Parameter usage is not properly propagated through trait aliases

[vandenheuvel]

It seems to me that both S1 and S2 should compile:

#![feature(trait_alias)]

// Works
struct S1<'a, T: 'a, I: Iterator<Item = &'a T> + Clone> {
    field: I,
}

trait SpecialIterator<'a, T: 'a> = Iterator<Item = &'a T> + Clone;
// Doesn't work
struct S2<'a, T: 'a, I: SpecialIterator<'a, T>> {
    field: I,
}

Playground. On 1.46.0-nightly (2020-07-05 2753fab7ce3647033146).

[estebank]

type alias doesn't propagate obligations, it's not only for impl Trait that this is the case. This might change in the future.