Projections fail to unify projections in related obligations

[AnthonyMikh]

I tried this code:

trait Add<Rhs> {
    type Sum;
}

impl Add<()> for () {
    type Sum = ();
}

type Unit = <() as Add<()>>::Sum;

trait Trait<C> {
    type Output;
}

fn f<T>()
where
    T: Trait<()>,
    <T as Trait<()>>::Output: Sized,
{}

fn g<T>()
where
    T: Trait<Unit>,
    <T as Trait<()>>::Output: Sized,
{}

fn h<T>()
where
    T: Trait<()>,
    <T as Trait<Unit>>::Output: Sized,
{}

Function g is f with () replaced with Unit in first where clause, while function h is f with () replaced with Unit in second where clause. Since Unit is effectively a type alias for (), I expected all three functions to compile.

Instead, the compiler complains about g:

error[E0277]: the trait bound `T: Trait<()>` is not satisfied
  --> src/lib.rs:25:1
   |
25 | / fn g<T>()
26 | | where
27 | |     T: Trait<Unit>,
28 | |     <T as Trait<()>>::Output: Sized,
   | |____________________________________^ the trait `Trait<()>` is not implemented for `T`
   |
help: consider further restricting this bound
   |
27 |     T: Trait<Unit> + Trait<()>,
   |                    +++++++++++

error[E0277]: the trait bound `T: Trait<()>` is not satisfied
  --> src/lib.rs:25:4
   |
25 | fn g<T>()
   |    ^ the trait `Trait<()>` is not implemented for `T`
   |
help: consider further restricting this bound
   |
27 |     T: Trait<Unit> + Trait<()>,
   |                    +++++++++++

The compiler suggests adding Trait<()> on T, which should not be necessary since Trait<Unit> has exactly the same meaning as Trait<()> (and it actually works that way for h). Having Unit type alias is not essential, inlining it yields the same error.

(Also it complains about the same unsatisfied bound twice, but it is less of an issue, I guess)

Meta

I used the latest stable at the moment, 1.67.1, but it can also be reproduced on the latest beta (2023-03-06 b955c8271da80a1af8a1) and latest nigthly (2023-03-06 b955c8271da80a1af8a1) at the moment.

Probably related to #43049.

@rustbot label: +A-associated-items

[lcnr]

this has the same root cause as #41118 (comment)