Confusing error caused by double projection in trait supertraits

[WaffleLapkin]

Given the following code: (play)

fn f<T>(_: impl A<X = T>) {}

trait A: C<Z = <<Self as A>::X as B>::Y> {
    type X: B;
}

trait B {
    type Y;
}

trait C {
    type Z;
}

The current output is:

error[E0277]: the trait bound `T: B` is not satisfied
 --> src/lib.rs:1:1
  |
1 | fn f<T>(_: impl A<X = T>) {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `B` is not implemented for `T`
  |
help: consider restricting type parameter `T`
  |
1 | fn f<T: B>(_: impl A<X = T>) {}
  |       +++

error[E0277]: the trait bound `T: B` is not satisfied
 --> src/lib.rs:1:17
  |
1 | fn f<T>(_: impl A<X = T>) {}
  |                 ^^^^^^^^ the trait `B` is not implemented for `T`
  |
help: consider restricting type parameter `T`
  |
1 | fn f<T: B>(_: impl A<X = T>) {}
  |       +++

This is confusing because it seems like this error is caused by type X: B in the trait, but actually it's caused by X as B in the supertraits, if your remove X as B the code compiles (as it should in any case, IMO!).

Ideally the code would compile or at least the output should look like:

error[E0277]: the trait bound `T: B` is not satisfied
 --> src/lib.rs:1:1
  |
1 | fn f<T>(_: impl A<X = T>) {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `B` is not implemented for `T`
  |
help: consider restricting type parameter `T`
  |
1 | fn f<T: B>(_: impl A<X = T>) {}
  |       +++
note: required because of this
  |
3 | trait A: C<Z = <<Self as A>::X as B>::Y> {
  |                  ^^^^^^^^^^^^^^^^^^

@rustbot label +D-confusing

[SparrowLii]

@rustbot claim

[estebank]

Triage: no change.

[WaffleLapkin]

@SparrowLii I've unassigned you, since #102502 was closed. Feel free to reclaim, in case you are working on another fix.