"escaping bound vars in predicate" with associated_type_bounds, HRTB

[comex]

Playground link

#![feature(unboxed_closures, associated_type_bounds)]

fn foo<F>(f: F) where
    F: for<'a> FnOnce<(&'a i32,), Output: 'a> {}

fn main() {
    foo(|x: &i32| -> &i32 { x });
}

produces:

error: internal compiler error: src/librustc_typeck/check/mod.rs:692: escaping bound vars in predicate Obligation(predicate=Binder(OutlivesPredicate(_, ReLateBound(DebruijnIndex(1), BrNamed(DefId(0:5 ~ playground[eb44]::foo[0]::'a[0]), 'a)))), depth=0)
 --> src/main.rs:7:5
  |
7 |     foo(|x: &i32| -> &i32 { x }); // OK
  |     ^^^

This does not happen if written out in this form, even though I think it should be equivalent:

    F: for<'a> FnOnce<(&'a i32,)>,
    for<'a> <F as FnOnce<(&'a i32,)>>::Output: 'a,

May or may not be related to #70263.

[Centril]

cc @alexreg

[alexreg]

@Centril Interesting. Is this (should this be) even supported right now? My point is, you can't do this just by concatenating where bounds, so...

[Centril]

Minimized

#![feature(associated_type_bounds)]

fn foo<F>(_: F)
where
    F: for<'a> Trait<Output: 'a>,
{
}

trait Trait {
    type Output;
}

impl<T> Trait for T {
    type Output = ();
}

fn main() {
    foo(());
}

The "desugared" version compiles fine.

[Centril]

Desugared version for reference: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=e52885892c45f3ab84d4f5d75ad46414

[JohnTitor]

Triage: This is no longer ICE with the latest nightly, marking as E-needs-test.