Trait bound in associated type isn't checked when normalizing type that doesn't require inspecting the AT

The below code shouldn't compile, since i64 does not implement Bound, so I shouldn't be allowed to write Output<i64>. However, it compiles without errors.

I'm guessing that rustc sees <.... as Super>::Assoc and immediately concludes that the type must be i32 without further checking.

trait Super {
    type Assoc;
}
trait Sub: Super<Assoc = i32> {}

// no impl for Bound anywhere
trait Bound {}

trait Apply {
    // Bound requirement here
    type Output<T: Bound>: Sub;
}

trait Trait {
    type What;
}

impl<A: Apply> Trait for A {
    // i64 doesn't implement Bound
    type What = <<A as Apply>::Output<i64> as Super>::Assoc;
}

cc @lcnr

Meta

Reproducible on the playground with version 1.96.0-nightly (2026-03-12 3102493c71626b5912d1)

Start by running the standalone Rust reproducer from the issue and confirm that the shown associated-type normalization accepts i64 despite its missing Bound implementation. Trace the compiler's trait-bound checking and normalization behavior for this case; done means the reproducer is rejected with an appropriate bound error and regression coverage exists.