cmd/gc: fix confusing error with broken types and defer/go

Fixes #5172.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/9614044

src/cmd/gc/typecheck.c

@@ -1793,6 +1793,11 @@ checkdefergo(Node *n)
 		break;
 	default:
 	conv:
+		// type is broken or missing, most likely a method call on a broken type
+		// we will warn about the broken type elsewhere. no need to emit a potentially confusing error
+		if(n->left->type == T || n->left->type->broke)
+			break;
+
 		if(!n->diag) {
 			// The syntax made sure it was a call, so this must be
 			// a conversion.

test/fixedbugs/issue5172.go

+// errorcheck
+
+// 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.
+
+// issue 5172: spurious warn about type conversion on broken type inside go and defer
+
+package main
+
+type foo struct {
+	x bar // ERROR "undefined"
+}
+
+func main() {
+	var f foo
+	go f.bar()
+	defer f.bar()
+}