[dev.typealias] go/types: improved Object printing

- added internal isAlias predicated and test
- use it for improved Object printing
- when printing a basic type object, don't repeat type name
  (i.e., print "type int" rather than "type int int")
- added another test to testdata/decls4.src

For #18130.

Change-Id: Ice9517c0065a2cc465c6d12f87cd27c01ef801e6
Reviewed-on: https://go-review.googlesource.com/35093
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/example_test.go

@@ -239,10 +239,10 @@ func fib(x int) int {
 	// type S string:
 	//   defined at fib.go:4:6
 	//   used at 6:23
-	// type int int:
+	// type int:
 	//   defined at -
 	//   used at 8:12, 8:17
-	// type string string:
+	// type string:
 	//   defined at -
 	//   used at 4:8
 	// var b S:

src/go/types/object.go

@@ -163,6 +163,30 @@ func NewTypeName(pos token.Pos, pkg *Package, name string, typ Type) *TypeName {
 	return &TypeName{object{nil, pos, pkg, name, typ, 0, token.NoPos}}
 }
 
+func (obj *TypeName) isAlias() bool {
+	switch t := obj.typ.(type) {
+	case nil:
+		return false
+	case *Basic:
+		// It would seem that we should be able to look for different names here;
+		// but the names of universeByte/Rune are "byte" and "rune", respectively.
+		// We do this so that we get better error messages. However, general alias
+		// types don't have that name information and thus behave differently when
+		// reporting errors (we won't see the alias name, only the original name).
+		// Maybe we should remove the special handling for the predeclared types
+		// as well to be consistent (at the cost of slightly less clear error
+		// messages when byte/rune are involved).
+		// This also plays out in the implementation of the Identical(Type, Type)
+		// predicate.
+		// TODO(gri) consider possible clean up
+		return 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
@@ -242,7 +266,9 @@ type Nil struct {
 }
 
 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())
@@ -255,8 +281,8 @@ func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
 		buf.WriteString("const")
 
 	case *TypeName:
+		tname = obj
 		buf.WriteString("type")
-		typ = typ.Underlying()
 
 	case *Var:
 		if obj.isField {
@@ -297,12 +323,26 @@ func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
 	}
 	buf.WriteString(obj.Name())
 
-	// TODO(gri) indicate type alias if we have one
+	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.Underlying()
+		}
+	}
 
-	if typ != nil {
 	buf.WriteByte(' ')
 	WriteType(buf, typ, qf)
-	}
 }
 
 func writePackage(buf *bytes.Buffer, pkg *Package, qf Qualifier) {

src/go/types/object_test.go

@@ -0,0 +1,36 @@
+// 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 types
+
+import "testing"
+
+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
+	for _, name := range Universe.Names() {
+		if obj, _ := Universe.Lookup(name).(*TypeName); obj != nil {
+			check(obj, name == "byte" || name == "rune")
+		}
+	}
+
+	// various other types
+	t0 := NewTypeName(0, nil, "t0", nil)
+	check(t0, false) // no type yet
+
+	t1 := NewTypeName(0, nil, "t1", nil)
+	n1 := NewNamed(t1, new(Struct), nil)
+	check(t1, false) // type name refers to named type and vice versa
+
+	t2 := NewTypeName(0, nil, "t2", new(Interface))
+	check(t2, true) // type name refers to unnamed type
+
+	t3 := NewTypeName(0, nil, "t3", n1)
+	check(t3, true) // type name refers to named type with different type name (true alias)
+}

src/go/types/predicates.go

@@ -139,7 +139,7 @@ func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 	case *Basic:
 		// Basic types are singletons except for the rune and byte
 		// aliases, thus we cannot solely rely on the x == y check
-		// above.
+		// above. See also comment in TypeName.isAlias.
 		if y, ok := y.(*Basic); ok {
 			return x.kind == y.kind
 		}

src/go/types/testdata/decls4.src

@@ -63,6 +63,7 @@ func (Ai /* ERROR "invalid receiver" */) m1() {}
 func (T0) m1() {}
 func (A0) m1 /* ERROR already declared */ () {}
 func (A0) m2 () {}
+func (A3 /* ERROR invalid receiver */ ) m1 () {}
 func (A10 /* ERROR invalid receiver */ ) m1() {}
 
 // x0 has methods m1, m2 declared via receiver type names T0 and A0