implied bounds: lifetime equality lost after normalization

[aliemjay]

This doesn't compile although it should

struct Equal<'a, 'b>(&'a &'b (), &'b &'a ()); // implies 'a == 'b
trait Trait { type Ty; }
impl<'x> Trait for Equal<'x, 'x> { type Ty = (); }
fn test<'a, 'b>(_: (<Equal<'a, 'b> as Trait>::Ty, Equal<'a, 'b>)) {}
//~^ ERROR lifetime may not live long enough

It compiles with this trivial change:

- fn test<'a, 'b>(_: (<Equal<'a, 'b> as Trait>::Ty, Equal<'a, 'b>)) {}
+ fn test<'a, 'b>(_: <Equal<'a, 'b> as Trait>::Ty, _: Equal<'a, 'b>) {}

@rustbot label C-bug T-types A-implied-bounds

[Caellian]

I had a somewhat similar issue, though my use case is even simpler as it involves only a single lifetime. Just the fact that the trait is implemented for SomeType<'a> causes issues.