[release-branch.go1.4] cmd/gc: don't unpack struct arguments to append

Fixes #9634.

Change-Id: I7b18f26c2fb812978fc7adc5bfd39ebfffe48701
Reviewed-on: https://go-review.googlesource.com/3080Reviewed-by: Minux Ma <minux@golang.org>
(cherry picked from commit f5b8813e)
Reviewed-on: https://go-review.googlesource.com/5000
Run-TryBot: Andrew Gerrand <adg@golang.org>

src/cmd/gc/typecheck.c

@@ -1335,7 +1335,7 @@ reswitch:
 			goto error;
 
 		// Unpack multiple-return result before type-checking.
-		if(istype(t, TSTRUCT)) {
+		if(istype(t, TSTRUCT) && t->funarg) {
 			t = t->type;
 			if(istype(t, TFIELD))
 				t = t->type;

test/fixedbugs/issue9634.go

+// errorcheck
+
+// Copyright 2015 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 9634: Structs are incorrectly unpacked when passed as an argument
+// to append.
+
+package main
+
+func main() {
+	s := struct{
+		t []int
+		u int
+	}{}
+	_ = append(s, 0) // ERROR "must be a slice|must be slice"
+}