Wrong (?) [E0631] signature mismatch for function with Fn trait arguments with GAT arguments #88382
Description
Compiling this code
#![feature(generic_associated_types)] trait Iterable { type Iterator<'a>; fn iter(&self) -> Self::Iterator<'_>; } struct SomeImplementation(); impl Iterable for SomeImplementation { type Iterator<'a> = std::iter::Empty<usize>; fn iter(&self) -> Self::Iterator<'_> { std::iter::empty() } } fn do_something<I: Iterable>(i: I, mut f: impl for<'a> Fn(&mut I::Iterator<'a>)) { f(&mut i.iter()); } fn main() { do_something(SomeImplementation(), |_| ()); do_something(SomeImplementation(), test); } fn test<'a, I: Iterable>(_: &mut I::Iterator<'a>) {}with the current nightly (rustc 1.56.0-nightly (0afc20860 2021-08-25)) yields an E0631 argument mismatch for both calls to do_something.
error[E0631]: type mismatch in closure arguments --> src/main.rs:22:5 | 22 | do_something(SomeImplementation(), |_| ()); | ^^^^^^^^^^^^ ------ found signature of `for<'r> fn(&'r mut std::iter::Empty<usize>) -> _` | | | expected signature of `for<'r, 'a> fn(&'r mut <SomeImplementation as Iterable>::Iterator<'a>) -> _` | note: required by a bound in `do_something` --> src/main.rs:17:48 | 17 | fn do_something<I: Iterable>(i: I, mut f: impl for<'a> Fn(&mut I::Iterator<'a>)) { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `do_something` error[E0631]: type mismatch in function arguments --> src/main.rs:23:40 | 23 | do_something(SomeImplementation(), test); | ^^^^ expected signature of `for<'a> fn(&mut <SomeImplementation as Iterable>::Iterator<'a>) -> _` ... 26 | fn test<'a, I: Iterable>(_: &mut I::Iterator<'a>) {} | ------------------------------------------------- found signature of `for<'r> fn(&'r mut std::iter::Empty<usize>) -> _` | note: required by a bound in `do_something` --> src/main.rs:17:56 | 17 | fn do_something<I: Iterable>(i: I, mut f: impl for<'a> Fn(&mut I::Iterator<'a>)) { | ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `do_something` I expected both calls to compile and from looking at the errors I also see no reasons why the signatures should be incompatible. In the case that my expectation is wrong I would expect the compiler to give me some hint, why this is not possible.
Sorry for the double post, I accidentally used the wrong template in #88355