go.tools/go/types: initialize local x early in case of bailout panic

- added respective test case

Fixes golang/go#5770.

R=adonovan
CC=golang-dev
https://golang.org/cl/10531043

go/types/expr.go

@@ -1031,6 +1031,11 @@ func (check *checker) callExpr(x *operand) {
 // cycleOk indicates whether it is ok for a type expression to refer to itself.
 //
 func (check *checker) rawExpr(x *operand, e ast.Expr, hint Type, iota int, cycleOk bool) {
+	// make sure x has a valid state for deferred functions in case of bailout
+	// (was issue 5770)
+	x.mode = invalid
+	x.typ = Typ[Invalid]
+
 	if trace {
 		c := ""
 		if cycleOk {

go/types/issues_test.go

@@ -0,0 +1,28 @@
+// Copyright 2013 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 file implements tests for various issues.
+
+package types
+
+import (
+	"go/parser"
+	"strings"
+	"testing"
+)
+
+func TestIssue5770(t *testing.T) {
+	src := `package p; type S struct{T}`
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	_, err = Check(f.Name.Name, fset, f) // do not crash
+	want := "undeclared name: T"
+	if err == nil || !strings.Contains(err.Error(), want) {
+		t.Errorf("got: %v; want: %s", err, want)
+	}
+}