Field(i))
+	}
+	offsets := sizes.Offsetsof(fields)
+	if offsets[0] != 0 || offsets[1] != 4 {
+		t.Errorf("OffsetsOf(%v) = %v want %v", ts, offsets, []int{0, 4})
+	}
+}
+
+func TestIssue16902(t *testing.T) {
+	const src = `
+package a
+
+import "unsafe"
+
+const _ = unsafe.Offsetof(struct{ x int64 }{}.x)
+`
+	f, err := parseSrc("x.go", src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	info := types2.Info{Types: make(map[syntax.Expr]types2.TypeAndValue)}
+	conf := types2.Config{
+		Importer: defaultImporter(),
+		Sizes:    &types2.StdSizes{WordSize: 8, MaxAlign: 8},
+	}
+	_, err = conf.Check("x", []*syntax.File{f}, &info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, tv := range info.Types {
+		_ = conf.Sizes.Sizeof(tv.Type)
+		_ = conf.Sizes.Alignof(tv.Type)
+	}
+}

src/cmd/compile/internal/types2/stdlib_test.go

@@ -0,0 +1,320 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file tests types.Check by using it to
+// typecheck the standard library and tests.
+
+package types2_test
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+	"go/build"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	. "cmd/compile/internal/types2"
+)
+
+var stdLibImporter = defaultImporter()
+
+func TestStdlib(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	pkgCount := 0
+	duration := walkPkgDirs(filepath.Join(runtime.GOROOT(), "src"), func(dir string, filenames []string) {
+		typecheck(t, dir, filenames)
+		pkgCount++
+	}, t.Error)
+
+	if testing.Verbose() {
+		fmt.Println(pkgCount, "packages typechecked in", duration)
+	}
+}
+
+// firstComment returns the contents of the first non-empty comment in
+// the given file, "skip", or the empty string. No matter the present
+// comments, if any of them contains a build tag, the result is always
+// "skip". Only comments within the first 4K of the file are considered.
+// TODO(gri) should only read until we see "package" token.
+func firstComment(filename string) (first string) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return ""
+	}
+	defer f.Close()
+
+	// read at most 4KB
+	var buf [4 << 10]byte
+	n, _ := f.Read(buf[:])
+	src := bytes.NewBuffer(buf[:n])
+
+	// TODO(gri) we need a better way to terminate CommentsDo
+	defer func() {
+		if p := recover(); p != nil {
+			if s, ok := p.(string); ok {
+				first = s
+			}
+		}
+	}()
+
+	syntax.CommentsDo(src, func(_, _ uint, text string) {
+		if text[0] != '/' {
+			return // not a comment
+		}
+
+		// extract comment text
+		if text[1] == '*' {
+			text = text[:len(text)-2]
+		}
+		text = strings.TrimSpace(text[2:])
+
+		if strings.HasPrefix(text, "+build ") {
+			panic("skip")
+		}
+		if first == "" {
+			first = text // text may be "" but that's ok
+		}
+		// continue as we may still see build tags
+	})
+
+	return
+}
+
+func testTestDir(t *testing.T, path string, ignore ...string) {
+	files, err := ioutil.ReadDir(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	excluded := make(map[string]bool)
+	for _, filename := range ignore {
+		excluded[filename] = true
+	}
+
+	for _, f := range files {
+		// filter directory contents
+		if f.IsDir() || !strings.HasSuffix(f.Name(), ".go") || excluded[f.Name()] {
+			continue
+		}
+
+		// get per-file instructions
+		expectErrors := false
+		filename := filepath.Join(path, f.Name())
+		if comment := firstComment(filename); comment != "" {
+			fields := strings.Fields(comment)
+			switch fields[0] {
+			case "skip", "compiledir":
+				continue // ignore this file
+			case "errorcheck":
+				expectErrors = true
+				for _, arg := range fields[1:] {
+					if arg == "-0" || arg == "-+" || arg == "-std" {
+						// Marked explicitly as not expected errors (-0),
+						// or marked as compiling runtime/stdlib, which is only done
+						// to trigger runtime/stdlib-only error output.
+						// In both cases, the code should typecheck.
+						expectErrors = false
+						break
+					}
+				}
+			}
+		}
+
+		// parse and type-check file
+		if testing.Verbose() {
+			fmt.Println("\t", filename)
+		}
+		file, err := syntax.ParseFile(filename, nil, nil, 0)
+		if err == nil {
+			conf := Config{Importer: stdLibImporter}
+			_, err = conf.Check(filename, []*syntax.File{file}, nil)
+		}
+
+		if expectErrors {
+			if err == nil {
+				t.Errorf("expected errors but found none in %s", filename)
+			}
+		} else {
+			if err != nil {
+				t.Error(err)
+			}
+		}
+	}
+}
+
+func TestStdTest(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if testing.Short() && testenv.Builder() == "" {
+		t.Skip("skipping in short mode")
+	}
+
+	testTestDir(t, filepath.Join(runtime.GOROOT(), "test"),
+		"cmplxdivide.go", // also needs file cmplxdivide1.go - ignore
+		"directive.go",   // tests compiler rejection of bad directive placement - ignore
+	)
+}
+
+func TestStdFixed(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if testing.Short() && testenv.Builder() == "" {
+		t.Skip("skipping in short mode")
+	}
+
+	testTestDir(t, filepath.Join(runtime.GOROOT(), "test", "fixedbugs"),
+		"bug248.go", "bug302.go", "bug369.go", // complex test instructions - ignore
+		"issue6889.go",   // gc-specific test
+		"issue7746.go",   // large constants - consumes too much memory
+		"issue11362.go",  // canonical import path check
+		"issue16369.go",  // go/types handles this correctly - not an issue
+		"issue18459.go",  // go/types doesn't check validity of //go:xxx directives
+		"issue18882.go",  // go/types doesn't check validity of //go:xxx directives
+		"issue20232.go",  // go/types handles larger constants than gc
+		"issue20529.go",  // go/types does not have constraints on stack size
+		"issue22200.go",  // go/types does not have constraints on stack size
+		"issue22200b.go", // go/types does not have constraints on stack size
+		"issue25507.go",  // go/types does not have constraints on stack size
+		"issue20780.go",  // go/types does not have constraints on stack size
+		"issue31747.go",  // go/types does not have constraints on language level (-lang=go1.12) (see #31793)
+		"issue34329.go",  // go/types does not have constraints on language level (-lang=go1.13) (see #31793)
+		"bug251.go",      // issue #34333 which was exposed with fix for #34151
+	)
+}
+
+func TestStdKen(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	testTestDir(t, filepath.Join(runtime.GOROOT(), "test", "ken"))
+}
+
+// Package paths of excluded packages.
+var excluded = map[string]bool{
+	"builtin": true,
+}
+
+// typecheck typechecks the given package files.
+func typecheck(t *testing.T, path string, filenames []string) {
+	// parse package files
+	var files []*syntax.File
+	for _, filename := range filenames {
+		errh := func(err error) { t.Error(err) }
+		file, err := syntax.ParseFile(filename, errh, nil, 0)
+		if err != nil {
+			return
+		}
+
+		if testing.Verbose() {
+			if len(files) == 0 {
+				fmt.Println("package", file.PkgName.Value)
+			}
+			fmt.Println("\t", filename)
+		}
+
+		files = append(files, file)
+	}
+
+	// typecheck package files
+	conf := Config{
+		Error:    func(err error) { t.Error(err) },
+		Importer: stdLibImporter,
+	}
+	info := Info{Uses: make(map[*syntax.Name]Object)}
+	conf.Check(path, files, &info)
+
+	// Perform checks of API invariants.
+
+	// All Objects have a package, except predeclared ones.
+	errorError := Universe.Lookup("error").Type().Interface().ExplicitMethod(0) // (error).Error
+	for id, obj := range info.Uses {
+		predeclared := obj == Universe.Lookup(obj.Name()) || obj == errorError
+		if predeclared == (obj.Pkg() != nil) {
+			posn := id.Pos()
+			if predeclared {
+				t.Errorf("%s: predeclared object with package: %s", posn, obj)
+			} else {
+				t.Errorf("%s: user-defined object without package: %s", posn, obj)
+			}
+		}
+	}
+}
+
+// pkgFilenames returns the list of package filenames for the given directory.
+func pkgFilenames(dir string) ([]string, error) {
+	ctxt := build.Default
+	ctxt.CgoEnabled = false
+	pkg, err := ctxt.ImportDir(dir, 0)
+	if err != nil {
+		if _, nogo := err.(*build.NoGoError); nogo {
+			return nil, nil // no *.go files, not an error
+		}
+		return nil, err
+	}
+	if excluded[pkg.ImportPath] {
+		return nil, nil
+	}
+	var filenames []string
+	for _, name := range pkg.GoFiles {
+		filenames = append(filenames, filepath.Join(pkg.Dir, name))
+	}
+	for _, name := range pkg.TestGoFiles {
+		filenames = append(filenames, filepath.Join(pkg.Dir, name))
+	}
+	return filenames, nil
+}
+
+func walkPkgDirs(dir string, pkgh func(dir string, filenames []string), errh func(args ...interface{})) time.Duration {
+	w := walker{time.Now(), 10 * time.Millisecond, pkgh, errh}
+	w.walk(dir)
+	return time.Since(w.start)
+}
+
+type walker struct {
+	start time.Time
+	dmax  time.Duration
+	pkgh  func(dir string, filenames []string)
+	errh  func(args ...interface{})
+}
+
+func (w *walker) walk(dir string) {
+	// limit run time for short tests
+	if testing.Short() && time.Since(w.start) >= w.dmax {
+		return
+	}
+
+	fis, err := ioutil.ReadDir(dir)
+	if err != nil {
+		w.errh(err)
+		return
+	}
+
+	// apply pkgh to the files in directory dir
+	// but ignore files directly under $GOROOT/src (might be temporary test files).
+	if dir != filepath.Join(runtime.GOROOT(), "src") {
+		files, err := pkgFilenames(dir)
+		if err != nil {
+			w.errh(err)
+			return
+		}
+		if files != nil {
+			w.pkgh(dir, files)
+		}
+	}
+
+	// traverse subdirectories, but don't walk into testdata
+	for _, fi := range fis {
+		if fi.IsDir() && fi.Name() != "testdata" {
+			w.walk(filepath.Join(dir, fi.Name()))
+		}
+	}
+}

src/cmd/compile/internal/types2/stmt.go

@@ -0,0 +1,919 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of statements.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"go/constant"
+	"sort"
+)
+
+func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body *syntax.BlockStmt, iota constant.Value) {
+	if check.conf.Trace {
+		check.trace(body.Pos(), "--- %s: %s", name, sig)
+		defer func() {
+			check.trace(endPos(body), "--- <end>")
+		}()
+	}
+
+	// set function scope extent
+	sig.scope.pos = body.Pos()
+	sig.scope.end = endPos(body)
+
+	// save/restore current context and setup function context
+	// (and use 0 indentation at function start)
+	defer func(ctxt context, indent int) {
+		check.context = ctxt
+		check.indent = indent
+	}(check.context, check.indent)
+	check.context = context{
+		decl:  decl,
+		scope: sig.scope,
+		iota:  iota,
+		sig:   sig,
+	}
+	check.indent = 0
+
+	check.stmtList(0, body.List)
+
+	if check.hasLabel {
+		check.labels(body)
+	}
+
+	if sig.results.Len() > 0 && !check.isTerminating(body, "") {
+		check.error(body.Rbrace, "missing return")
+	}
+
+	// TODO(gri) Should we make it an error to declare generic functions
+	//           where the type parameters are not used?
+	// 12/19/2018: Probably not - it can make sense to have an API with
+	//           all functions uniformly sharing the same type parameters.
+
+	// spec: "Implementation restriction: A compiler may make it illegal to
+	// declare a variable inside a function body if the variable is never used."
+	check.usage(sig.scope)
+}
+
+func (check *Checker) usage(scope *Scope) {
+	var unused []*Var
+	for _, elem := range scope.elems {
+		if v, _ := elem.(*Var); v != nil && !v.used {
+			unused = append(unused, v)
+		}
+	}
+	sort.Slice(unused, func(i, j int) bool {
+		return cmpPos(unused[i].pos, unused[j].pos) < 0
+	})
+	for _, v := range unused {
+		check.softErrorf(v.pos, "%s declared but not used", v.name)
+	}
+
+	for _, scope := range scope.children {
+		// Don't go inside function literal scopes a second time;
+		// they are handled explicitly by funcBody.
+		if !scope.isFunc {
+			check.usage(scope)
+		}
+	}
+}
+
+// stmtContext is a bitset describing which
+// control-flow statements are permissible,
+// and provides additional context information
+// for better error messages.
+type stmtContext uint
+
+const (
+	// permissible control-flow statements
+	breakOk stmtContext = 1 << iota
+	continueOk
+	fallthroughOk
+
+	// additional context information
+	finalSwitchCase
+)
+
+func (check *Checker) simpleStmt(s syntax.Stmt) {
+	if s != nil {
+		check.stmt(0, s)
+	}
+}
+
+func trimTrailingEmptyStmts(list []syntax.Stmt) []syntax.Stmt {
+	for i := len(list); i > 0; i-- {
+		if _, ok := list[i-1].(*syntax.EmptyStmt); !ok {
+			return list[:i]
+		}
+	}
+	return nil
+}
+
+func (check *Checker) stmtList(ctxt stmtContext, list []syntax.Stmt) {
+	ok := ctxt&fallthroughOk != 0
+	inner := ctxt &^ fallthroughOk
+	list = trimTrailingEmptyStmts(list) // trailing empty statements are "invisible" to fallthrough analysis
+	for i, s := range list {
+		inner := inner
+		if ok && i+1 == len(list) {
+			inner |= fallthroughOk
+		}
+		check.stmt(inner, s)
+	}
+}
+
+func (check *Checker) multipleSwitchDefaults(list []*syntax.CaseClause) {
+	var first *syntax.CaseClause
+	for _, c := range list {
+		if c.Cases == nil {
+			if first != nil {
+				check.errorf(c, "multiple defaults (first at %s)", first.Pos())
+				// TODO(gri) probably ok to bail out after first error (and simplify this code)
+			} else {
+				first = c
+			}
+		}
+	}
+}
+
+func (check *Checker) multipleSelectDefaults(list []*syntax.CommClause) {
+	var first *syntax.CommClause
+	for _, c := range list {
+		if c.Comm == nil {
+			if first != nil {
+				check.errorf(c, "multiple defaults (first at %s)", first.Pos())
+				// TODO(gri) probably ok to bail out after first error (and simplify this code)
+			} else {
+				first = c
+			}
+		}
+	}
+}
+
+func (check *Checker) openScope(node syntax.Node, comment string) {
+	scope := NewScope(check.scope, node.Pos(), endPos(node), comment)
+	check.recordScope(node, scope)
+	check.scope = scope
+}
+
+func (check *Checker) closeScope() {
+	check.scope = check.scope.Parent()
+}
+
+func (check *Checker) suspendedCall(keyword string, call *syntax.CallExpr) {
+	var x operand
+	var msg string
+	switch check.rawExpr(&x, call, nil) {
+	case conversion:
+		msg = "requires function call, not conversion"
+	case expression:
+		msg = "discards result of"
+	case statement:
+		return
+	default:
+		unreachable()
+	}
+	check.errorf(&x, "%s %s %s", keyword, msg, &x)
+}
+
+// goVal returns the Go value for val, or nil.
+func goVal(val constant.Value) interface{} {
+	// val should exist, but be conservative and check
+	if val == nil {
+		return nil
+	}
+	// Match implementation restriction of other compilers.
+	// gc only checks duplicates for integer, floating-point
+	// and string values, so only create Go values for these
+	// types.
+	switch val.Kind() {
+	case constant.Int:
+		if x, ok := constant.Int64Val(val); ok {
+			return x
+		}
+		if x, ok := constant.Uint64Val(val); ok {
+			return x
+		}
+	case constant.Float:
+		if x, ok := constant.Float64Val(val); ok {
+			return x
+		}
+	case constant.String:
+		return constant.StringVal(val)
+	}
+	return nil
+}
+
+// A valueMap maps a case value (of a basic Go type) to a list of positions
+// where the same case value appeared, together with the corresponding case
+// types.
+// Since two case values may have the same "underlying" value but different
+// types we need to also check the value's types (e.g., byte(1) vs myByte(1))
+// when the switch expression is of interface type.
+type (
+	valueMap  map[interface{}][]valueType // underlying Go value -> valueType
+	valueType struct {
+		pos syntax.Pos
+		typ Type
+	}
+)
+
+func (check *Checker) caseValues(x *operand, values []syntax.Expr, seen valueMap) {
+L:
+	for _, e := range values {
+		var v operand
+		check.expr(&v, e)
+		if x.mode == invalid || v.mode == invalid {
+			continue L
+		}
+		check.convertUntyped(&v, x.typ)
+		if v.mode == invalid {
+			continue L
+		}
+		// Order matters: By comparing v against x, error positions are at the case values.
+		res := v // keep original v unchanged
+		check.comparison(&res, x, syntax.Eql)
+		if res.mode == invalid {
+			continue L
+		}
+		if v.mode != constant_ {
+			continue L // we're done
+		}
+		// look for duplicate values
+		if val := goVal(v.val); val != nil {
+			// look for duplicate types for a given value
+			// (quadratic algorithm, but these lists tend to be very short)
+			for _, vt := range seen[val] {
+				if check.identical(v.typ, vt.typ) {
+					check.errorf(&v, "duplicate case %s in expression switch", &v)
+					check.error(vt.pos, "\tprevious case") // secondary error, \t indented
+					continue L
+				}
+			}
+			seen[val] = append(seen[val], valueType{v.Pos(), v.typ})
+		}
+	}
+}
+
+func (check *Checker) caseTypes(x *operand, xtyp *Interface, types []syntax.Expr, seen map[Type]syntax.Pos, strict bool) (T Type) {
+L:
+	for _, e := range types {
+		T = check.typOrNil(e)
+		if T == Typ[Invalid] {
+			continue L
+		}
+		if T != nil {
+			check.ordinaryType(e.Pos(), T)
+		}
+		// look for duplicate types
+		// (quadratic algorithm, but type switches tend to be reasonably small)
+		for t, pos := range seen {
+			if T == nil && t == nil || T != nil && t != nil && check.identical(T, t) {
+				// talk about "case" rather than "type" because of nil case
+				Ts := "nil"
+				if T != nil {
+					Ts = T.String()
+				}
+				check.errorf(e, "duplicate case %s in type switch", Ts)
+				check.error(pos, "\tprevious case") // secondary error, \t indented
+				continue L
+			}
+		}
+		seen[T] = e.Pos()
+		if T != nil {
+			check.typeAssertion(e.Pos(), x, xtyp, T, strict)
+		}
+	}
+	return
+}
+
+// stmt typechecks statement s.
+func (check *Checker) stmt(ctxt stmtContext, s syntax.Stmt) {
+	// statements must end with the same top scope as they started with
+	if debug {
+		defer func(scope *Scope) {
+			// don't check if code is panicking
+			if p := recover(); p != nil {
+				panic(p)
+			}
+			assert(scope == check.scope)
+		}(check.scope)
+	}
+
+	// process collected function literals before scope changes
+	defer check.processDelayed(len(check.delayed))
+
+	inner := ctxt &^ (fallthroughOk | finalSwitchCase)
+	switch s := s.(type) {
+	case *syntax.EmptyStmt:
+		// ignore
+
+	case *syntax.DeclStmt:
+		check.declStmt(s.DeclList)
+
+	case *syntax.LabeledStmt:
+		check.hasLabel = true
+		check.stmt(ctxt, s.Stmt)
+
+	case *syntax.ExprStmt:
+		// spec: "With the exception of specific built-in functions,
+		// function and method calls and receive operations can appear
+		// in statement context. Such statements may be parenthesized."
+		var x operand
+		kind := check.rawExpr(&x, s.X, nil)
+		var msg string
+		switch x.mode {
+		default:
+			if kind == statement {
+				return
+			}
+			msg = "is not used"
+		case builtin:
+			msg = "must be called"
+		case typexpr:
+			msg = "is not an expression"
+		}
+		check.errorf(&x, "%s %s", &x, msg)
+
+	case *syntax.SendStmt:
+		var ch, x operand
+		check.expr(&ch, s.Chan)
+		check.expr(&x, s.Value)
+		if ch.mode == invalid || x.mode == invalid {
+			return
+		}
+
+		tch := ch.typ.Chan()
+		if tch == nil {
+			check.invalidOpf(s, "cannot send to non-chan type %s", ch.typ)
+			return
+		}
+
+		if tch.dir == RecvOnly {
+			check.invalidOpf(s, "cannot send to receive-only type %s", tch)
+			return
+		}
+
+		check.assignment(&x, tch.elem, "send")
+
+	case *syntax.AssignStmt:
+		lhs := unpackExpr(s.Lhs)
+		rhs := unpackExpr(s.Rhs)
+		if s.Op == 0 || s.Op == syntax.Def {
+			// regular assignment or short variable declaration
+			if len(lhs) == 0 {
+				check.invalidASTf(s, "missing lhs in assignment")
+				return
+			}
+			if s.Op == syntax.Def {
+				check.shortVarDecl(s.Pos(), lhs, rhs)
+			} else {
+				// regular assignment
+				check.assignVars(lhs, rhs)
+			}
+		} else {
+			// assignment operations
+			if len(lhs) != 1 || len(rhs) != 1 {
+				check.errorf(s, "assignment operation %s requires single-valued expressions", s.Op)
+				return
+			}
+
+			// provide better error messages for x++ and x--
+			if rhs[0] == syntax.ImplicitOne {
+				var x operand
+				check.expr(&x, lhs[0])
+				if x.mode == invalid {
+					return
+				}
+				if !isNumeric(x.typ) {
+					check.invalidOpf(lhs[0], "%s%s%s (non-numeric type %s)", lhs[0], s.Op, s.Op, x.typ)
+					return
+				}
+			}
+
+			var x operand
+			check.binary(&x, nil, lhs[0], rhs[0], s.Op)
+			if x.mode == invalid {
+				return
+			}
+			check.assignVar(lhs[0], &x)
+		}
+
+	// case *syntax.GoStmt:
+	// 	check.suspendedCall("go", s.Call)
+
+	// case *syntax.DeferStmt:
+	// 	check.suspendedCall("defer", s.Call)
+	case *syntax.CallStmt:
+		// TODO(gri) get rid of this conversion to string
+		kind := "go"
+		if s.Tok == syntax.Defer {
+			kind = "defer"
+		}
+		check.suspendedCall(kind, s.Call)
+
+	case *syntax.ReturnStmt:
+		res := check.sig.results
+		results := unpackExpr(s.Results)
+		if res.Len() > 0 {
+			// function returns results
+			// (if one, say the first, result parameter is named, all of them are named)
+			if len(results) == 0 && res.vars[0].name != "" {
+				// spec: "Implementation restriction: A compiler may disallow an empty expression
+				// list in a "return" statement if a different entity (constant, type, or variable)
+				// with the same name as a result parameter is in scope at the place of the return."
+				for _, obj := range res.vars {
+					if alt := check.lookup(obj.name); alt != nil && alt != obj {
+						check.errorf(s, "result parameter %s not in scope at return", obj.name)
+						check.errorf(alt, "\tinner declaration of %s", obj)
+						// ok to continue
+					}
+				}
+			} else {
+				// return has results or result parameters are unnamed
+				check.initVars(res.vars, results, s.Pos())
+			}
+		} else if len(results) > 0 {
+			check.error(results[0], "no result values expected")
+			check.use(results...)
+		}
+
+	case *syntax.BranchStmt:
+		if s.Label != nil {
+			check.hasLabel = true
+			return // checked in 2nd pass (check.labels)
+		}
+		switch s.Tok {
+		case syntax.Break:
+			if ctxt&breakOk == 0 {
+				check.error(s, "break not in for, switch, or select statement")
+			}
+		case syntax.Continue:
+			if ctxt&continueOk == 0 {
+				check.error(s, "continue not in for statement")
+			}
+		case syntax.Fallthrough:
+			if ctxt&fallthroughOk == 0 {
+				msg := "fallthrough statement out of place"
+				if ctxt&finalSwitchCase != 0 {
+					msg = "cannot fallthrough final case in switch"
+				}
+				check.error(s, msg)
+			}
+		default:
+			check.invalidASTf(s, "branch statement: %s", s.Tok)
+		}
+
+	case *syntax.BlockStmt:
+		check.openScope(s, "block")
+		defer check.closeScope()
+
+		check.stmtList(inner, s.List)
+
+	case *syntax.IfStmt:
+		check.openScope(s, "if")
+		defer check.closeScope()
+
+		check.simpleStmt(s.Init)
+		var x operand
+		check.expr(&x, s.Cond)
+		if x.mode != invalid && !isBoolean(x.typ) {
+			check.error(s.Cond, "non-boolean condition in if statement")
+		}
+		check.stmt(inner, s.Then)
+		// The parser produces a correct AST but if it was modified
+		// elsewhere the else branch may be invalid. Check again.
+		switch s.Else.(type) {
+		case nil:
+			// valid or error already reported
+		case *syntax.IfStmt, *syntax.BlockStmt:
+			check.stmt(inner, s.Else)
+		default:
+			check.error(s.Else, "invalid else branch in if statement")
+		}
+
+	case *syntax.SwitchStmt:
+		inner |= breakOk
+		check.openScope(s, "switch")
+		defer check.closeScope()
+
+		check.simpleStmt(s.Init)
+
+		if g, _ := s.Tag.(*syntax.TypeSwitchGuard); g != nil {
+			check.typeSwitchStmt(inner, s, g)
+		} else {
+			check.switchStmt(inner, s)
+		}
+
+	case *syntax.SelectStmt:
+		inner |= breakOk
+
+		check.multipleSelectDefaults(s.Body)
+
+		for _, clause := range s.Body {
+			if clause == nil {
+				continue // error reported before
+			}
+
+			// clause.Comm must be a SendStmt, RecvStmt, or default case
+			valid := false
+			var rhs syntax.Expr // rhs of RecvStmt, or nil
+			switch s := clause.Comm.(type) {
+			case nil, *syntax.SendStmt:
+				valid = true
+			case *syntax.AssignStmt:
+				if _, ok := s.Rhs.(*syntax.ListExpr); !ok {
+					rhs = s.Rhs
+				}
+			case *syntax.ExprStmt:
+				rhs = s.X
+			}
+
+			// if present, rhs must be a receive operation
+			if rhs != nil {
+				if x, _ := unparen(rhs).(*syntax.Operation); x != nil && x.Y == nil && x.Op == syntax.Recv {
+					valid = true
+				}
+			}
+
+			if !valid {
+				check.error(clause.Comm, "select case must be send or receive (possibly with assignment)")
+				continue
+			}
+
+			check.openScope(s, "case")
+			if clause.Comm != nil {
+				check.stmt(inner, clause.Comm)
+			}
+			check.stmtList(inner, clause.Body)
+			check.closeScope()
+		}
+
+	case *syntax.ForStmt:
+		inner |= breakOk | continueOk
+		check.openScope(s, "for")
+		defer check.closeScope()
+
+		if rclause, _ := s.Init.(*syntax.RangeClause); rclause != nil {
+			check.rangeStmt(inner, s, rclause)
+			break
+		}
+
+		check.simpleStmt(s.Init)
+		if s.Cond != nil {
+			var x operand
+			check.expr(&x, s.Cond)
+			if x.mode != invalid && !isBoolean(x.typ) {
+				check.error(s.Cond, "non-boolean condition in for statement")
+			}
+		}
+		check.simpleStmt(s.Post)
+		// spec: "The init statement may be a short variable
+		// declaration, but the post statement must not."
+		if s, _ := s.Post.(*syntax.AssignStmt); s != nil && s.Op == syntax.Def {
+			// The parser already reported an error.
+			// Don't call useLHS here because we want to use the lhs in
+			// this erroneous statement so that we don't get errors about
+			// these lhs variables being declared but not used.
+			check.use(s.Lhs) // avoid follow-up errors
+		}
+		check.stmt(inner, s.Body)
+
+	default:
+		check.error(s, "invalid statement")
+	}
+}
+
+func newName(pos syntax.Pos, value string) *syntax.Name {
+	n := new(syntax.Name)
+	// TODO(gri) why does this not work?
+	//n.pos = pos
+	n.Value = value
+	return n
+}
+
+func (check *Checker) switchStmt(inner stmtContext, s *syntax.SwitchStmt) {
+	// init statement already handled
+
+	var x operand
+	if s.Tag != nil {
+		check.expr(&x, s.Tag)
+		// By checking assignment of x to an invisible temporary
+		// (as a compiler would), we get all the relevant checks.
+		check.assignment(&x, nil, "switch expression")
+	} else {
+		// spec: "A missing switch expression is
+		// equivalent to the boolean value true."
+		x.mode = constant_
+		x.typ = Typ[Bool]
+		x.val = constant.MakeBool(true)
+		// TODO(gri) should have a better position here
+		pos := s.Rbrace
+		if len(s.Body) > 0 {
+			pos = s.Body[0].Pos()
+		}
+		x.expr = newName(pos, "true")
+	}
+
+	check.multipleSwitchDefaults(s.Body)
+
+	seen := make(valueMap) // map of seen case values to positions and types
+	for i, clause := range s.Body {
+		if clause == nil {
+			check.invalidASTf(clause, "incorrect expression switch case")
+			continue
+		}
+		check.caseValues(&x, unpackExpr(clause.Cases), seen)
+		check.openScope(clause, "case")
+		inner := inner
+		if i+1 < len(s.Body) {
+			inner |= fallthroughOk
+		} else {
+			inner |= finalSwitchCase
+		}
+		check.stmtList(inner, clause.Body)
+		check.closeScope()
+	}
+}
+
+func (check *Checker) typeSwitchStmt(inner stmtContext, s *syntax.SwitchStmt, guard *syntax.TypeSwitchGuard) {
+	// init statement already handled
+
+	// A type switch guard must be of the form:
+	//
+	//     TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
+	//                          \__lhs__/        \___rhs___/
+
+	// check lhs, if any
+	lhs := guard.Lhs
+	if lhs != nil {
+		if lhs.Value == "_" {
+			// _ := x.(type) is an invalid short variable declaration
+			check.softErrorf(lhs, "no new variable on left side of :=")
+			lhs = nil // avoid declared but not used error below
+		} else {
+			check.recordDef(lhs, nil) // lhs variable is implicitly declared in each cause clause
+		}
+	}
+
+	// check rhs
+	var x operand
+	check.expr(&x, guard.X)
+	if x.mode == invalid {
+		return
+	}
+	var xtyp *Interface
+	var strict bool
+	switch t := x.typ.Under().(type) {
+	case *Interface:
+		xtyp = t
+	// Disabled for now. See comment in the implementation of type assertions (expr.go).
+	// case *TypeParam:
+	// 	xtyp = t.Bound()
+	// 	strict = true
+	default:
+		check.errorf(&x, "%s is not an interface or generic type", &x)
+		return
+	}
+
+	check.multipleSwitchDefaults(s.Body)
+
+	var lhsVars []*Var                // list of implicitly declared lhs variables
+	seen := make(map[Type]syntax.Pos) // map of seen types to positions
+	for _, clause := range s.Body {
+		if clause == nil {
+			check.invalidASTf(s, "incorrect type switch case")
+			continue
+		}
+		// Check each type in this type switch case.
+		cases := unpackExpr(clause.Cases)
+		T := check.caseTypes(&x, xtyp, cases, seen, strict)
+		check.openScope(clause, "case")
+		// If lhs exists, declare a corresponding variable in the case-local scope.
+		if lhs != nil {
+			// spec: "The TypeSwitchGuard may include a short variable declaration.
+			// When that form is used, the variable is declared at the beginning of
+			// the implicit block in each clause. In clauses with a case listing
+			// exactly one type, the variable has that type; otherwise, the variable
+			// has the type of the expression in the TypeSwitchGuard."
+			if len(cases) != 1 || T == nil {
+				T = x.typ
+			}
+			obj := NewVar(lhs.Pos(), check.pkg, lhs.Value, T)
+			scopePos := clause.Pos() // for default clause (len(List) == 0)
+			if n := len(cases); n > 0 {
+				scopePos = endPos(cases[n-1])
+			}
+			check.declare(check.scope, nil, obj, scopePos)
+			check.recordImplicit(clause, obj)
+			// For the "declared but not used" error, all lhs variables act as
+			// one; i.e., if any one of them is 'used', all of them are 'used'.
+			// Collect them for later analysis.
+			lhsVars = append(lhsVars, obj)
+		}
+		check.stmtList(inner, clause.Body)
+		check.closeScope()
+	}
+
+	// If lhs exists, we must have at least one lhs variable that was used.
+	if lhs != nil {
+		var used bool
+		for _, v := range lhsVars {
+			if v.used {
+				used = true
+			}
+			v.used = true // avoid usage error when checking entire function
+		}
+		if !used {
+			check.softErrorf(lhs, "%s declared but not used", lhs.Value)
+		}
+	}
+}
+
+func (check *Checker) rangeStmt(inner stmtContext, s *syntax.ForStmt, rclause *syntax.RangeClause) {
+	// scope already opened
+
+	// check expression to iterate over
+	var x operand
+	check.expr(&x, rclause.X)
+
+	// determine lhs, if any
+	sKey := rclause.Lhs // possibly nil
+	var sValue syntax.Expr
+	if p, _ := sKey.(*syntax.ListExpr); p != nil {
+		if len(p.ElemList) != 2 {
+			check.invalidASTf(s, "invalid lhs in range clause")
+			return
+		}
+		sKey = p.ElemList[0]
+		sValue = p.ElemList[1]
+	}
+
+	// determine key/value types
+	var key, val Type
+	if x.mode != invalid {
+		typ := optype(x.typ.Under())
+		if _, ok := typ.(*Chan); ok && sValue != nil {
+			// TODO(gri) this also needs to happen for channels in generic variables
+			check.softErrorf(sValue, "range over %s permits only one iteration variable", &x)
+			// ok to continue
+		}
+		var msg string
+		key, val, msg = rangeKeyVal(typ, isVarName(sKey), isVarName(sValue))
+		if key == nil || msg != "" {
+			if msg != "" {
+				msg = ": " + msg
+			}
+			check.softErrorf(&x, "cannot range over %s%s", &x, msg)
+			// ok to continue
+		}
+	}
+
+	// check assignment to/declaration of iteration variables
+	// (irregular assignment, cannot easily map to existing assignment checks)
+
+	// lhs expressions and initialization value (rhs) types
+	lhs := [2]syntax.Expr{sKey, sValue}
+	rhs := [2]Type{key, val} // key, val may be nil
+
+	if rclause.Def {
+		// short variable declaration; variable scope starts after the range clause
+		// (the for loop opens a new scope, so variables on the lhs never redeclare
+		// previously declared variables)
+		var vars []*Var
+		for i, lhs := range lhs {
+			if lhs == nil {
+				continue
+			}
+
+			// determine lhs variable
+			var obj *Var
+			if ident, _ := lhs.(*syntax.Name); ident != nil {
+				// declare new variable
+				name := ident.Value
+				obj = NewVar(ident.Pos(), check.pkg, name, nil)
+				check.recordDef(ident, obj)
+				// _ variables don't count as new variables
+				if name != "_" {
+					vars = append(vars, obj)
+				}
+			} else {
+				check.errorf(lhs, "cannot declare %s", lhs)
+				obj = NewVar(lhs.Pos(), check.pkg, "_", nil) // dummy variable
+			}
+
+			// initialize lhs variable
+			if typ := rhs[i]; typ != nil {
+				x.mode = value
+				x.expr = lhs // we don't have a better rhs expression to use here
+				x.typ = typ
+				check.initVar(obj, &x, "range clause")
+			} else {
+				obj.typ = Typ[Invalid]
+				obj.used = true // don't complain about unused variable
+			}
+		}
+
+		// declare variables
+		if len(vars) > 0 {
+			scopePos := endPos(rclause.X) // TODO(gri) should this just be s.Body.Pos (spec clarification)?
+			for _, obj := range vars {
+				// spec: "The scope of a constant or variable identifier declared inside
+				// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
+				// for short variable declarations) and ends at the end of the innermost
+				// containing block."
+				check.declare(check.scope, nil /* recordDef already called */, obj, scopePos)
+			}
+		} else {
+			check.error(s, "no new variables on left side of :=")
+		}
+	} else {
+		// ordinary assignment
+		for i, lhs := range lhs {
+			if lhs == nil {
+				continue
+			}
+			if typ := rhs[i]; typ != nil {
+				x.mode = value
+				x.expr = lhs // we don't have a better rhs expression to use here
+				x.typ = typ
+				check.assignVar(lhs, &x)
+			}
+		}
+	}
+
+	check.stmt(inner, s.Body)
+}
+
+// isVarName reports whether x is a non-nil, non-blank (_) expression.
+func isVarName(x syntax.Expr) bool {
+	if x == nil {
+		return false
+	}
+	ident, _ := unparen(x).(*syntax.Name)
+	return ident == nil || ident.Value != "_"
+}
+
+// rangeKeyVal returns the key and value type produced by a range clause
+// over an expression of type typ, and possibly an error message. If the
+// range clause is not permitted the returned key is nil or msg is not
+// empty (in that case we still may have a non-nil key type which can be
+// used to reduce the chance for follow-on errors).
+// The wantKey, wantVal, and hasVal flags indicate which of the iteration
+// variables are used or present; this matters if we range over a generic
+// type where not all keys or values are of the same type.
+func rangeKeyVal(typ Type, wantKey, wantVal bool) (Type, Type, string) {
+	switch typ := typ.(type) {
+	case *Basic:
+		if isString(typ) {
+			return Typ[Int], universeRune, "" // use 'rune' name
+		}
+	case *Array:
+		return Typ[Int], typ.elem, ""
+	case *Slice:
+		return Typ[Int], typ.elem, ""
+	case *Pointer:
+		if typ := typ.base.Array(); typ != nil {
+			return Typ[Int], typ.elem, ""
+		}
+	case *Map:
+		return typ.key, typ.elem, ""
+	case *Chan:
+		var msg string
+		if typ.dir == SendOnly {
+			msg = "send-only channel"
+		}
+		return typ.elem, Typ[Invalid], msg
+	case *Sum:
+		first := true
+		var key, val Type
+		var msg string
+		typ.is(func(t Type) bool {
+			k, v, m := rangeKeyVal(t.Under(), wantKey, wantVal)
+			if k == nil || m != "" {
+				key, val, msg = k, v, m
+				return false
+			}
+			if first {
+				key, val, msg = k, v, m
+				first = false
+				return true
+			}
+			if wantKey && !Identical(key, k) {
+				key, val, msg = nil, nil, "all possible values must have the same key type"
+				return false
+			}
+			if wantVal && !Identical(val, v) {
+				key, val, msg = nil, nil, "all possible values must have the same element type"
+				return false
+			}
+			return true
+		})
+		return key, val, msg
+	}
+	return nil, nil, ""
+}

src/cmd/compile/internal/types2/subst.go

@@ -0,0 +1,554 @@
+// UNREVIEWED
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements instantiation of generic types
+// through substitution of type parameters by actual
+// types.
+
+package types2
+
+import (
+	"bytes"
+	"cmd/compile/internal/syntax"
+	"fmt"
+)
+
+type substMap struct {
+	// The targs field is currently needed for *Named type substitution.
+	// TODO(gri) rewrite that code, get rid of this field, and make this
+	//           struct just the map (proj)
+	targs []Type
+	proj  map[*TypeParam]Type
+}
+
+// makeSubstMap creates a new substitution map mapping tpars[i] to targs[i].
+// If targs[i] is nil, tpars[i] is not substituted.
+func makeSubstMap(tpars []*TypeName, targs []Type) *substMap {
+	assert(len(tpars) == len(targs))
+	proj := make(map[*TypeParam]Type, len(tpars))
+	for i, tpar := range tpars {
+		// We must expand type arguments otherwise *Instance
+		// types end up as components in composite types.
+		// TODO(gri) explain why this causes problems, if it does
+		targ := expand(targs[i]) // possibly nil
+		targs[i] = targ
+		proj[tpar.typ.(*TypeParam)] = targ
+	}
+	return &substMap{targs, proj}
+}
+
+func (m *substMap) String() string {
+	return fmt.Sprintf("%s", m.proj)
+}
+
+func (m *substMap) empty() bool {
+	return len(m.proj) == 0
+}
+
+func (m *substMap) lookup(tpar *TypeParam) Type {
+	if t := m.proj[tpar]; t != nil {
+		return t
+	}
+	return tpar
+}
+
+func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslist []syntax.Pos) (res Type) {
+	if check.conf.Trace {
+		check.trace(pos, "-- instantiating %s with %s", typ, typeListString(targs))
+		check.indent++
+		defer func() {
+			check.indent--
+			var under Type
+			if res != nil {
+				// Calling Under() here may lead to endless instantiations.
+				// Test case: type T[P any] T[P]
+				// TODO(gri) investigate if that's a bug or to be expected.
+				under = res.Underlying()
+			}
+			check.trace(pos, "=> %s (under = %s)", res, under)
+		}()
+	}
+
+	assert(poslist == nil || len(poslist) <= len(targs))
+
+	// TODO(gri) What is better here: work with TypeParams, or work with TypeNames?
+	var tparams []*TypeName
+	switch t := typ.(type) {
+	case *Named:
+		tparams = t.tparams
+	case *Signature:
+		tparams = t.tparams
+		defer func() {
+			// If we had an unexpected failure somewhere don't
+			// panic below when asserting res.(*Signature).
+			if res == nil {
+				return
+			}
+			// If the signature doesn't use its type parameters, subst
+			// will not make a copy. In that case, make a copy now (so
+			// we can set tparams to nil w/o causing side-effects).
+			if t == res {
+				copy := *t
+				res = &copy
+			}
+			// After instantiating a generic signature, it is not generic
+			// anymore; we need to set tparams to nil.
+			res.(*Signature).tparams = nil
+		}()
+
+	default:
+		check.dump("%v: cannot instantiate %v", pos, typ)
+		unreachable() // only defined types and (defined) functions can be generic
+
+	}
+
+	// the number of supplied types must match the number of type parameters
+	if len(targs) != len(tparams) {
+		// TODO(gri) provide better error message
+		check.errorf(pos, "got %d arguments but %d type parameters", len(targs), len(tparams))
+		return Typ[Invalid]
+	}
+
+	if len(tparams) == 0 {
+		return typ // nothing to do (minor optimization)
+	}
+
+	smap := makeSubstMap(tparams, targs)
+
+	// check bounds
+	for i, tname := range tparams {
+		tpar := tname.typ.(*TypeParam)
+		iface := tpar.Bound()
+		if iface.Empty() {
+			continue // no type bound
+		}
+
+		targ := targs[i]
+
+		// best position for error reporting
+		pos := pos
+		if i < len(poslist) {
+			pos = poslist[i]
+		}
+
+		// The type parameter bound is parameterized with the same type parameters
+		// as the instantiated type; before we can use it for bounds checking we
+		// need to instantiate it with the type arguments with which we instantiate
+		// the parameterized type.
+		iface = check.subst(pos, iface, smap).(*Interface)
+
+		// targ must implement iface (methods)
+		// - check only if we have methods
+		check.completeInterface(nopos, iface)
+		if len(iface.allMethods) > 0 {
+			// If the type argument is a type parameter itself, its pointer designation
+			// must match the pointer designation of the callee's type parameter.
+			// If the type argument is a pointer to a type parameter, the type argument's
+			// method set is empty.
+			// TODO(gri) is this what we want? (spec question)
+			if tparg := targ.TypeParam(); tparg != nil {
+				if tparg.ptr != tpar.ptr {
+					check.errorf(pos, "pointer designation mismatch")
+					break
+				}
+			} else if base, isPtr := deref(targ); isPtr && base.TypeParam() != nil {
+				check.errorf(pos, "%s has no methods", targ)
+				break
+			}
+			// If a type parameter is marked as a pointer type, the type bound applies
+			// to a pointer of the type argument.
+			actual := targ
+			if tpar.ptr {
+				actual = NewPointer(targ)
+			}
+			if m, wrong := check.missingMethod(actual, iface, true); m != nil {
+				// TODO(gri) needs to print updated name to avoid major confusion in error message!
+				//           (print warning for now)
+				// check.softErrorf(pos, "%s does not satisfy %s (warning: name not updated) = %s (missing method %s)", targ, tpar.bound, iface, m)
+				if m.name == "==" {
+					// We don't want to report "missing method ==".
+					check.softErrorf(pos, "%s does not satisfy comparable", targ)
+				} else if wrong != nil {
+					// TODO(gri) This can still report uninstantiated types which makes the error message
+					//           more difficult to read then necessary.
+					check.softErrorf(pos,
+						"%s does not satisfy %s: wrong method signature\n\tgot  %s\n\twant %s",
+						targ, tpar.bound, wrong, m,
+					)
+				} else {
+					check.softErrorf(pos, "%s does not satisfy %s (missing method %s)", targ, tpar.bound, m.name)
+				}
+				break
+			}
+		}
+
+		// targ's underlying type must also be one of the interface types listed, if any
+		if iface.allTypes == nil {
+			continue // nothing to do
+		}
+		// iface.allTypes != nil
+
+		// If targ is itself a type parameter, each of its possible types, but at least one, must be in the
+		// list of iface types (i.e., the targ type list must be a non-empty subset of the iface types).
+		if targ := targ.TypeParam(); targ != nil {
+			targBound := targ.Bound()
+			if targBound.allTypes == nil {
+				check.softErrorf(pos, "%s does not satisfy %s (%s has no type constraints)", targ, tpar.bound, targ)
+				break
+			}
+			for _, t := range unpack(targBound.allTypes) {
+				if !iface.isSatisfiedBy(t.Under()) {
+					// TODO(gri) match this error message with the one below (or vice versa)
+					check.softErrorf(pos, "%s does not satisfy %s (%s type constraint %s not found in %s)", targ, tpar.bound, targ, t, iface.allTypes)
+					break
+				}
+			}
+			break
+		}
+
+		// Otherwise, targ's type or underlying type must also be one of the interface types listed, if any.
+		if !iface.isSatisfiedBy(targ) {
+			check.softErrorf(pos, "%s does not satisfy %s (%s not found in %s)", targ, tpar.bound, targ.Under(), iface.allTypes)
+			break
+		}
+	}
+
+	return check.subst(pos, typ, smap)
+}
+
+// subst returns the type typ with its type parameters tpars replaced by
+// the corresponding type arguments targs, recursively.
+// subst is functional in the sense that it doesn't modify the incoming
+// type. If a substitution took place, the result type is different from
+// from the incoming type.
+func (check *Checker) subst(pos syntax.Pos, typ Type, smap *substMap) Type {
+	if smap.empty() {
+		return typ
+	}
+
+	// common cases
+	switch t := typ.(type) {
+	case *Basic:
+		return typ // nothing to do
+	case *TypeParam:
+		return smap.lookup(t)
+	}
+
+	// general case
+	subst := subster{check, pos, make(map[Type]Type), smap}
+	return subst.typ(typ)
+}
+
+type subster struct {
+	check *Checker
+	pos   syntax.Pos
+	cache map[Type]Type
+	smap  *substMap
+}
+
+func (subst *subster) typ(typ Type) Type {
+	switch t := typ.(type) {
+	case nil:
+		// Call typOrNil if it's possible that typ is nil.
+		panic("nil typ")
+
+	case *Basic, *bottom, *top:
+		// nothing to do
+
+	case *Array:
+		elem := subst.typOrNil(t.elem)
+		if elem != t.elem {
+			return &Array{len: t.len, elem: elem}
+		}
+
+	case *Slice:
+		elem := subst.typOrNil(t.elem)
+		if elem != t.elem {
+			return &Slice{elem: elem}
+		}
+
+	case *Struct:
+		if fields, copied := subst.varList(t.fields); copied {
+			return &Struct{fields: fields, tags: t.tags}
+		}
+
+	case *Pointer:
+		base := subst.typ(t.base)
+		if base != t.base {
+			return &Pointer{base: base}
+		}
+
+	case *Tuple:
+		return subst.tuple(t)
+
+	case *Signature:
+		// TODO(gri) rethink the recv situation with respect to methods on parameterized types
+		// recv := subst.var_(t.recv) // TODO(gri) this causes a stack overflow - explain
+		recv := t.recv
+		params := subst.tuple(t.params)
+		results := subst.tuple(t.results)
+		if recv != t.recv || params != t.params || results != t.results {
+			return &Signature{
+				rparams:  t.rparams,
+				tparams:  t.tparams,
+				scope:    t.scope,
+				recv:     recv,
+				params:   params,
+				results:  results,
+				variadic: t.variadic,
+			}
+		}
+
+	case *Sum:
+		types, copied := subst.typeList(t.types)
+		if copied {
+			// Don't do it manually, with a Sum literal: the new
+			// types list may not be unique and NewSum may remove
+			// duplicates.
+			return NewSum(types)
+		}
+
+	case *Interface:
+		methods, mcopied := subst.funcList(t.methods)
+		types := t.types
+		if t.types != nil {
+			types = subst.typ(t.types)
+		}
+		embeddeds, ecopied := subst.typeList(t.embeddeds)
+		if mcopied || types != t.types || ecopied {
+			iface := &Interface{methods: methods, types: types, embeddeds: embeddeds}
+			subst.check.posMap[iface] = subst.check.posMap[t] // satisfy completeInterface requirement
+			subst.check.completeInterface(nopos, iface)
+			return iface
+		}
+
+	case *Map:
+		key := subst.typ(t.key)
+		elem := subst.typ(t.elem)
+		if key != t.key || elem != t.elem {
+			return &Map{key: key, elem: elem}
+		}
+
+	case *Chan:
+		elem := subst.typ(t.elem)
+		if elem != t.elem {
+			return &Chan{dir: t.dir, elem: elem}
+		}
+
+	case *Named:
+		subst.check.indent++
+		defer func() {
+			subst.check.indent--
+		}()
+		dump := func(format string, args ...interface{}) {
+			if subst.check.conf.Trace {
+				subst.check.trace(subst.pos, format, args...)
+			}
+		}
+
+		if t.tparams == nil {
+			dump(">>> %s is not parameterized", t)
+			return t // type is not parameterized
+		}
+
+		var new_targs []Type
+
+		if len(t.targs) > 0 {
+
+			// already instantiated
+			dump(">>> %s already instantiated", t)
+			assert(len(t.targs) == len(t.tparams))
+			// For each (existing) type argument targ, determine if it needs
+			// to be substituted; i.e., if it is or contains a type parameter
+			// that has a type argument for it.
+			for i, targ := range t.targs {
+				dump(">>> %d targ = %s", i, targ)
+				new_targ := subst.typ(targ)
+				if new_targ != targ {
+					dump(">>> substituted %d targ %s => %s", i, targ, new_targ)
+					if new_targs == nil {
+						new_targs = make([]Type, len(t.tparams))
+						copy(new_targs, t.targs)
+					}
+					new_targs[i] = new_targ
+				}
+			}
+
+			if new_targs == nil {
+				dump(">>> nothing to substitute in %s", t)
+				return t // nothing to substitute
+			}
+
+		} else {
+
+			// not yet instantiated
+			dump(">>> first instantiation of %s", t)
+			new_targs = subst.smap.targs
+
+		}
+
+		// before creating a new named type, check if we have this one already
+		h := instantiatedHash(t, new_targs)
+		dump(">>> new type hash: %s", h)
+		if named, found := subst.check.typMap[h]; found {
+			dump(">>> found %s", named)
+			subst.cache[t] = named
+			return named
+		}
+
+		// create a new named type and populate caches to avoid endless recursion
+		tname := NewTypeName(subst.pos, t.obj.pkg, t.obj.name, nil)
+		named := subst.check.NewNamed(tname, t.underlying, t.methods) // method signatures are updated lazily
+		named.tparams = t.tparams                                     // new type is still parameterized
+		named.targs = new_targs
+		subst.check.typMap[h] = named
+		subst.cache[t] = named
+
+		// do the substitution
+		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, new_targs)
+		named.underlying = subst.typOrNil(t.underlying)
+		named.orig = named.underlying // for cycle detection (Checker.validType)
+
+		return named
+
+	case *TypeParam:
+		return subst.smap.lookup(t)
+
+	case *instance:
+		// TODO(gri) can we avoid the expansion here and just substitute the type parameters?
+		return subst.typ(t.expand())
+
+	default:
+		unimplemented()
+	}
+
+	return typ
+}
+
+// TODO(gri) Eventually, this should be more sophisticated.
+//           It won't work correctly for locally declared types.
+func instantiatedHash(typ *Named, targs []Type) string {
+	var buf bytes.Buffer
+	writeTypeName(&buf, typ.obj, nil)
+	buf.WriteByte('(')
+	writeTypeList(&buf, targs, nil, nil)
+	buf.WriteByte(')')
+
+	// With respect to the represented type, whether a
+	// type is fully expanded or stored as instance
+	// does not matter - they are the same types.
+	// Remove the instanceMarkers printed for instances.
+	res := buf.Bytes()
+	i := 0
+	for _, b := range res {
+		if b != instanceMarker {
+			res[i] = b
+			i++
+		}
+	}
+
+	return string(res[:i])
+}
+
+func typeListString(list []Type) string {
+	var buf bytes.Buffer
+	writeTypeList(&buf, list, nil, nil)
+	return buf.String()
+}
+
+// typOrNil is like typ but if the argument is nil it is replaced with Typ[Invalid].
+// A nil type may appear in pathological cases such as type T[P any] []func(_ T([]_))
+// where an array/slice element is accessed before it is set up.
+func (subst *subster) typOrNil(typ Type) Type {
+	if typ == nil {
+		return Typ[Invalid]
+	}
+	return subst.typ(typ)
+}
+
+func (subst *subster) var_(v *Var) *Var {
+	if v != nil {
+		if typ := subst.typ(v.typ); typ != v.typ {
+			copy := *v
+			copy.typ = typ
+			return &copy
+		}
+	}
+	return v
+}
+
+func (subst *subster) tuple(t *Tuple) *Tuple {
+	if t != nil {
+		if vars, copied := subst.varList(t.vars); copied {
+			return &Tuple{vars: vars}
+		}
+	}
+	return t
+}
+
+func (subst *subster) varList(in []*Var) (out []*Var, copied bool) {
+	out = in
+	for i, v := range in {
+		if w := subst.var_(v); w != v {
+			if !copied {
+				// first variable that got substituted => allocate new out slice
+				// and copy all variables
+				new := make([]*Var, len(in))
+				copy(new, out)
+				out = new
+				copied = true
+			}
+			out[i] = w
+		}
+	}
+	return
+}
+
+func (subst *subster) func_(f *Func) *Func {
+	if f != nil {
+		if typ := subst.typ(f.typ); typ != f.typ {
+			copy := *f
+			copy.typ = typ
+			return &copy
+		}
+	}
+	return f
+}
+
+func (subst *subster) funcList(in []*Func) (out []*Func, copied bool) {
+	out = in
+	for i, f := range in {
+		if g := subst.func_(f); g != f {
+			if !copied {
+				// first function that got substituted => allocate new out slice
+				// and copy all functions
+				new := make([]*Func, len(in))
+				copy(new, out)
+				out = new
+				copied = true
+			}
+			out[i] = g
+		}
+	}
+	return
+}
+
+func (subst *subster) typeList(in []Type) (out []Type, copied bool) {
+	out = in
+	for i, t := range in {
+		if u := subst.typ(t); u != t {
+			if !copied {
+				// first function that got substituted => allocate new out slice
+				// and copy all functions
+				new := make([]Type, len(in))
+				copy(new, out)
+				out = new
+				copied = true
+			}
+			out[i] = u
+		}
+	}
+	return
+}

src/cmd/compile/internal/types2/typestring.go

@@ -0,0 +1,480 @@
+// UNREVIEWED
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements printing of types.
+
+package types2
+
+import (
+	"bytes"
+	"fmt"
+	"unicode/utf8"
+)
+
+// A Qualifier controls how named package-level objects are printed in
+// calls to TypeString, ObjectString, and SelectionString.
+//
+// These three formatting routines call the Qualifier for each
+// package-level object O, and if the Qualifier returns a non-empty
+// string p, the object is printed in the form p.O.
+// If it returns an empty string, only the object name O is printed.
+//
+// Using a nil Qualifier is equivalent to using (*Package).Path: the
+// object is qualified by the import path, e.g., "encoding/json.Marshal".
+//
+type Qualifier func(*Package) string
+
+// RelativeTo returns a Qualifier that fully qualifies members of
+// all packages other than pkg.
+func RelativeTo(pkg *Package) Qualifier {
+	if pkg == nil {
+		return nil
+	}
+	return func(other *Package) string {
+		if pkg == other {
+			return "" // same package; unqualified
+		}
+		return other.Path()
+	}
+}
+
+// If gcCompatibilityMode is set, printing of types is modified
+// to match the representation of some types in the gc compiler:
+//
+//	- byte and rune lose their alias name and simply stand for
+//	  uint8 and int32 respectively
+//	- embedded interfaces get flattened (the embedding info is lost,
+//	  and certain recursive interface types cannot be printed anymore)
+//
+// This makes it easier to compare packages computed with the type-
+// checker vs packages imported from gc export data.
+//
+// Caution: This flag affects all uses of WriteType, globally.
+// It is only provided for testing in conjunction with
+// gc-generated data.
+//
+// This flag is exported in the x/tools/go/types package. We don't
+// need it at the moment in the std repo and so we don't export it
+// anymore. We should eventually try to remove it altogether.
+// TODO(gri) remove this
+var gcCompatibilityMode bool
+
+// TypeString returns the string representation of typ.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func TypeString(typ Type, qf Qualifier) string {
+	var buf bytes.Buffer
+	WriteType(&buf, typ, qf)
+	return buf.String()
+}
+
+// WriteType writes the string representation of typ to buf.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func WriteType(buf *bytes.Buffer, typ Type, qf Qualifier) {
+	writeType(buf, typ, qf, make([]Type, 0, 8))
+}
+
+// instanceMarker is the prefix for an instantiated type
+// in "non-evaluated" instance form.
+const instanceMarker = '#'
+
+func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
+	// Theoretically, this is a quadratic lookup algorithm, but in
+	// practice deeply nested composite types with unnamed component
+	// types are uncommon. This code is likely more efficient than
+	// using a map.
+	for _, t := range visited {
+		if t == typ {
+			fmt.Fprintf(buf, "○%T", goTypeName(typ)) // cycle to typ
+			return
+		}
+	}
+	visited = append(visited, typ)
+
+	switch t := typ.(type) {
+	case nil:
+		buf.WriteString("<nil>")
+
+	case *Basic:
+		if t.kind == UnsafePointer {
+			buf.WriteString("unsafe.")
+		}
+		if gcCompatibilityMode {
+			// forget the alias names
+			switch t.kind {
+			case Byte:
+				t = Typ[Uint8]
+			case Rune:
+				t = Typ[Int32]
+			}
+		}
+		buf.WriteString(t.name)
+
+	case *Array:
+		fmt.Fprintf(buf, "[%d]", t.len)
+		writeType(buf, t.elem, qf, visited)
+
+	case *Slice:
+		buf.WriteString("[]")
+		writeType(buf, t.elem, qf, visited)
+
+	case *Struct:
+		buf.WriteString("struct{")
+		for i, f := range t.fields {
+			if i > 0 {
+				buf.WriteString("; ")
+			}
+			buf.WriteString(f.name)
+			if f.embedded {
+				// emphasize that the embedded field's name
+				// doesn't match the field's type name
+				if f.name != embeddedFieldName(f.typ) {
+					buf.WriteString(" /* = ")
+					writeType(buf, f.typ, qf, visited)
+					buf.WriteString(" */")
+				}
+			} else {
+				buf.WriteByte(' ')
+				writeType(buf, f.typ, qf, visited)
+			}
+			if tag := t.Tag(i); tag != "" {
+				fmt.Fprintf(buf, " %q", tag)
+			}
+		}
+		buf.WriteByte('}')
+
+	case *Pointer:
+		buf.WriteByte('*')
+		writeType(buf, t.base, qf, visited)
+
+	case *Tuple:
+		writeTuple(buf, t, false, qf, visited)
+
+	case *Signature:
+		buf.WriteString("func")
+		writeSignature(buf, t, qf, visited)
+
+	case *Sum:
+		for i, t := range t.types {
+			if i > 0 {
+				buf.WriteString(", ")
+			}
+			writeType(buf, t, qf, visited)
+		}
+
+	case *Interface:
+		// We write the source-level methods and embedded types rather
+		// than the actual method set since resolved method signatures
+		// may have non-printable cycles if parameters have embedded
+		// interface types that (directly or indirectly) embed the
+		// current interface. For instance, consider the result type
+		// of m:
+		//
+		//     type T interface{
+		//         m() interface{ T }
+		//     }
+		//
+		buf.WriteString("interface{")
+		empty := true
+		if gcCompatibilityMode {
+			// print flattened interface
+			// (useful to compare against gc-generated interfaces)
+			for i, m := range t.allMethods {
+				if i > 0 {
+					buf.WriteString("; ")
+				}
+				buf.WriteString(m.name)
+				writeSignature(buf, m.typ.(*Signature), qf, visited)
+				empty = false
+			}
+			if !empty && t.allTypes != nil {
+				buf.WriteString("; ")
+			}
+			if t.allTypes != nil {
+				buf.WriteString("type ")
+				writeType(buf, t.allTypes, qf, visited)
+			}
+		} else {
+			// print explicit interface methods and embedded types
+			for i, m := range t.methods {
+				if i > 0 {
+					buf.WriteString("; ")
+				}
+				buf.WriteString(m.name)
+				writeSignature(buf, m.typ.(*Signature), qf, visited)
+				empty = false
+			}
+			if !empty && t.types != nil {
+				buf.WriteString("; ")
+			}
+			if t.types != nil {
+				buf.WriteString("type ")
+				writeType(buf, t.types, qf, visited)
+				empty = false
+			}
+			if !empty && len(t.embeddeds) > 0 {
+				buf.WriteString("; ")
+			}
+			for i, typ := range t.embeddeds {
+				if i > 0 {
+					buf.WriteString("; ")
+				}
+				writeType(buf, typ, qf, visited)
+				empty = false
+			}
+		}
+		if t.allMethods == nil || len(t.methods) > len(t.allMethods) {
+			if !empty {
+				buf.WriteByte(' ')
+			}
+			buf.WriteString("/* incomplete */")
+		}
+		buf.WriteByte('}')
+
+	case *Map:
+		buf.WriteString("map[")
+		writeType(buf, t.key, qf, visited)
+		buf.WriteByte(']')
+		writeType(buf, t.elem, qf, visited)
+
+	case *Chan:
+		var s string
+		var parens bool
+		switch t.dir {
+		case SendRecv:
+			s = "chan "
+			// chan (<-chan T) requires parentheses
+			if c, _ := t.elem.(*Chan); c != nil && c.dir == RecvOnly {
+				parens = true
+			}
+		case SendOnly:
+			s = "chan<- "
+		case RecvOnly:
+			s = "<-chan "
+		default:
+			panic("unreachable")
+		}
+		buf.WriteString(s)
+		if parens {
+			buf.WriteByte('(')
+		}
+		writeType(buf, t.elem, qf, visited)
+		if parens {
+			buf.WriteByte(')')
+		}
+
+	case *Named:
+		writeTypeName(buf, t.obj, qf)
+		if t.targs != nil {
+			// instantiated type
+			buf.WriteByte('[')
+			writeTypeList(buf, t.targs, qf, visited)
+			buf.WriteByte(']')
+		} else if t.tparams != nil {
+			// parameterized type
+			writeTParamList(buf, t.tparams, qf, visited)
+		}
+
+	case *TypeParam:
+		s := "?"
+		if t.obj != nil {
+			s = t.obj.name
+		}
+		buf.WriteString(s + subscript(t.id))
+
+	case *instance:
+		buf.WriteByte(instanceMarker) // indicate "non-evaluated" syntactic instance
+		writeTypeName(buf, t.base.obj, qf)
+		buf.WriteByte('[')
+		writeTypeList(buf, t.targs, qf, visited)
+		buf.WriteByte(']')
+
+	case *bottom:
+		buf.WriteString("⊥")
+
+	case *top:
+		buf.WriteString("⊤")
+
+	default:
+		// For externally defined implementations of Type.
+		buf.WriteString(t.String())
+	}
+}
+
+func writeTypeList(buf *bytes.Buffer, list []Type, qf Qualifier, visited []Type) {
+	for i, typ := range list {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		writeType(buf, typ, qf, visited)
+	}
+}
+
+func writeTParamList(buf *bytes.Buffer, list []*TypeName, qf Qualifier, visited []Type) {
+	// bound returns the type bound for tname. The result is never nil.
+	bound := func(tname *TypeName) Type {
+		// be careful to avoid crashes in case of inconsistencies
+		if t, _ := tname.typ.(*TypeParam); t != nil && t.bound != nil {
+			return t.bound
+		}
+		return &emptyInterface
+	}
+
+	// If a single type bound is not the empty interface, we have to write them all.
+	var writeBounds bool
+	for _, p := range list {
+		// bound(p) should be an interface but be careful (it may be invalid)
+		b := bound(p).Interface()
+		if b != nil && !b.Empty() {
+			writeBounds = true
+			break
+		}
+	}
+	writeBounds = true // always write the bounds for new type parameter list syntax
+
+	buf.WriteString("[")
+	var prev Type
+	for i, p := range list {
+		b := bound(p)
+		if i > 0 {
+			if writeBounds && b != prev {
+				// type bound changed - write previous one before advancing
+				buf.WriteByte(' ')
+				writeType(buf, prev, qf, visited)
+			}
+			buf.WriteString(", ")
+		}
+		prev = b
+
+		if t, _ := p.typ.(*TypeParam); t != nil {
+			if t.ptr {
+				buf.WriteByte('*')
+			}
+			writeType(buf, t, qf, visited)
+		} else {
+			buf.WriteString(p.name)
+		}
+	}
+	if writeBounds && prev != nil {
+		buf.WriteByte(' ')
+		writeType(buf, prev, qf, visited)
+	}
+	buf.WriteByte(']')
+}
+
+func writeTypeName(buf *bytes.Buffer, obj *TypeName, qf Qualifier) {
+	s := "<Named w/o object>"
+	if obj != nil {
+		if obj.pkg != nil {
+			writePackage(buf, obj.pkg, qf)
+		}
+		// TODO(gri): function-local named types should be displayed
+		// differently from named types at package level to avoid
+		// ambiguity.
+		s = obj.name
+	}
+	buf.WriteString(s)
+}
+
+func writeTuple(buf *bytes.Buffer, tup *Tuple, variadic bool, qf Qualifier, visited []Type) {
+	buf.WriteByte('(')
+	if tup != nil {
+		for i, v := range tup.vars {
+			if i > 0 {
+				buf.WriteString(", ")
+			}
+			if v.name != "" {
+				buf.WriteString(v.name)
+				buf.WriteByte(' ')
+			}
+			typ := v.typ
+			if variadic && i == len(tup.vars)-1 {
+				if s, ok := typ.(*Slice); ok {
+					buf.WriteString("...")
+					typ = s.elem
+				} else {
+					// special case:
+					// append(s, "foo"...) leads to signature func([]byte, string...)
+					if t := typ.Basic(); t == nil || t.kind != String {
+						panic("internal error: string type expected")
+					}
+					writeType(buf, typ, qf, visited)
+					buf.WriteString("...")
+					continue
+				}
+			}
+			writeType(buf, typ, qf, visited)
+		}
+	}
+	buf.WriteByte(')')
+}
+
+// WriteSignature writes the representation of the signature sig to buf,
+// without a leading "func" keyword.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func WriteSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier) {
+	writeSignature(buf, sig, qf, make([]Type, 0, 8))
+}
+
+func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []Type) {
+	if sig.tparams != nil {
+		writeTParamList(buf, sig.tparams, qf, visited)
+	}
+
+	writeTuple(buf, sig.params, sig.variadic, qf, visited)
+
+	n := sig.results.Len()
+	if n == 0 {
+		// no result
+		return
+	}
+
+	buf.WriteByte(' ')
+	if n == 1 && sig.results.vars[0].name == "" {
+		// single unnamed result
+		writeType(buf, sig.results.vars[0].typ, qf, visited)
+		return
+	}
+
+	// multiple or named result(s)
+	writeTuple(buf, sig.results, false, qf, visited)
+}
+
+// embeddedFieldName returns an embedded field's name given its type.
+// The result is "" if the type doesn't have an embedded field name.
+func embeddedFieldName(typ Type) string {
+	switch t := typ.(type) {
+	case *Basic:
+		return t.name
+	case *Named:
+		return t.obj.name
+	case *Pointer:
+		// *T is ok, but **T is not
+		if _, ok := t.base.(*Pointer); !ok {
+			return embeddedFieldName(t.base)
+		}
+	case *instance:
+		return t.base.obj.name
+	}
+	return "" // not a (pointer to) a defined type
+}
+
+// subscript returns the decimal (utf8) representation of x using subscript digits.
+func subscript(x uint64) string {
+	const w = len("₀") // all digits 0...9 have the same utf8 width
+	var buf [32 * w]byte
+	i := len(buf)
+	for {
+		i -= w
+		utf8.EncodeRune(buf[i:], '₀'+rune(x%10)) // '₀' == U+2080
+		x /= 10
+		if x == 0 {
+			break
+		}
+	}
+	return string(buf[i:])
+}

src/cmd/compile/internal/types2/typestring_test.go

@@ -0,0 +1,221 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2_test
+
+import (
+	"internal/testenv"
+	"testing"
+
+	"cmd/compile/internal/syntax"
+	. "cmd/compile/internal/types2"
+)
+
+const filename = "<src>"
+
+func makePkg(src string) (*Package, error) {
+	file, err := parseSrc(filename, src)
+	if err != nil {
+		return nil, err
+	}
+	// use the package name as package path
+	conf := Config{Importer: defaultImporter()}
+	return conf.Check(file.PkgName.Value, []*syntax.File{file}, nil)
+}
+
+type testEntry struct {
+	src, str string
+}
+
+// dup returns a testEntry where both src and str are the same.
+func dup(s string) testEntry {
+	return testEntry{s, s}
+}
+
+// types that don't depend on any other type declarations
+var independentTestTypes = []testEntry{
+	// basic types
+	dup("int"),
+	dup("float32"),
+	dup("string"),
+
+	// arrays
+	dup("[10]int"),
+
+	// slices
+	dup("[]int"),
+	dup("[][]int"),
+
+	// structs
+	dup("struct{}"),
+	dup("struct{x int}"),
+	{`struct {
+		x, y int
+		z float32 "foo"
+	}`, `struct{x int; y int; z float32 "foo"}`},
+	{`struct {
+		string
+		elems []complex128
+	}`, `struct{string; elems []complex128}`},
+
+	// pointers
+	dup("*int"),
+	dup("***struct{}"),
+	dup("*struct{a int; b float32}"),
+
+	// functions
+	dup("func()"),
+	dup("func(x int)"),
+	{"func(x, y int)", "func(x int, y int)"},
+	{"func(x, y int, z string)", "func(x int, y int, z string)"},
+	dup("func(int)"),
+	{"func(int, string, byte)", "func(int, string, byte)"},
+
+	dup("func() int"),
+	{"func() (string)", "func() string"},
+	dup("func() (u int)"),
+	{"func() (u, v int, w string)", "func() (u int, v int, w string)"},
+
+	dup("func(int) string"),
+	dup("func(x int) string"),
+	dup("func(x int) (u string)"),
+	{"func(x, y int) (u string)", "func(x int, y int) (u string)"},
+
+	dup("func(...int) string"),
+	dup("func(x ...int) string"),
+	dup("func(x ...int) (u string)"),
+	{"func(x int, y ...int) (u string)", "func(x int, y ...int) (u string)"},
+
+	// interfaces
+	dup("interface{}"),
+	dup("interface{m()}"),
+	dup(`interface{String() string; m(int) float32}`),
+	dup(`interface{type int, float32, complex128}`),
+
+	// maps
+	dup("map[string]int"),
+	{"map[struct{x, y int}][]byte", "map[struct{x int; y int}][]byte"},
+
+	// channels
+	dup("chan<- chan int"),
+	dup("chan<- <-chan int"),
+	dup("<-chan <-chan int"),
+	dup("chan (<-chan int)"),
+	dup("chan<- func()"),
+	dup("<-chan []func() int"),
+}
+
+// types that depend on other type declarations (src in TestTypes)
+var dependentTestTypes = []testEntry{
+	// interfaces
+	dup(`interface{io.Reader; io.Writer}`),
+	dup(`interface{m() int; io.Writer}`),
+	{`interface{m() interface{T}}`, `interface{m() interface{p.T}}`},
+}
+
+func TestTypeString(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	var tests []testEntry
+	tests = append(tests, independentTestTypes...)
+	tests = append(tests, dependentTestTypes...)
+
+	for _, test := range tests {
+		src := `package p; import "io"; type _ io.Writer; type T ` + test.src
+		pkg, err := makePkg(src)
+		if err != nil {
+			t.Errorf("%s: %s", src, err)
+			continue
+		}
+		typ := pkg.Scope().Lookup("T").Type().Underlying()
+		if got := typ.String(); got != test.str {
+			t.Errorf("%s: got %s, want %s", test.src, got, test.str)
+		}
+	}
+}
+
+var nopos syntax.Pos
+
+func TestIncompleteInterfaces(t *testing.T) {
+	sig := NewSignature(nil, nil, nil, false)
+	m := NewFunc(nopos, nil, "m", sig)
+	for _, test := range []struct {
+		typ  *Interface
+		want string
+	}{
+		{new(Interface), "interface{/* incomplete */}"},
+		{new(Interface).Complete(), "interface{}"},
+
+		{NewInterface(nil, nil), "interface{}"},
+		{NewInterface(nil, nil).Complete(), "interface{}"},
+		{NewInterface([]*Func{}, nil), "interface{}"},
+		{NewInterface([]*Func{}, nil).Complete(), "interface{}"},
+		{NewInterface(nil, []*Named{}), "interface{}"},
+		{NewInterface(nil, []*Named{}).Complete(), "interface{}"},
+		{NewInterface([]*Func{m}, nil), "interface{m() /* incomplete */}"},
+		{NewInterface([]*Func{m}, nil).Complete(), "interface{m()}"},
+		{NewInterface(nil, []*Named{newDefined(new(Interface).Complete())}), "interface{T /* incomplete */}"},
+		{NewInterface(nil, []*Named{newDefined(new(Interface).Complete())}).Complete(), "interface{T}"},
+		{NewInterface(nil, []*Named{newDefined(NewInterface([]*Func{m}, nil))}), "interface{T /* incomplete */}"},
+		{NewInterface(nil, []*Named{newDefined(NewInterface([]*Func{m}, nil).Complete())}), "interface{T /* incomplete */}"},
+		{NewInterface(nil, []*Named{newDefined(NewInterface([]*Func{m}, nil).Complete())}).Complete(), "interface{T}"},
+
+		{NewInterfaceType(nil, nil), "interface{}"},
+		{NewInterfaceType(nil, nil).Complete(), "interface{}"},
+		{NewInterfaceType([]*Func{}, nil), "interface{}"},
+		{NewInterfaceType([]*Func{}, nil).Complete(), "interface{}"},
+		{NewInterfaceType(nil, []Type{}), "interface{}"},
+		{NewInterfaceType(nil, []Type{}).Complete(), "interface{}"},
+		{NewInterfaceType([]*Func{m}, nil), "interface{m() /* incomplete */}"},
+		{NewInterfaceType([]*Func{m}, nil).Complete(), "interface{m()}"},
+		{NewInterfaceType(nil, []Type{new(Interface).Complete()}), "interface{interface{} /* incomplete */}"},
+		{NewInterfaceType(nil, []Type{new(Interface).Complete()}).Complete(), "interface{interface{}}"},
+		{NewInterfaceType(nil, []Type{NewInterfaceType([]*Func{m}, nil)}), "interface{interface{m() /* incomplete */} /* incomplete */}"},
+		{NewInterfaceType(nil, []Type{NewInterfaceType([]*Func{m}, nil).Complete()}), "interface{interface{m()} /* incomplete */}"},
+		{NewInterfaceType(nil, []Type{NewInterfaceType([]*Func{m}, nil).Complete()}).Complete(), "interface{interface{m()}}"},
+	} {
+		got := test.typ.String()
+		if got != test.want {
+			t.Errorf("got: %s, want: %s", got, test.want)
+		}
+	}
+}
+
+// newDefined creates a new defined type named T with the given underlying type.
+// Helper function for use with TestIncompleteInterfaces only.
+func newDefined(underlying Type) *Named {
+	tname := NewTypeName(nopos, nil, "T", nil)
+	return NewNamed(tname, underlying, nil)
+}
+
+func TestQualifiedTypeString(t *testing.T) {
+	p, _ := pkgFor("p.go", "package p; type T int", nil)
+	q, _ := pkgFor("q.go", "package q", nil)
+
+	pT := p.Scope().Lookup("T").Type()
+	for _, test := range []struct {
+		typ  Type
+		this *Package
+		want string
+	}{
+		{nil, nil, "<nil>"},
+		{pT, nil, "p.T"},
+		{pT, p, "T"},
+		{pT, q, "p.T"},
+		{NewPointer(pT), p, "*T"},
+		{NewPointer(pT), q, "*p.T"},
+	} {
+		qualifier := func(pkg *Package) string {
+			if pkg != test.this {
+				return pkg.Name()
+			}
+			return ""
+		}
+		if got := TypeString(test.typ, qualifier); got != test.want {
+			t.Errorf("TypeString(%s, %s) = %s, want %s",
+				test.this, test.typ, got, test.want)
+		}
+	}
+}

src/cmd/compile/internal/types2/unify.go

@@ -0,0 +1,454 @@
+// UNREVIEWED
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements type unification.
+
+package types2
+
+import "sort"
+
+// The unifier maintains two separate sets of type parameters x and y
+// which are used to resolve type parameters in the x and y arguments
+// provided to the unify call. For unidirectional unification, only
+// one of these sets (say x) is provided, and then type parameters are
+// only resolved for the x argument passed to unify, not the y argument
+// (even if that also contains possibly the same type parameters). This
+// is crucial to infer the type parameters of self-recursive calls:
+//
+//	func f[type P](a P) { f(a) }
+//
+// For the call f(a) we want to infer that the type argument for P is P.
+// During unification, the parameter type P must be resolved to the type
+// parameter P ("x" side), but the argument type P must be left alone so
+// that unification resolves the type parameter P to P.
+//
+// For bidirection unification, both sets are provided. This enables
+// unification to go from argument to parameter type and vice versa.
+// For constraint type inference, we use bidirectional unification
+// where both the x and y type parameters are identical. This is done
+// by setting up one of them (using init) and then assigning its value
+// to the other.
+
+// A unifier maintains the current type parameters for x and y
+// and the respective types inferred for each type parameter.
+// A unifier is created by calling newUnifier.
+type unifier struct {
+	check *Checker
+	exact bool
+	x, y  tparamsList // x and y must initialized via tparamsList.init
+	types []Type      // inferred types, shared by x and y
+}
+
+// newUnifier returns a new unifier.
+// If exact is set, unification requires unified types to match
+// exactly. If exact is not set, a named type's underlying type
+// is considered if unification would fail otherwise, and the
+// direction of channels is ignored.
+func newUnifier(check *Checker, exact bool) *unifier {
+	u := &unifier{check: check, exact: exact}
+	u.x.unifier = u
+	u.y.unifier = u
+	return u
+}
+
+// unify attempts to unify x and y and reports whether it succeeded.
+func (u *unifier) unify(x, y Type) bool {
+	return u.nify(x, y, nil)
+}
+
+// A tparamsList describes a list of type parameters and the types inferred for them.
+type tparamsList struct {
+	unifier *unifier
+	tparams []*TypeName
+	// For each tparams element, there is a corresponding type slot index in indices.
+	// index  < 0: unifier.types[-index] == nil
+	// index == 0: no type slot allocated yet
+	// index  > 0: unifier.types[index] == typ
+	// Joined tparams elements share the same type slot and thus have the same index.
+	// By using a negative index for nil types we don't need to check unifier.types
+	// to see if we have a type or not.
+	indices []int // len(d.indices) == len(d.tparams)
+}
+
+// init initializes d with the given type parameters.
+// The type parameters must be in the order in which they appear in their declaration
+// (this ensures that the tparams indices match the respective type parameter index).
+func (d *tparamsList) init(tparams []*TypeName) {
+	if len(tparams) == 0 {
+		return
+	}
+	if debug {
+		for i, tpar := range tparams {
+			assert(i == tpar.typ.(*TypeParam).index)
+		}
+	}
+	d.tparams = tparams
+	d.indices = make([]int, len(tparams))
+}
+
+// join unifies the i'th type parameter of x with the j'th type parameter of y.
+// If both type parameters already have a type associated with them and they are
+// not joined, join fails and return false.
+func (u *unifier) join(i, j int) bool {
+	ti := u.x.indices[i]
+	tj := u.y.indices[j]
+	switch {
+	case ti == 0 && tj == 0:
+		// Neither type parameter has a type slot associated with them.
+		// Allocate a new joined nil type slot (negative index).
+		u.types = append(u.types, nil)
+		u.x.indices[i] = -len(u.types)
+		u.y.indices[j] = -len(u.types)
+	case ti == 0:
+		// The type parameter for x has no type slot yet. Use slot of y.
+		u.x.indices[i] = tj
+	case tj == 0:
+		// The type parameter for y has no type slot yet. Use slot of x.
+		u.y.indices[j] = ti
+
+	// Both type parameters have a slot: ti != 0 && tj != 0.
+	case ti == tj:
+		// Both type parameters already share the same slot. Nothing to do.
+		break
+	case ti > 0 && tj > 0:
+		// Both type parameters have (possibly different) inferred types. Cannot join.
+		return false
+	case ti > 0:
+		// Only the type parameter for x has an inferred type. Use x slot for y.
+		u.y.setIndex(j, ti)
+	// This case is handled like the default case.
+	// case tj > 0:
+	// 	// Only the type parameter for y has an inferred type. Use y slot for x.
+	// 	u.x.setIndex(i, tj)
+	default:
+		// Neither type parameter has an inferred type. Use y slot for x
+		// (or x slot for y, it doesn't matter).
+		u.x.setIndex(i, tj)
+	}
+	return true
+}
+
+// If typ is a type parameter of d, index returns the type parameter index.
+// Otherwise, the result is < 0.
+func (d *tparamsList) index(typ Type) int {
+	if t, ok := typ.(*TypeParam); ok {
+		if i := t.index; i < len(d.tparams) && d.tparams[i].typ == t {
+			return i
+		}
+	}
+	return -1
+}
+
+// setIndex sets the type slot index for the i'th type parameter
+// (and all its joined parameters) to tj. The type parameter
+// must have a (possibly nil) type slot associated with it.
+func (d *tparamsList) setIndex(i, tj int) {
+	ti := d.indices[i]
+	assert(ti != 0 && tj != 0)
+	for k, tk := range d.indices {
+		if tk == ti {
+			d.indices[k] = tj
+		}
+	}
+}
+
+// at returns the type set for the i'th type parameter; or nil.
+func (d *tparamsList) at(i int) Type {
+	if ti := d.indices[i]; ti > 0 {
+		return d.unifier.types[ti-1]
+	}
+	return nil
+}
+
+// set sets the type typ for the i'th type parameter;
+// typ must not be nil and it must not have been set before.
+func (d *tparamsList) set(i int, typ Type) {
+	assert(typ != nil)
+	u := d.unifier
+	switch ti := d.indices[i]; {
+	case ti < 0:
+		u.types[-ti-1] = typ
+		d.setIndex(i, -ti)
+	case ti == 0:
+		u.types = append(u.types, typ)
+		d.indices[i] = len(u.types)
+	default:
+		panic("type already set")
+	}
+}
+
+// types returns the list of inferred types (via unification) for the type parameters
+// described by d, and an index. If all types were inferred, the returned index is < 0.
+// Otherwise, it is the index of the first type parameter which couldn't be inferred;
+// i.e., for which list[index] is nil.
+func (d *tparamsList) types() (list []Type, index int) {
+	list = make([]Type, len(d.tparams))
+	index = -1
+	for i := range d.tparams {
+		t := d.at(i)
+		list[i] = t
+		if index < 0 && t == nil {
+			index = i
+		}
+	}
+	return
+}
+
+func (u *unifier) nifyEq(x, y Type, p *ifacePair) bool {
+	return x == y || u.nify(x, y, p)
+}
+
+// nify implements the core unification algorithm which is an
+// adapted version of Checker.identical0. For changes to that
+// code the corresponding changes should be made here.
+// Must not be called directly from outside the unifier.
+func (u *unifier) nify(x, y Type, p *ifacePair) bool {
+	// types must be expanded for comparison
+	x = expand(x)
+	y = expand(y)
+
+	if !u.exact {
+		// If exact unification is known to fail because we attempt to
+		// match a type name against an unnamed type literal, consider
+		// the underlying type of the named type.
+		// (Subtle: We use isNamed to include any type with a name (incl.
+		// basic types and type parameters. We use Named() because we only
+		// want *Named types.)
+		switch {
+		case !isNamed(x) && y != nil && y.Named() != nil:
+			return u.nify(x, y.Under(), p)
+		case x != nil && x.Named() != nil && !isNamed(y):
+			return u.nify(x.Under(), y, p)
+		}
+	}
+
+	// Cases where at least one of x or y is a type parameter.
+	switch i, j := u.x.index(x), u.y.index(y); {
+	case i >= 0 && j >= 0:
+		// both x and y are type parameters
+		if u.join(i, j) {
+			return true
+		}
+		// both x and y have an inferred type - they must match
+		return u.nifyEq(u.x.at(i), u.y.at(j), p)
+
+	case i >= 0:
+		// x is a type parameter, y is not
+		if tx := u.x.at(i); tx != nil {
+			return u.nifyEq(tx, y, p)
+		}
+		// otherwise, infer type from y
+		u.x.set(i, y)
+		return true
+
+	case j >= 0:
+		// y is a type parameter, x is not
+		if ty := u.y.at(j); ty != nil {
+			return u.nifyEq(x, ty, p)
+		}
+		// otherwise, infer type from x
+		u.y.set(j, x)
+		return true
+	}
+
+	// For type unification, do not shortcut (x == y) for identical
+	// types. Instead keep comparing them element-wise to unify the
+	// matching (and equal type parameter types). A simple test case
+	// where this matters is: func f[P any](a P) { f(a) } .
+
+	switch x := x.(type) {
+	case *Basic:
+		// Basic types are singletons except for the rune and byte
+		// aliases, thus we cannot solely rely on the x == y check
+		// above. See also comment in TypeName.IsAlias.
+		if y, ok := y.(*Basic); ok {
+			return x.kind == y.kind
+		}
+
+	case *Array:
+		// Two array types are identical if they have identical element types
+		// and the same array length.
+		if y, ok := y.(*Array); ok {
+			// If one or both array lengths are unknown (< 0) due to some error,
+			// assume they are the same to avoid spurious follow-on errors.
+			return (x.len < 0 || y.len < 0 || x.len == y.len) && u.nify(x.elem, y.elem, p)
+		}
+
+	case *Slice:
+		// Two slice types are identical if they have identical element types.
+		if y, ok := y.(*Slice); ok {
+			return u.nify(x.elem, y.elem, p)
+		}
+
+	case *Struct:
+		// Two struct types are identical if they have the same sequence of fields,
+		// and if corresponding fields have the same names, and identical types,
+		// and identical tags. Two embedded fields are considered to have the same
+		// name. Lower-case field names from different packages are always different.
+		if y, ok := y.(*Struct); ok {
+			if x.NumFields() == y.NumFields() {
+				for i, f := range x.fields {
+					g := y.fields[i]
+					if f.embedded != g.embedded ||
+						x.Tag(i) != y.Tag(i) ||
+						!f.sameId(g.pkg, g.name) ||
+						!u.nify(f.typ, g.typ, p) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Pointer:
+		// Two pointer types are identical if they have identical base types.
+		if y, ok := y.(*Pointer); ok {
+			return u.nify(x.base, y.base, p)
+		}
+
+	case *Tuple:
+		// Two tuples types are identical if they have the same number of elements
+		// and corresponding elements have identical types.
+		if y, ok := y.(*Tuple); ok {
+			if x.Len() == y.Len() {
+				if x != nil {
+					for i, v := range x.vars {
+						w := y.vars[i]
+						if !u.nify(v.typ, w.typ, p) {
+							return false
+						}
+					}
+				}
+				return true
+			}
+		}
+
+	case *Signature:
+		// Two function types are identical if they have the same number of parameters
+		// and result values, corresponding parameter and result types are identical,
+		// and either both functions are variadic or neither is. Parameter and result
+		// names are not required to match.
+		// TODO(gri) handle type parameters or document why we can ignore them.
+		if y, ok := y.(*Signature); ok {
+			return x.variadic == y.variadic &&
+				u.nify(x.params, y.params, p) &&
+				u.nify(x.results, y.results, p)
+		}
+
+	case *Sum:
+		// This should not happen with the current internal use of sum types.
+		panic("type inference across sum types not implemented")
+
+	case *Interface:
+		// Two interface types are identical if they have the same set of methods with
+		// the same names and identical function types. Lower-case method names from
+		// different packages are always different. The order of the methods is irrelevant.
+		if y, ok := y.(*Interface); ok {
+			// If identical0 is called (indirectly) via an external API entry point
+			// (such as Identical, IdenticalIgnoreTags, etc.), check is nil. But in
+			// that case, interfaces are expected to be complete and lazy completion
+			// here is not needed.
+			if u.check != nil {
+				u.check.completeInterface(nopos, x)
+				u.check.completeInterface(nopos, y)
+			}
+			a := x.allMethods
+			b := y.allMethods
+			if len(a) == len(b) {
+				// Interface types are the only types where cycles can occur
+				// that are not "terminated" via named types; and such cycles
+				// can only be created via method parameter types that are
+				// anonymous interfaces (directly or indirectly) embedding
+				// the current interface. Example:
+				//
+				//    type T interface {
+				//        m() interface{T}
+				//    }
+				//
+				// If two such (differently named) interfaces are compared,
+				// endless recursion occurs if the cycle is not detected.
+				//
+				// If x and y were compared before, they must be equal
+				// (if they were not, the recursion would have stopped);
+				// search the ifacePair stack for the same pair.
+				//
+				// This is a quadratic algorithm, but in practice these stacks
+				// are extremely short (bounded by the nesting depth of interface
+				// type declarations that recur via parameter types, an extremely
+				// rare occurrence). An alternative implementation might use a
+				// "visited" map, but that is probably less efficient overall.
+				q := &ifacePair{x, y, p}
+				for p != nil {
+					if p.identical(q) {
+						return true // same pair was compared before
+					}
+					p = p.prev
+				}
+				if debug {
+					assert(sort.IsSorted(byUniqueMethodName(a)))
+					assert(sort.IsSorted(byUniqueMethodName(b)))
+				}
+				for i, f := range a {
+					g := b[i]
+					if f.Id() != g.Id() || !u.nify(f.typ, g.typ, q) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Map:
+		// Two map types are identical if they have identical key and value types.
+		if y, ok := y.(*Map); ok {
+			return u.nify(x.key, y.key, p) && u.nify(x.elem, y.elem, p)
+		}
+
+	case *Chan:
+		// Two channel types are identical if they have identical value types.
+		if y, ok := y.(*Chan); ok {
+			return (!u.exact || x.dir == y.dir) && u.nify(x.elem, y.elem, p)
+		}
+
+	case *Named:
+		// Two named types are identical if their type names originate
+		// in the same type declaration.
+		// if y, ok := y.(*Named); ok {
+		// 	return x.obj == y.obj
+		// }
+		if y, ok := y.(*Named); ok {
+			// TODO(gri) This is not always correct: two types may have the same names
+			//           in the same package if one of them is nested in a function.
+			//           Extremely unlikely but we need an always correct solution.
+			if x.obj.pkg == y.obj.pkg && x.obj.name == y.obj.name {
+				assert(len(x.targs) == len(y.targs))
+				for i, x := range x.targs {
+					if !u.nify(x, y.targs[i], p) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *TypeParam:
+		// Two type parameters (which are not part of the type parameters of the
+		// enclosing type as those are handled in the beginning of this function)
+		// are identical if they originate in the same declaration.
+		return x == y
+
+	// case *instance:
+	//	unreachable since types are expanded
+
+	case nil:
+		// avoid a crash in case of nil type
+
+	default:
+		u.check.dump("### u.nify(%s, %s), u.x.tparams = %s", x, y, u.x.tparams)
+		unreachable()
+	}
+
+	return false
+}

src/cmd/compile/internal/types2/universe.go

@@ -0,0 +1,282 @@
+// UNREVIEWED
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file sets up the universe scope and the unsafe package.
+
+package types2
+
+import (
+	"go/constant"
+	"strings"
+)
+
+// The Universe scope contains all predeclared objects of Go.
+// It is the outermost scope of any chain of nested scopes.
+var Universe *Scope
+
+// The Unsafe package is the package returned by an importer
+// for the import path "unsafe".
+var Unsafe *Package
+
+var (
+	universeIota  *Const
+	universeByte  *Basic // uint8 alias, but has name "byte"
+	universeRune  *Basic // int32 alias, but has name "rune"
+	universeAny   *Named
+	universeError *Named
+)
+
+// Typ contains the predeclared *Basic types indexed by their
+// corresponding BasicKind.
+//
+// The *Basic type for Typ[Byte] will have the name "uint8".
+// Use Universe.Lookup("byte").Type() to obtain the specific
+// alias basic type named "byte" (and analogous for "rune").
+var Typ = []*Basic{
+	Invalid: {Invalid, 0, "invalid type", aType{}},
+
+	Bool:          {Bool, IsBoolean, "bool", aType{}},
+	Int:           {Int, IsInteger, "int", aType{}},
+	Int8:          {Int8, IsInteger, "int8", aType{}},
+	Int16:         {Int16, IsInteger, "int16", aType{}},
+	Int32:         {Int32, IsInteger, "int32", aType{}},
+	Int64:         {Int64, IsInteger, "int64", aType{}},
+	Uint:          {Uint, IsInteger | IsUnsigned, "uint", aType{}},
+	Uint8:         {Uint8, IsInteger | IsUnsigned, "uint8", aType{}},
+	Uint16:        {Uint16, IsInteger | IsUnsigned, "uint16", aType{}},
+	Uint32:        {Uint32, IsInteger | IsUnsigned, "uint32", aType{}},
+	Uint64:        {Uint64, IsInteger | IsUnsigned, "uint64", aType{}},
+	Uintptr:       {Uintptr, IsInteger | IsUnsigned, "uintptr", aType{}},
+	Float32:       {Float32, IsFloat, "float32", aType{}},
+	Float64:       {Float64, IsFloat, "float64", aType{}},
+	Complex64:     {Complex64, IsComplex, "complex64", aType{}},
+	Complex128:    {Complex128, IsComplex, "complex128", aType{}},
+	String:        {String, IsString, "string", aType{}},
+	UnsafePointer: {UnsafePointer, 0, "Pointer", aType{}},
+
+	UntypedBool:    {UntypedBool, IsBoolean | IsUntyped, "untyped bool", aType{}},
+	UntypedInt:     {UntypedInt, IsInteger | IsUntyped, "untyped int", aType{}},
+	UntypedRune:    {UntypedRune, IsInteger | IsUntyped, "untyped rune", aType{}},
+	UntypedFloat:   {UntypedFloat, IsFloat | IsUntyped, "untyped float", aType{}},
+	UntypedComplex: {UntypedComplex, IsComplex | IsUntyped, "untyped complex", aType{}},
+	UntypedString:  {UntypedString, IsString | IsUntyped, "untyped string", aType{}},
+	UntypedNil:     {UntypedNil, IsUntyped, "untyped nil", aType{}},
+}
+
+var aliases = [...]*Basic{
+	{Byte, IsInteger | IsUnsigned, "byte", aType{}},
+	{Rune, IsInteger, "rune", aType{}},
+}
+
+func defPredeclaredTypes() {
+	for _, t := range Typ {
+		def(NewTypeName(nopos, nil, t.name, t))
+	}
+	for _, t := range aliases {
+		def(NewTypeName(nopos, nil, t.name, t))
+	}
+
+	// any
+	// (Predeclared and entered into universe scope so we do all the
+	// usual checks; but removed again from scope later since it's
+	// only visible as constraint in a type parameter list.)
+	{
+		typ := &Named{underlying: &emptyInterface}
+		def(NewTypeName(nopos, nil, "any", typ))
+	}
+
+	// Error has a nil package in its qualified name since it is in no package
+	{
+		res := NewVar(nopos, nil, "", Typ[String])
+		sig := &Signature{results: NewTuple(res)}
+		err := NewFunc(nopos, nil, "Error", sig)
+		typ := &Named{underlying: NewInterfaceType([]*Func{err}, nil).Complete()}
+		sig.recv = NewVar(nopos, nil, "", typ)
+		def(NewTypeName(nopos, nil, "error", typ))
+	}
+}
+
+var predeclaredConsts = [...]struct {
+	name string
+	kind BasicKind
+	val  constant.Value
+}{
+	{"true", UntypedBool, constant.MakeBool(true)},
+	{"false", UntypedBool, constant.MakeBool(false)},
+	{"iota", UntypedInt, constant.MakeInt64(0)},
+}
+
+func defPredeclaredConsts() {
+	for _, c := range predeclaredConsts {
+		def(NewConst(nopos, nil, c.name, Typ[c.kind], c.val))
+	}
+}
+
+func defPredeclaredNil() {
+	def(&Nil{object{name: "nil", typ: Typ[UntypedNil], color_: black}})
+}
+
+// A builtinId is the id of a builtin function.
+type builtinId int
+
+const (
+	// universe scope
+	_Append builtinId = iota
+	_Cap
+	_Close
+	_Complex
+	_Copy
+	_Delete
+	_Imag
+	_Len
+	_Make
+	_New
+	_Panic
+	_Print
+	_Println
+	_Real
+	_Recover
+
+	// package unsafe
+	_Alignof
+	_Offsetof
+	_Sizeof
+
+	// testing support
+	_Assert
+	_Trace
+)
+
+var predeclaredFuncs = [...]struct {
+	name     string
+	nargs    int
+	variadic bool
+	kind     exprKind
+}{
+	_Append:  {"append", 1, true, expression},
+	_Cap:     {"cap", 1, false, expression},
+	_Close:   {"close", 1, false, statement},
+	_Complex: {"complex", 2, false, expression},
+	_Copy:    {"copy", 2, false, statement},
+	_Delete:  {"delete", 2, false, statement},
+	_Imag:    {"imag", 1, false, expression},
+	_Len:     {"len", 1, false, expression},
+	_Make:    {"make", 1, true, expression},
+	_New:     {"new", 1, false, expression},
+	_Panic:   {"panic", 1, false, statement},
+	_Print:   {"print", 0, true, statement},
+	_Println: {"println", 0, true, statement},
+	_Real:    {"real", 1, false, expression},
+	_Recover: {"recover", 0, false, statement},
+
+	_Alignof:  {"Alignof", 1, false, expression},
+	_Offsetof: {"Offsetof", 1, false, expression},
+	_Sizeof:   {"Sizeof", 1, false, expression},
+
+	_Assert: {"assert", 1, false, statement},
+	_Trace:  {"trace", 0, true, statement},
+}
+
+func defPredeclaredFuncs() {
+	for i := range predeclaredFuncs {
+		id := builtinId(i)
+		if id == _Assert || id == _Trace {
+			continue // only define these in testing environment
+		}
+		def(newBuiltin(id))
+	}
+}
+
+// DefPredeclaredTestFuncs defines the assert and trace built-ins.
+// These built-ins are intended for debugging and testing of this
+// package only.
+func DefPredeclaredTestFuncs() {
+	if Universe.Lookup("assert") != nil {
+		return // already defined
+	}
+	def(newBuiltin(_Assert))
+	def(newBuiltin(_Trace))
+}
+
+func defPredeclaredComparable() {
+	// The "comparable" interface can be imagined as defined like
+	//
+	// type comparable interface {
+	//         == () untyped bool
+	//         != () untyped bool
+	// }
+	//
+	// == and != cannot be user-declared but we can declare
+	// a magic method == and check for its presence when needed.
+
+	// Define interface { == () }. We don't care about the signature
+	// for == so leave it empty except for the receiver, which is
+	// set up later to match the usual interface method assumptions.
+	sig := new(Signature)
+	eql := NewFunc(nopos, nil, "==", sig)
+	iface := NewInterfaceType([]*Func{eql}, nil).Complete()
+
+	// set up the defined type for the interface
+	obj := NewTypeName(nopos, nil, "comparable", nil)
+	named := NewNamed(obj, iface, nil)
+	obj.color_ = black
+	sig.recv = NewVar(nopos, nil, "", named) // complete == signature
+
+	def(obj)
+}
+
+func init() {
+	Universe = NewScope(nil, nopos, nopos, "universe")
+	Unsafe = NewPackage("unsafe", "unsafe")
+	Unsafe.complete = true
+
+	defPredeclaredTypes()
+	defPredeclaredConsts()
+	defPredeclaredNil()
+	defPredeclaredFuncs()
+	defPredeclaredComparable()
+
+	universeIota = Universe.Lookup("iota").(*Const)
+	universeByte = Universe.Lookup("byte").(*TypeName).typ.(*Basic)
+	universeRune = Universe.Lookup("rune").(*TypeName).typ.(*Basic)
+	universeAny = Universe.Lookup("any").(*TypeName).typ.(*Named)
+	universeError = Universe.Lookup("error").(*TypeName).typ.(*Named)
+
+	// "any" is only visible as constraint in a type parameter list
+	delete(Universe.elems, "any")
+}
+
+// Objects with names containing blanks are internal and not entered into
+// a scope. Objects with exported names are inserted in the unsafe package
+// scope; other objects are inserted in the universe scope.
+//
+func def(obj Object) {
+	assert(obj.color() == black)
+	name := obj.Name()
+	if strings.Contains(name, " ") {
+		return // nothing to do
+	}
+	// fix Obj link for named types
+	if typ := obj.Type().Named(); typ != nil {
+		typ.obj = obj.(*TypeName)
+	}
+	// exported identifiers go into package unsafe
+	scope := Universe
+	if obj.Exported() {
+		scope = Unsafe.scope
+		// set Pkg field
+		switch obj := obj.(type) {
+		case *TypeName:
+			obj.pkg = Unsafe
+		case *Builtin:
+			obj.pkg = Unsafe
+		default:
+			unreachable()
+		}
+	}
+	if scope.Insert(obj) != nil {
+		panic("internal error: double declaration")
+	}
+}

src/cmd/compile/internal/types2/walk.go

@@ -0,0 +1,322 @@
+// UNREVIEWED
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements syntax tree walking.
+// TODO(gri) A more general API should probably be in
+//           the syntax package.
+
+package types2
+
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+)
+
+// Walk traverses a syntax in pre-order: It starts by calling f(root);
+// root must not be nil. If f returns false (== "continue"), Walk calls
+// f recursively for each of the non-nil children of that node; if f
+// returns true (== "stop"), Walk does not traverse the respective node's
+// children.
+// Some nodes may be shared among multiple parent nodes (e.g., types in
+// field lists such as type T in "a, b, c T"). Such shared nodes are
+// walked multiple times.
+// TODO(gri) Revisit this design. It may make sense to walk those nodes
+//           only once. A place where this matters is TestResolveIdents.
+func Walk(root syntax.Node, f func(syntax.Node) bool) {
+	w := walker{f}
+	w.node(root)
+}
+
+type walker struct {
+	f func(syntax.Node) bool
+}
+
+func (w *walker) node(n syntax.Node) {
+	if n == nil {
+		panic("invalid syntax tree: nil node")
+	}
+
+	if w.f(n) {
+		return
+	}
+
+	switch n := n.(type) {
+	// packages
+	case *syntax.File:
+		w.node(n.PkgName)
+		w.declList(n.DeclList)
+
+	// declarations
+	case *syntax.ImportDecl:
+		if n.LocalPkgName != nil {
+			w.node(n.LocalPkgName)
+		}
+		w.node(n.Path)
+
+	case *syntax.ConstDecl:
+		w.nameList(n.NameList)
+		if n.Type != nil {
+			w.node(n.Type)
+		}
+		if n.Values != nil {
+			w.node(n.Values)
+		}
+
+	case *syntax.TypeDecl:
+		w.node(n.Name)
+		w.fieldList(n.TParamList)
+		w.node(n.Type)
+
+	case *syntax.VarDecl:
+		w.nameList(n.NameList)
+		if n.Type != nil {
+			w.node(n.Type)
+		}
+		if n.Values != nil {
+			w.node(n.Values)
+		}
+
+	case *syntax.FuncDecl:
+		if n.Recv != nil {
+			w.node(n.Recv)
+		}
+		w.node(n.Name)
+		w.fieldList(n.TParamList)
+		w.node(n.Type)
+		if n.Body != nil {
+			w.node(n.Body)
+		}
+
+	// expressions
+	case *syntax.BadExpr: // nothing to do
+	case *syntax.Name:
+	case *syntax.BasicLit: // nothing to do
+
+	case *syntax.CompositeLit:
+		if n.Type != nil {
+			w.node(n.Type)
+		}
+		w.exprList(n.ElemList)
+
+	case *syntax.KeyValueExpr:
+		w.node(n.Key)
+		w.node(n.Value)
+
+	case *syntax.FuncLit:
+		w.node(n.Type)
+		w.node(n.Body)
+
+	case *syntax.ParenExpr:
+		w.node(n.X)
+
+	case *syntax.SelectorExpr:
+		w.node(n.X)
+		w.node(n.Sel)
+
+	case *syntax.IndexExpr:
+		w.node(n.X)
+		w.node(n.Index)
+
+	case *syntax.SliceExpr:
+		w.node(n.X)
+		for _, x := range n.Index {
+			if x != nil {
+				w.node(x)
+			}
+		}
+
+	case *syntax.AssertExpr:
+		w.node(n.X)
+		w.node(n.Type)
+
+	case *syntax.TypeSwitchGuard:
+		if n.Lhs != nil {
+			w.node(n.Lhs)
+		}
+		w.node(n.X)
+
+	case *syntax.Operation:
+		w.node(n.X)
+		if n.Y != nil {
+			w.node(n.Y)
+		}
+
+	case *syntax.CallExpr:
+		w.node(n.Fun)
+		w.exprList(n.ArgList)
+
+	case *syntax.ListExpr:
+		w.exprList(n.ElemList)
+
+	// types
+	case *syntax.ArrayType:
+		if n.Len != nil {
+			w.node(n.Len)
+		}
+		w.node(n.Elem)
+
+	case *syntax.SliceType:
+		w.node(n.Elem)
+
+	case *syntax.DotsType:
+		w.node(n.Elem)
+
+	case *syntax.StructType:
+		w.fieldList(n.FieldList)
+		for _, t := range n.TagList {
+			if t != nil {
+				w.node(t)
+			}
+		}
+
+	case *syntax.Field:
+		if n.Name != nil {
+			w.node(n.Name)
+		}
+		w.node(n.Type)
+
+	case *syntax.InterfaceType:
+		w.fieldList(n.MethodList)
+
+	case *syntax.FuncType:
+		w.fieldList(n.ParamList)
+		w.fieldList(n.ResultList)
+
+	case *syntax.MapType:
+		w.node(n.Key)
+		w.node(n.Value)
+
+	case *syntax.ChanType:
+		w.node(n.Elem)
+
+	// statements
+	case *syntax.EmptyStmt: // nothing to do
+
+	case *syntax.LabeledStmt:
+		w.node(n.Label)
+		w.node(n.Stmt)
+
+	case *syntax.BlockStmt:
+		w.stmtList(n.List)
+
+	case *syntax.ExprStmt:
+		w.node(n.X)
+
+	case *syntax.SendStmt:
+		w.node(n.Chan)
+		w.node(n.Value)
+
+	case *syntax.DeclStmt:
+		w.declList(n.DeclList)
+
+	case *syntax.AssignStmt:
+		w.node(n.Lhs)
+		w.node(n.Rhs)
+
+	case *syntax.BranchStmt:
+		if n.Label != nil {
+			w.node(n.Label)
+		}
+		// Target points to nodes elsewhere in the syntax tree
+
+	case *syntax.CallStmt:
+		w.node(n.Call)
+
+	case *syntax.ReturnStmt:
+		if n.Results != nil {
+			w.node(n.Results)
+		}
+
+	case *syntax.IfStmt:
+		if n.Init != nil {
+			w.node(n.Init)
+		}
+		w.node(n.Cond)
+		w.node(n.Then)
+		if n.Else != nil {
+			w.node(n.Else)
+		}
+
+	case *syntax.ForStmt:
+		if n.Init != nil {
+			w.node(n.Init)
+		}
+		if n.Cond != nil {
+			w.node(n.Cond)
+		}
+		if n.Post != nil {
+			w.node(n.Post)
+		}
+		w.node(n.Body)
+
+	case *syntax.SwitchStmt:
+		if n.Init != nil {
+			w.node(n.Init)
+		}
+		if n.Tag != nil {
+			w.node(n.Tag)
+		}
+		for _, s := range n.Body {
+			w.node(s)
+		}
+
+	case *syntax.SelectStmt:
+		for _, s := range n.Body {
+			w.node(s)
+		}
+
+	// helper nodes
+	case *syntax.RangeClause:
+		if n.Lhs != nil {
+			w.node(n.Lhs)
+		}
+		w.node(n.X)
+
+	case *syntax.CaseClause:
+		if n.Cases != nil {
+			w.node(n.Cases)
+		}
+		w.stmtList(n.Body)
+
+	case *syntax.CommClause:
+		if n.Comm != nil {
+			w.node(n.Comm)
+		}
+		w.stmtList(n.Body)
+
+	default:
+		panic(fmt.Sprintf("internal error: unknown node type %T", n))
+	}
+}
+
+func (w *walker) declList(list []syntax.Decl) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) exprList(list []syntax.Expr) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) stmtList(list []syntax.Stmt) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) nameList(list []*syntax.Name) {
+	for _, n := range list {
+		w.node(n)
+	}
+}
+
+func (w *walker) fieldList(list []*syntax.Field) {
+	for _, n := range list {
+		w.node(n)
+	}
+}