Add a regression test for #19538.

Closes #19538.

src/test/compile-fail/issue-19538.rs

@@ -0,0 +1,29 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+trait Foo {
+    fn foo<T>(&self, val: T);
+}
+
+trait Bar: Foo { }
+
+pub struct Thing;
+
+impl Foo for Thing {
+    fn foo<T>(&self, val: T) { }
+}
+
+impl Bar for Thing { }
+
+fn main() {
+    let mut thing = Thing;
+    let test: &mut Bar = &mut thing;
+    //~^ ERROR cannot convert to a trait object because trait `Bar` is not object-safe
+}