rollup merge of #20207: jroesch/issue-18906-testcase

Closes issue #18906. This is what we talked about last night/early this morning. r? @nikomatsakis.

src/test/run-pass/issue-18906.rs

@@ -0,0 +1,36 @@
+// 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 Borrow<Sized? Borrowed> {
+        fn borrow(&self) -> &Borrowed;
+}
+
+impl<T: Sized> Borrow<T> for T {
+        fn borrow(&self) -> &T { self }
+}
+
+trait Foo {
+        fn foo(&self, other: &Self);
+}
+
+fn bar<K, Q>(k: &K, q: &Q) where K: Borrow<Q>, Q: Foo {
+    q.foo(k.borrow())
+}
+
+struct MyTree<K>;
+
+impl<K> MyTree<K> {
+    // This caused a failure in #18906
+    fn bar<Q>(k: &K, q: &Q) where K: Borrow<Q>, Q: Foo {
+        q.foo(k.borrow())
+    }
+}
+
+fn main() {}