ICE:`Unimplemented` when trying to select an implementation for constant trait item reference.

[Ygg01]

When running following code:

#![feature(associated_consts)]
trait HasNumber<T> {
    const Number: usize;
}

enum One {}
enum Two {}

enum Foo {}

impl<T> HasNumber<T> for One {
    const Number: usize = 1;
}

impl<T> HasNumber<T> for Two {
    const Number: usize = 2;
}

On playground nightly and beta channels I get the following error:

<anon>:3:5: 3:25 error: internal compiler error: Encountered error `Unimplemented` when trying to select an implementation for constant trait item reference.
<anon>:3     const Number: usize;
             ^~~~~~~~~~~~~~~~~~~~

I expected to not get that error.

[quantheory]

This does not seem to happen on the latest nightly.

[arielb1]

This does not affect 1.5.