Overflow on requirement evaluation with associated types

trait Foo { }

trait Bar<P> {
    type X;
}

impl<T, U> Foo for (T, U)
    where T: Bar<(T, U), X=U>
{ }

impl<T, P: Foo> Bar<P> for T {
    type X = u32;
}

fn main() { }

produces on stable and nightly:

error[E0275]: overflow evaluating the requirement `<T as Bar<(T, U)>>::X`
 --> src/lol.rs:7:1
  |
7 |   impl<T, U> Foo for (T, U)
  |  _^ starting here...
8 | |     where T: Bar<(T, U), X=U>
9 | | { }
  | |___^ ...ending here
  |
  = note: required because of the requirements on the impl of `Foo` for `(T, U)`
  = note: required because of the requirements on the impl of `Bar<(T, U)>` for `T`

error: aborting due to previous error

The impl's compile individually but not together. I expect it to be able to compile.

1. Note that in the absense of associated types, the above works fine.

   You can cheat (for some reason?) and put a bound on T in the Bar<P> impl, but you just end up running into the same overflow somewhere else:

   trait Foo {
       fn whatever() { }
   }
   
   trait Bar<P> {
       type X;
   }
   
   trait Baz { }
   impl Baz for usize { }
   
   impl<T, U> Foo for (T, U)
       where T: Bar<(T, U), X=U>
   { }
   
   impl<T: Baz, P: Foo> Bar<P> for T {
       type X = u32;
   }
   
   fn main() {
       <(usize, u32) as Foo>::whatever();
   }
   

   produces

   error[E0275]: overflow evaluating the requirement `(usize, u32): Foo`
     --> <anon>:21:5
      |
   21 |     <(usize, u32) as Foo>::whatever();
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: required by `Foo::whatever`
   
   error: aborting due to previous error
   

   ebfull at 2017-03-17 03:21:18

2. Triage: this still overflows.

   Steve Klabnik at 2020-03-27 13:34:22