Rustc can't infer GAT type when used as argument to Fn closure outside of trait

[geom3trik]

I tried this code:

playgorund

#![feature(generic_associated_types)]

trait Trait {
    type Assoc<'a>;
}

fn f<T: Trait>(_: T, _: impl Fn(T::Assoc<'_>)) {}

struct Type;

impl Trait for Type {
    type Assoc<'a> = ();
}

fn main() {
    f(Type, |_|());
}

I expected to see this happen: compiles with no errors.

Instead, this happened:

error[E0631]: type mismatch in closure arguments
  --> src/main.rs:16:5
   |
16 |     f(Type, |_|());
   |     ^       ----- found signature of `fn(()) -> _`
   |     |
   |     expected signature of `for<'r> fn(<Type as Trait>::Assoc<'r>) -> _`
   |
note: required by a bound in `f`
  --> src/main.rs:7:30
   |
7  | fn f<T: Trait>(_: T, _: impl Fn(T::Assoc<'_>)) {}
   |                              ^^^^^^^^^^^^^^^^ required by this bound in `f`

The error seems to suggest that rustc can't figure out that the associated type is () which should satisfy the bound in f.

Meta

1.56.0-nightly ( 5eacec9 )

This could be related to #85921, but unlike that issue this one does not seem to be fixed on latest nightly.

[b-naber]

This seems related to #88382

[jackh726]

GATs issue triage: not blocking. Not really a GATs issue, but there's a PR open that will fix this regardless.