trait selection fails for projections under binders

[aliemjay]

This fails to compile, but it shouldn't:

pub trait Trait {}

pub trait WithAssoc<'a> {
    type Assoc;
}

impl<T> Trait for for<'a> fn(T, <T as WithAssoc<'a>>::Assoc) where T: for<'a> WithAssoc<'a> {}

impl WithAssoc<'_> for u8 {
    type Assoc = u8;
}

fn main() {
    fn impls_trait<T: Trait>() {}
    impls_trait::<fn(u8, u8)>;
    //~^ ERROR the trait bound `fn(u8, u8): Trait` is not satisfied
}

@rustbot label C-bug T-types A-traits A-associated-items