Rollup merge of #36941 - martinhath:issue-21837, r=alexcrichton

Add regression test for Issue #21837

This PR adds a regression test for Issue #21837, as explained in the comments of the issue.

Author: Manishearth

src/test/compile-fail/issue-21837.rs

@@ -0,0 +1,20 @@
+// 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.
+
+pub trait Bound {}
+pub struct Foo<T: Bound>(T);
+
+pub trait Trait1 {}
+impl<T: Bound> Trait1 for Foo<T> {}
+
+pub trait Trait2 {}
+impl<T> Trait2 for Foo<T> {} //~ ERROR the trait bound `T: Bound` is not satisfied
+
+fn main() {}