[release-branch.go1.7] cmd/compile: rewrite literal.method to ensure full initialization

CALLPART of STRUCTLIT did not check for incomplete initialization
of struct; modify PTRLIT treatment to force zeroing.

Test for structlit, believe this might have also failed for
arraylit.

Fixes #18410.

Change-Id: I511abf8ef850e300996d40568944665714efe1fc
Reviewed-on: https://go-review.googlesource.com/34622
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-on: https://go-review.googlesource.com/35636
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/sinit.go

@@ -1042,6 +1042,8 @@ func anylit(ctxt int, n *Node, var_ *Node, init *Nodes) {
 
 		var r *Node
 		if n.Right != nil {
+			// n.Right is stack temporary used as backing store.
+			init.Append(Nod(OAS, n.Right, nil)) // zero backing store, just in case (#18410)
 			r = Nod(OADDR, n.Right, nil)
 			r = typecheck(r, Erv)
 		} else {

test/fixedbugs/issue18410.go

+// run
+
+// Copyright 2016 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.
+
+// This checks partially initialized structure literals
+// used to create value.method functions have their
+// non-initialized fields properly zeroed/nil'd
+
+package main
+
+type X struct {
+	A, B, C *int
+}
+
+//go:noinline
+func (t X) Print() {
+	if t.B != nil {
+		panic("t.B must be nil")
+	}
+}
+
+//go:noinline
+func caller(f func()) {
+	f()
+}
+
+//go:noinline
+func test() {
+	var i, j int
+	x := X{A: &i, C: &j}
+	caller(func() { X{A: &i, C: &j}.Print() })
+	caller(X{A: &i, C: &j}.Print)
+	caller(x.Print)
+}
+
+func main() {
+	test()
+}