Wrong (?) [E0631] signature mismatch for function with Fn trait arguments with GAT arguments (original) (raw)
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; 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