incompletely relating alias args may cause borrowck ICE

[lcnr]

trait Id {
    type This<'a>;
}

#[derive(Copy, Clone)]
struct W<T>(T);
impl Id for W<u32> {
    type This<'a> = W<u32>;
}
impl Id for W<i32> {
    type This<'a> = W<i32>;
}

trait Trait<T> {}
// when using this impl we only normalize after instantiating
// with infer vars, at this point `<?t as Id>::This<'a>` is still
// ambig, so we keep the alias around and require it to structurally
// relate.
impl<T: Id> Trait<for<'a> fn(T::This<'a>)> for T {}
fn is_trait<T: Trait<U>, U>(_x: T) {}

fn main() {
    let x = W(1);
    // We prove `W<_>: Trait<for<'a> fn(<W<_>>::This<'a>)>` during
    // typeck, and `W<i32>: Trait<for<'a> fn(<W<i32>)>` during borrowck.
    // This then fails.
    is_trait(x);
    let _: W<i32> = x;
}

ICEs with

error: internal compiler error: error performing operation: fully_perform
  --> src/main.rs:20:5
   |
20 |     is_trait::<_, _>(x);
   |     ^^^^^^^^^^^^^^^^^^^
   |
note: delayed at /rustc/05f9846f893b09a1be1fc8560e33fc3c815cfecb/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs:87:25 - disabled backtrace
  --> src/main.rs:20:5
   |
20 |     is_trait::<_, _>(x);
   |     ^^^^^^^^^^^^^^^^^^^