cmd/compile: implement package-level aliases (no export yet)

Requires -newparser=1.

For #17487.
For #16339.

Change-Id: I156fb0c0f8a97e8c72dbbfbd7fe821efee12b957
Reviewed-on: https://go-review.googlesource.com/31597
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/main.go

@@ -943,6 +943,8 @@ func mkpackage(pkgname string) {
 					s.Def.Name.Pack.Used = true
 				}
 
+				// TODO(gri) This will also affect exported aliases.
+				// Need to fix this.
 				s.Def = nil
 				continue
 			}

src/cmd/compile/internal/gc/noder.go

@@ -61,7 +61,7 @@ func (p *noder) decls(decls []syntax.Decl) (l []*Node) {
 			p.importDecl(decl)
 
 		case *syntax.AliasDecl:
-			yyerror("alias declarations not yet implemented")
+			p.aliasDecl(decl)
 
 		case *syntax.VarDecl:
 			l = append(l, p.varDecl(decl)...)
@@ -90,8 +90,9 @@ func (p *noder) decls(decls []syntax.Decl) (l []*Node) {
 			lastConstGroup = decl.Group
 
 		case *syntax.TypeDecl:
+			// TODO(gri) remove this notation - we're not going to use it after all
 			if decl.Alias {
-				yyerror("alias declarations not yet implemented")
+				yyerror("type aliases using = not supported")
 				break
 			}
 			l = append(l, p.typeDecl(decl))
@@ -153,6 +154,92 @@ func (p *noder) importDecl(imp *syntax.ImportDecl) {
 	my.Block = 1 // at top level
 }
 
+func (p *noder) aliasDecl(decl *syntax.AliasDecl) {
+	// Because alias declarations must refer to imported entities
+	// which are already set up, we can do all checks right here.
+	// We won't know anything about entities that have not been
+	// declared yet, but since they cannot have been imported, we
+	// know there's an error and we don't care about the details.
+
+	// The original entity must be denoted by a qualified identifier.
+	// (The parser doesn't make this restriction to be more error-
+	// tolerant.)
+	qident, ok := decl.Orig.(*syntax.SelectorExpr)
+	if !ok {
+		// TODO(gri) This prints a dot-imported object with qualification
+		//           (confusing error). Fix this.
+		yyerror("invalid alias: %v is not a package-qualified identifier", p.expr(decl.Orig))
+		return
+	}
+
+	pkg := p.expr(qident.X)
+	if pkg.Op != OPACK {
+		yyerror("invalid alias: %v is not a package", pkg)
+		return
+	}
+	pkg.Used = true
+
+	orig := oldname(restrictlookup(qident.Sel.Value, pkg.Name.Pkg))
+
+	// An alias declaration must not refer to package unsafe.
+	if orig.Sym.Pkg == unsafepkg {
+		yyerror("invalid alias: %v refers to package unsafe (%v)", decl.Name.Value, orig)
+		return
+	}
+
+	// The aliased entity must be from a matching constant, type, variable,
+	// or function declaration, respectively.
+	var what string
+	switch decl.Tok {
+	case syntax.Const:
+		if orig.Op != OLITERAL {
+			what = "constant"
+		}
+	case syntax.Type:
+		if orig.Op != OTYPE {
+			what = "type"
+		}
+	case syntax.Var:
+		if orig.Op != ONAME || orig.Class != PEXTERN {
+			what = "variable"
+		}
+	case syntax.Func:
+		if orig.Op != ONAME || orig.Class != PFUNC {
+			what = "function"
+		}
+	default:
+		Fatalf("unexpected token: %s", decl.Tok)
+	}
+	if what != "" {
+		yyerror("invalid alias: %v is not a %s", orig, what)
+		return
+	}
+
+	// don't declare blank aliases
+	if decl.Name.Value == "_" {
+		return
+	}
+
+	// declare alias
+	// (this is similar to handling dot imports)
+	asym := p.name(decl.Name)
+	if asym.Def != nil {
+		redeclare(asym, "in alias declaration")
+		return
+	}
+	asym.Def = orig
+	asym.Block = block
+	asym.Lastlineno = lineno
+
+	if exportname(asym.Name) {
+		yyerror("cannot export alias %v: not yet implemented", asym)
+		// TODO(gri) newname(asym) is only needed to satisfy exportsym
+		// (and indirectly, exportlist). We should be able to just
+		// collect the Syms, eventually.
+		// exportsym(newname(asym))
+	}
+}
+
 func (p *noder) varDecl(decl *syntax.VarDecl) []*Node {
 	names := p.declNames(decl.NameList)
 

src/go/types/stdlib_test.go

@@ -140,6 +140,7 @@ func TestStdTest(t *testing.T) {
 	}
 
 	testTestDir(t, filepath.Join(runtime.GOROOT(), "test"),
+		"alias2.go",      // excluded until we can handle alias declarations
 		"cmplxdivide.go", // also needs file cmplxdivide1.go - ignore
 		"sigchld.go",     // don't work on Windows; testTestDir should consult build tags
 	)

test/alias2.go

@@ -0,0 +1,96 @@
+// errorcheck -newparser=1
+
+// 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.
+
+// Test basic restrictions on alias declarations.
+
+package p
+
+import (
+	"fmt" // use at most once (to test "imported but not used" error)
+	"go/build"
+	. "go/build"
+	"io"
+	"math"
+	"unsafe"
+)
+
+// helper
+var before struct {
+	f
+}
+
+// aliases must refer to package-qualified identifiers
+// TODO(gri) should only see one error for declaration below - fix this
+const _ => 0 // ERROR "unexpected literal 0|_ is not a package-qualified identifier"
+
+type _ => _ // ERROR "_ is not a package-qualified identifier"
+type t => _ // ERROR "_ is not a package-qualified identifier"
+
+const _ => iota // ERROR "iota is not a package-qualified identifier"
+type _ => int   // ERROR "int is not a package-qualified identifier"
+
+const c => iota // ERROR "iota is not a package-qualified identifier"
+type t => int   // ERROR "int is not a package-qualified identifier"
+
+// dot-imported identifiers are not qualified identifiers
+// TODO(gri) fix error printing - should not print a qualified identifier...
+var _ => Default // ERROR "build\.Default is not a package-qualified identifier"
+
+// qualified identifiers must start with a package
+var _ => before.f  // ERROR "before is not a package"
+func _ => before.f // ERROR "before is not a package"
+var _ => after.m   // ERROR "after is not a package"
+func _ => after.m  // ERROR "after is not a package"
+
+var v => before.f  // ERROR "before is not a package"
+func f => before.f // ERROR "before is not a package"
+var v => after.m   // ERROR "after is not a package"
+func f => after.m  // ERROR "after is not a package"
+
+// TODO(gri) fix error printing - should not print a qualified identifier...
+var _ => Default.ARCH // ERROR "build.Default is not a package"
+
+// aliases may not refer to package unsafe
+type ptr => unsafe.Pointer // ERROR "ptr refers to package unsafe"
+func size => unsafe.Sizeof // ERROR "size refers to package unsafe"
+
+// aliases must refer to entities of the same kind
+const _ => math.Pi
+const pi => math.Pi
+const pi1 => math.Sin // ERROR "math.Sin is not a constant"
+
+type _ => io.Writer
+type writer => io.Writer
+type writer1 => math.Sin // ERROR "math.Sin is not a type"
+
+var _ => build.Default
+var def => build.Default
+var def1 => build.Import // ERROR "build.Import is not a variable"
+
+func _ => math.Sin
+func sin => math.Sin
+func sin1 => math.Pi // ERROR "math.Pi is not a function"
+
+// alias reference to a package marks package as used
+func _ => fmt.Println
+
+// TODO(gri) aliased cannot be exported yet - fix this
+const Pi => math.Pi      // ERROR "cannot export alias Pi"
+type Writer => io.Writer // ERROR "cannot export alias Writer"
+var Def => build.Default // ERROR "cannot export alias Def"
+func Sin => math.Sin     // ERROR "cannot export alias Sin"
+
+// type aliases denote identical types
+type myPackage => build.Package
+
+var pkg myPackage
+var _ build.Package = pkg   // valid assignment
+var _ *build.Package = &pkg // valid assignment
+
+// helper
+type after struct{}
+
+func (after) m() {}