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

// 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() {}