[E0631] signature mismatch for function with Fn trait arguments with GAT arguments · Issue #88355 · rust-lang/rust

tim3z · 2021-08-26T09:53:49.000Z

Compiling this code ```rust #![feature(generic_associated_types)] trait Iterable { type Iterator ; fn iter(&self) -> Self::Iterator ; } struct SomeImplementation(); impl Iterable for SomeImplementation { type Iterator = std::iter::Empty ; fn iter(&self) -> Self::Iterator { std::iter::empty() } } fn do_something (i: I, mut f: impl for Fn(&mut I::Iterator )) { f(&mut i.iter()); } fn main() { do_something(SomeImplementation(), |_| ()); do_something(SomeImplementation(), test); } fn test (_: &mut I::Iterator ) {} ``` 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 fn(&'r mut std::iter::Empty ) -> _` | | | expected signature of `for fn(&'r mut ::Iterator ) -> _` | note: required by a bound in `do_something` --> src/main.rs:17:48 | 17 | fn do_something (i: I, mut f: impl for Fn(&mut I::Iterator )) { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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 fn(&mut ::Iterator ) -> _` ... 26 | fn test (_: &mut I::Iterator ) {} | ------------------------------------------------- found signature of `for fn(&'r mut std::iter::Empty ) -> _` | note: required by a bound in `do_something` --> src/main.rs:17:56 | 17 | fn do_something (i: I, mut f: impl for Fn(&mut I::Iterator )) { | ^^^^^^^^^^^^^^^^^^^^^^^^ 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.