cmd/gc: correctly typecheck expression lists in returns.

Invalid return statements were accidentally compiling or
triggering internal errors.
Fixes #3044.

R=golang-dev, rsc
CC=golang-dev, remy
https://golang.org/cl/5673074

src/cmd/gc/typecheck.c

@@ -1410,7 +1410,10 @@ reswitch:
 
 	case ORETURN:
 		ok |= Etop;
+		if(count(n->list) == 1)
 			typechecklist(n->list, Erv | Efnstruct);
+		else
+			typechecklist(n->list, Erv);
 		if(curfn == N) {
 			yyerror("return outside function");
 			goto error;

test/fixedbugs/bug418.go

+// errchk $G $D/$F.go
+
+// Copyright 2012 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 3044.
+// Multiple valued expressions in return lists.
+
+package p
+
+func Two() (a, b int)
+
+// F used to compile.
+func F() (x interface{}, y int) {
+	return Two(), 0 // ERROR "single-value context"
+}
+
+// Recursive used to trigger an internal compiler error.
+func Recursive() (x interface{}, y int) {
+	return Recursive(), 0 // ERROR "single-value context"
+}