Inconsistent vtable layout with HRTBs

[steffahn]

trait Supertrait<T> {
    fn _print_numbers(&self, mem: &[usize; 100]) {
        println!("{mem:?}");
    }
}
impl<T> Supertrait<T> for () {}

trait Trait<T, U>: Supertrait<T> + Supertrait<U> {
    fn say_hello(&self, _: &usize) {
        println!("Hello!");
    }
}
impl<T, U> Trait<T, U> for () {}

fn main() {
    (&() as &'static dyn for<'a> Trait<&'static (), &'a ()>
        as &'static dyn Trait<&'static (), &'static ()>)
        .say_hello(&0);
}

(playground)

example output (shortened):

[0, 2338324182462507040, 7738151096083899748, 438881233243824, 439624262586284, 439667212259195, 439765996507179…

so apparently, we're calling _print_numbers actually, because the vtable of dyn for<'a> Trait<&'static (), &'a ()> and dyn Trait<&'static (), &'static ()> aren't compatible.

@rustbot label I-unsound, T-compiler, A-trait-objects, A-coercions, A-higher-ranked

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This code example is technically a regression, starting to misbehave somewhere between 1.55 and 1.56 (haven't further bisected yet…)