associated type not normalized when a where-clause is present

[arielb1]

Affected Versions

At least 1.3, 1.4, rustc 1.5.0-nightly (11a612795 2015-10-04)

STR

trait Foo { type Bar; }
impl<T> Foo for T { type Bar = u64; }
fn foo<T>() -> <T as Foo>::Bar
    where T: Foo // <- the code compiles if this is removed
{ 1 }
fn main() {}

Expected Result

the code should compile

Actual Result

<anon>:7:14: 7:15 error: mismatched types:
 expected `<T as Foo>::Bar`,
    found `_`
(expected associated type,
    found integral variable) [E0308]
<anon>:7     x = Some(1);
                      ^

cc @nikomatsakis

[arielb1]

The root cause is that selection picks the where-clause candidate over any impl candidate, and this does not allow projection to make progress. This looks to be hard to fix.

[nikomatsakis]

Yes, this is to some extent a known limitation and the "expected result", but I would still classify it as a bug.