[dev.typealias] go/ast, go/parser, go/printer, go/types: initial type alias support

Parsing and printing support for type aliases complete.
go/types recognizes them an issues an "unimplemented" error for now.

For #18130.

Change-Id: I9f2f7b1971b527276b698d9347bcd094ef0012ee
Reviewed-on: https://go-review.googlesource.com/34986
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/ast/ast.go

@@ -848,6 +848,7 @@ type (
 	TypeSpec struct {
 		Doc     *CommentGroup // associated documentation; or nil
 		Name    *Ident        // type name
+		Assign  token.Pos     // position of '=', if any
 		Type    Expr          // *Ident, *ParenExpr, *SelectorExpr, *StarExpr, or any of the *XxxTypes
 		Comment *CommentGroup // line comments; or nil
 	}

src/go/parser/parser.go

@@ -2327,7 +2327,10 @@ func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.
 	// (Global identifiers are resolved in a separate phase after parsing.)
 	spec := &ast.TypeSpec{Doc: doc, Name: ident}
 	p.declare(spec, nil, p.topScope, ast.Typ, ident)
-
+	if p.tok == token.ASSIGN {
+		spec.Assign = p.pos
+		p.next()
+	}
 	spec.Type = p.parseType()
 	p.expectSemi() // call before accessing p.linecomment
 	spec.Comment = p.lineComment

src/go/parser/short_test.go

@@ -46,6 +46,8 @@ var valids = []string{
 	`package p; const (x = 0; y; z)`, // issue 9639
 	`package p; var _ = map[P]int{P{}:0, {}:1}`,
 	`package p; var _ = map[*P]int{&P{}:0, {}:1}`,
+	`package p; type T = int`,
+	`package p; type (T = p.T; _ = struct{}; x = *T)`,
 }
 
 func TestValid(t *testing.T) {

src/go/printer/nodes.go

@@ -1445,6 +1445,9 @@ func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
 		} else {
 			p.print(vtab)
 		}
+		if s.Assign.IsValid() {
+			p.print(token.ASSIGN, blank)
+		}
 		p.expr(s.Type)
 		p.setComment(s.Comment)
 

src/go/printer/testdata/declarations.golden

@@ -985,3 +985,18 @@ func _(struct {
 	x	int
 	y	int
 })	// no extra comma between } and )
+
+// alias declarations
+
+type c0 struct{}
+type c1 = C
+type c2 = struct{ x int }
+type c3 = p.C
+type (
+	s	struct{}
+	a	= A
+	b	= A
+	c	= foo
+	d	= interface{}
+	ddd	= p.Foo
+)

src/go/printer/testdata/declarations.input

@@ -999,3 +999,18 @@ func _(struct {
 	x int
 	y int
 }) // no extra comma between } and )
+
+// alias declarations
+
+type c0 struct{}
+type c1 = C
+type c2 = struct{ x int}
+type c3 = p.C
+type (
+	s struct{}
+	a = A
+	b = A
+	c = foo
+	d = interface{}
+	ddd = p.Foo
+)

src/go/types/decl.go

@@ -534,6 +534,9 @@ func (check *Checker) declStmt(decl ast.Decl) {
 				}
 
 			case *ast.TypeSpec:
+				if s.Assign.IsValid() {
+					check.errorf(s.Assign, "type alias declarations not yet implemented")
+				}
 				obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Name, 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

src/go/types/resolver.go

@@ -346,6 +346,9 @@ func (check *Checker) collectObjects() {
 						}
 
 					case *ast.TypeSpec:
+						if s.Assign.IsValid() {
+							check.errorf(s.Assign, "type alias declarations not yet implemented")
+						}
 						obj := NewTypeName(s.Name.Pos(), pkg, s.Name.Name, nil)
 						check.declarePkgObj(s.Name, obj, &declInfo{file: fileScope, typ: s.Type})
 

src/go/types/testdata/decls0.src

@@ -208,3 +208,11 @@ func (BlankT) _() {}
 func (BlankT) _(int) {}
 func (BlankT) _() int { return 0 }
 func (BlankT) _(int) int { return 0}
+
+// type alias declarations
+// TODO(gri) complete this
+type (
+	__ = /* ERROR not yet implemented */ int
+	a0 = /* ERROR not yet implemented */ int
+	a1 = /* ERROR not yet implemented */ struct{}
+)