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//@ compile-flags: -Znext-solver
//@ check-pass
// A regression test for a fairly subtle issue with how we
// generalize aliases referencing higher-ranked regions
// which previously caused unexpected ambiguity errors.
//
// The explanations in the test may end up being out of date
// in the future as we may refine our approach to generalization
// going forward.
//
// cc trait-system-refactor-initiative#108
trait Trait<'a> {
type Assoc;
}
impl<'a> Trait<'a> for () {
type Assoc = ();
}
fn foo<T: for<'a> Trait<'a>>(x: T) -> for<'a> fn(<T as Trait<'a>>::Assoc) {
|_| ()
}
fn unconstrained<T>() -> T {
todo!()
}
fn main() {
// create `?x.0` in the root universe
let mut x = unconstrained();
// bump the current universe of the inference context
let bump: for<'a, 'b> fn(&'a (), &'b ()) = |_, _| ();
let _: for<'a> fn(&'a (), &'a ()) = bump;
// create `?y.1` in a higher universe
let mut y = Default::default();
// relate `?x.0` with `for<'a> fn(<?y.1 as Trait<'a>>::Assoc)`
// -> instantiate `?x.0` with `for<'a> fn(<?y_new.0 as Trait<'a>>::Assoc)`
x = foo(y);
// Constrain `?y.1` to `()`
let _: () = y;
// `AliasRelate(<?y_new.0 as Trait<'a>>::Assoc, <() as Trait<'a>>::Assoc)`
// remains ambiguous unless we somehow constrain `?y_new.0` during
// generalization to be equal to `?y.1`, which is exactly what we
// did to fix this issue.
}
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