cmd/gc: always treat the output parameters as having been assigned in capturevars.

Fixes #9738.

Change-Id: Iab75de2d78335d4e31c3dce6a0e1826d8cddf5f3
Reviewed-on: https://go-review.googlesource.com/3690Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/cmd/gc/closure.c

@@ -209,7 +209,8 @@ capturevars(Node *xfunc)
 		dowidth(v->type);
 		outer = v->outerexpr;
 		v->outerexpr = N;
-		if(!v->closure->addrtaken && !v->closure->assigned && v->type->width <= 128)
+		// out parameters will be assigned to implicitly upon return.
+		if(outer->class != PPARAMOUT && !v->closure->addrtaken && !v->closure->assigned && v->type->width <= 128)
 			v->byval = 1;
 		else {
 			outer = nod(OADDR, outer, N);

test/fixedbugs/issue9738.go

+// run
+
+// 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.
+
+package main
+
+func F() (x int) {
+	defer func() {
+		if x != 42 {
+			println("BUG: x =", x)
+		}
+	}()
+	return 42
+}
+
+func main() {
+	F()
+}