Associated types of supertraits aren't accepted or completed in `impl Subtrait<...>`

```rust
trait Super {
    type AssocSuper;
}

trait Sub: Super {
    type AssocSub;
}

fn f() -> impl Sub<AssocSuper = ()> {}
```

Here in the `impl Sub<...>` the `AssocSuper` remains an unresolved reference, but rustc has no problem with this code. `AssocSuper` is also not completed, while `AssocSub` is.