test: add new test that gofrontend failed to handle

The gofrontend code would in some circumstances incorrectly generate a
type descriptor for an alias type, causing the type to fail to be
equal to the unaliased type.

Change-Id: I47d33b0bfde3c72a9a186049539732bdd5a6a96e
Reviewed-on: https://go-review.googlesource.com/c/go/+/275632
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/bug510.dir/a.go

@@ -0,0 +1,13 @@
+// Copyright 2020 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 a
+
+import "reflect"
+
+type A = map[int] bool
+
+func F() interface{} {
+	return reflect.New(reflect.TypeOf((*A)(nil))).Elem().Interface()
+}

test/fixedbugs/bug510.dir/b.go

@@ -0,0 +1,14 @@
+// Copyright 2020 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
+
+import "./a"
+
+func main() {
+	_, ok := a.F().(*map[int]bool)
+	if !ok {
+		panic("bad type")
+	}
+}

test/fixedbugs/bug510.go

@@ -0,0 +1,9 @@
+// rundir
+
+// Copyright 2020 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.
+
+// Gccgo confused type descriptors for aliases.
+
+package ignored