False ambiguity due to overlap between higher-ranked and other where clause

[nikomatsakis]

The test at http://is.gd/xHYV2z was extracted from the qcollect-traits package on crates.io. It fails to compile due to a false ambiguity between the higher-ranked where-clause and the other one. This is particularly frustrating because the ambiguity occurs as part of projection and the higher-ranked where clause doesn't even list a binding for Output.

The correct fix is probably to prune one where clause or the other as part of selection, but this is a bit tricky with the current region setup due to #21974.

pub trait ImmutableSequenceTypes<'a, T:'a>{
    type Output;
}

pub trait MutableSequenceTypes<'a, T:'a>: ImmutableSequenceTypes<'a, T> {
}

fn foo<'t,X,T:'t>()
    where X: for<'a> ImmutableSequenceTypes<'a, T>,
          X: ImmutableSequenceTypes<'t, T, Output=&'t T>
{
    bar::<<X as ImmutableSequenceTypes<'t,T>>::Output>();
}

fn bar<A>() { }

fn main() { }

(Due to this bug, PR #27641 causes a regression in qcollect-traits, but the problem is pre-existing.)

[steveklabnik]

Triage: this still fails to compile today.

[steveklabnik]

Triage: this still fails to compile, with

error[E0282]: type annotations needed
  --> src/main.rs:8:1
   |
8  | / fn foo<'t,X,T:'t>()
9  | |     where X: for<'a> ImmutableSequenceTypes<'a, T>,
10 | |           X: ImmutableSequenceTypes<'t, T, Output=&'t T>
11 | | {
12 | |     bar::<<X as ImmutableSequenceTypes<'t,T>>::Output>();
13 | | }
   | |_^ cannot infer type

[estebank]

Current output:

error[E0283]: type annotations needed: cannot satisfy `X: ImmutableSequenceTypes<'t, T>`
  --> f71.rs:10:14
   |
10 |           X: ImmutableSequenceTypes<'t, T, Output=&'t T>
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: multiple `impl`s or `where` clauses satisfying `X: ImmutableSequenceTypes<'t, T>` found
  --> f71.rs:9:14
   |
9  |     where X: for<'a> ImmutableSequenceTypes<'a, T>,
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
10 |           X: ImmutableSequenceTypes<'t, T, Output=&'t T>
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^