Associated type check for trait objects fires incorrectly on supertrait/Self bounds

[aturon]

Here's an example (reproducible on Rust 1.9+):

trait FutureInternals {
    type Item;
    type Error;

    fn poll(&mut self);
}

trait Future<T, E> where Self: FutureInternals<Item=T, Error=E> {
    fn boxed(self) -> Box<Future<T, E>> {
        Box::new(self)
    }
}

impl<T: FutureInternals> Future<T::Item, T::Error> for T {}

This code produces the error:

error: the value of the associated type `Error` (from the trait `FutureInternals`) must be specified [--explain E0191]
 --> <anon>:9:27
  |>
9 |>     fn boxed(self) -> Box<Future<T, E>> {
  |>                           ^^^^^^^^^^^^

error: the value of the associated type `Item` (from the trait `FutureInternals`) must be specified [--explain E0191]
 --> <anon>:9:27
  |>
9 |>     fn boxed(self) -> Box<Future<T, E>> {
  |>                           ^^^^^^^^^^^^

despite the fact that the associated types are fully constrained.

[aturon]

cc @eddyb @arielb1 @nikomatsakis

[aturon]

Note: the same error occurs if you use a supertrait bound instead.

[nikomatsakis]

Dup of #23856 I believe ... this probably isn't too hard to fix. Just something we never got around to.

[nikomatsakis]

Closing in favor of #23856

[nikomatsakis]

Note that:

trait Future<T, E> where Self: FutureInternals<Item=T, Error=E> {

is a super-trait bound.