x/tools/go/ssa: Accept struct conversions that ignore tags

This is now allowed in go1.8.

Fixes golang/go#19646.

Change-Id: Iece4fd2a881144bdbe841e0a26ba4348d6b8828e
Reviewed-on: https://go-review.googlesource.com/38452
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

go/ssa/emit.go

@@ -147,7 +147,7 @@ func emitCompare(f *Function, op token.Token, x, y Value, pos token.Pos) Value {
 //
 func isValuePreserving(ut_src, ut_dst types.Type) bool {
 	// Identical underlying types?
-	if types.Identical(ut_dst, ut_src) {
+	if structTypesIdentical(ut_dst, ut_src) {
 		return true
 	}
 

go/ssa/identical.go

@@ -0,0 +1,7 @@
+// +build go1.8
+
+package ssa
+
+import "go/types"
+
+var structTypesIdentical = types.IdenticalIgnoreTags

go/ssa/identical_17.go

@@ -0,0 +1,7 @@
+// +build !go1.8
+
+package ssa
+
+import "go/types"
+
+var structTypesIdentical = types.Identical

go/ssa/identical_test.go

@@ -0,0 +1,9 @@
+//+build go1.8
+
+package ssa_test
+
+import "testing"
+
+func TestValueForExprStructConv(t *testing.T) {
+	testValueForExpr(t, "testdata/structconv.go")
+}

go/ssa/source_test.go

@@ -194,12 +194,16 @@ func checkVarValue(t *testing.T, prog *ssa.Program, pkg *ssa.Package, ref []ast.
 // Ensure that, in debug mode, we can determine the ssa.Value
 // corresponding to every ast.Expr.
 func TestValueForExpr(t *testing.T) {
+	testValueForExpr(t, "testdata/valueforexpr.go")
+}
+
+func testValueForExpr(t *testing.T, testfile string) {
 	if runtime.GOOS == "android" {
 		t.Skipf("no testdata dir on %s", runtime.GOOS)
 	}
 
 	conf := loader.Config{ParserMode: parser.ParseComments}
-	f, err := conf.ParseFile("testdata/valueforexpr.go", nil)
+	f, err := conf.ParseFile(testfile, nil)
 	if err != nil {
 		t.Error(err)
 		return

go/ssa/testdata/structconv.go

@@ -0,0 +1,24 @@
+//+build ignore
+
+// This file is the input to TestValueForExprStructConv in identical_test.go,
+// which uses the same framework as TestValueForExpr does in source_test.go.
+//
+// In Go 1.8, struct conversions are permitted even when the struct types have
+// different tags. This wasn't permitted in earlier versions of Go, so this file
+// exists separately from valueforexpr.go to just test this behavior in Go 1.8
+// and later.
+
+package main
+
+type t1 struct {
+	x int
+}
+type t2 struct {
+	x int `tag`
+}
+
+func main() {
+	var tv1 t1
+	var tv2 t2 = /*@ChangeType*/ (t2(tv1))
+	_ = tv2
+}