auto merge of #12023 : nick29581/rust/err_res, r=alexcrichton

closes #3512

src/test/compile-fail/conflicting-implementations-aux.rs

@@ -0,0 +1,24 @@
+// Copyright 2014 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.
+
+// Regression test for #3512 - conflicting trait impls in different crates should give a
+// 'conflicting implementations' error message.
+
+// aux-build:trait_impl_conflict.rs
+extern mod trait_impl_conflict;
+use trait_impl_conflict::Foo;
+
+impl<A> Foo for A {
+//~^ ERROR conflicting implementations for trait `trait_impl_conflict::Foo`
+//~^^ ERROR cannot provide an extension implementation where both trait and type are not defined in this crate
+}
+
+fn main() {
+}