cmd/gc: squelch spurious "invalid recursive type" error

R=ken2
CC=golang-dev
https://golang.org/cl/13512047

src/cmd/gc/align.c

@@ -119,7 +119,8 @@ dowidth(Type *t)
 	if(t->width == -2) {
 		lno = lineno;
 		lineno = t->lineno;
-		yyerror("invalid recursive type %T", t);
+		if(!t->broke)
+			yyerror("invalid recursive type %T", t);
 		t->width = 0;
 		lineno = lno;
 		return;
@@ -219,7 +220,8 @@ dowidth(Type *t)
 		checkwidth(t->down);
 		break;
 	case TFORW:		// should have been filled in
-		yyerror("invalid recursive type %T", t);
+		if(!t->broke)
+			yyerror("invalid recursive type %T", t);
 		w = 1;	// anything will do
 		break;
 	case TANY:

src/cmd/gc/typecheck.c

@@ -3046,7 +3046,7 @@ queuemethod(Node *n)
 Node*
 typecheckdef(Node *n)
 {
-	int lno;
+	int lno, nerrors0;
 	Node *e;
 	Type *t;
 	NodeList *l;
@@ -3174,7 +3174,13 @@ typecheckdef(Node *n)
 		n->walkdef = 1;
 		n->type = typ(TFORW);
 		n->type->sym = n->sym;
+		nerrors0 = nerrors;
 		typecheckdeftype(n);
+		if(n->type->etype == TFORW && nerrors > nerrors0) {
+			// Something went wrong during type-checking,
+			// but it was reported. Silence future errors.
+			n->type->broke = 1;
+		}
 		if(curfn)
 			resumecheckwidth();
 		break;

test/fixedbugs/issue5581.go

+// errorcheck
+
+// Used to emit a spurious "invalid recursive type" error.
+// See golang.org/issue/5581.
+
+// 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 main
+
+import "fmt"
+
+func NewBar() *Bar { return nil }
+
+func (x *Foo) Method() (int, error) {
+	for y := range x.m {
+		_ = y.A
+	}
+	return 0, nil
+}
+
+type Foo struct {
+	m map[*Bar]int
+}
+
+type Bar struct {
+	A *Foo
+	B chan Blah // ERROR "undefined: Blah"
+}
+
+func main() {
+	fmt.Println("Hello, playground")
+}