add a new test featuring two impl traits to show what it looks like

src/test/ui/impl-trait/universal-two-impl-traits.rs

@@ -0,0 +1,21 @@
+// Copyright 2017 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.
+
+#![feature(universal_impl_trait)]
+
+use std::fmt::Debug;
+
+fn foo(x: impl Debug, y: impl Debug) -> String {
+    let mut a = x;
+    a = y;
+    format!("{:?}", a)
+}
+
+fn main() { }

src/test/ui/impl-trait/universal-two-impl-traits.stderr

@@ -0,0 +1,11 @@
+error[E0308]: mismatched types
+  --> $DIR/universal-two-impl-traits.rs:17:9
+   |
+17 |     a = y;
+   |         ^ expected type parameter, found a different type parameter
+   |
+   = note: expected type `impl Debug` (type parameter)
+              found type `impl Debug` (type parameter)
+
+error: aborting due to previous error
+