cmd/gc: do not typecheck nil types in multiple assignment

Fixes #6572.

LGTM=rsc, daniel.morsing, rsc
R=golang-codereviews, bradfitz, minux.ma, iant, rsc, gobot, daniel.morsing
CC=golang-codereviews
https://golang.org/cl/14516055

src/cmd/gc/typecheck.c

@@ -2814,7 +2814,7 @@ typecheckas2(Node *n)
 			n->op = OAS2FUNC;
 			t = structfirst(&s, &r->type);
 			for(ll=n->list; ll; ll=ll->next) {
-				if(ll->n->type != T)
+				if(t->type != T && ll->n->type != T)
 					checkassignto(t->type, ll->n);
 				if(ll->n->defn == n && ll->n->ntype == N)
 					ll->n->type = t->type;

test/fixedbugs/issue6572.go

+// errorcheck
+
+// Copyright 2014 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
+
+func foo() (T, T) { // ERROR "undefined"
+	return 0, 0
+}
+
+func bar() (T, string, T) { // ERROR "undefined"
+	return 0, "", 0
+}
+
+func main() {
+	var x, y, z int
+	x, y = foo()
+	x, y, z = bar() // ERROR "cannot (use type|assign) string"
+}