gc: suppress non-root cause message for bad receivers.

Fixed issue 2500

R=rsc
CC=golang-dev
https://golang.org/cl/5485053

src/cmd/gc/dcl.c

@@ -1273,6 +1273,8 @@ addmethod(Sym *sf, Type *t, int local)
 				t = t->type;
 			}
 		}
+		if(t->broke) // rely on typecheck having complained before
+			return;
 		if(t != T) {
 			if(t->sym == S) {
 				yyerror("invalid receiver type %T (%T is an unnamed type)", pa, t);

test/fixedbugs/bug384.go

+// errchk $G $D/$F.go
+
+// 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.
+
+// Issue 2500
+
+package foo
+
+// Check that we only get root cause message, no further complaints about r undefined
+func (r *indexWriter) foo() {}  // ERROR "undefined.*indexWriter"