Closed
Description
I tried this code:
trait Variable<'a> {
type Type;
}
impl Variable<'_> for () {
type Type = ();
}
fn check<F, T>(_: F)
where
F: Fn(T), // <- if removed, all fn_* then require type annotations
F: for<'a> Fn(<T as Variable<'a>>::Type),
T: for<'a> Variable<'a>,
{
}
fn test(arg: impl Fn(())) {
fn fn_1(_: ()) {}
let fn_2 = |_: ()| ();
let fn_3 = |a| fn_1(a);
let fn_4 = arg;
check(fn_1); // Error
check(fn_2); // Ok
check(fn_3); // Ok
check(fn_4); // Error
check::<_, ()>(fn_1); // Ok
check::<_, ()>(fn_4); // Ok
}I expected to see this happen: the code compiles fine.
Instead, this happened: check(fn_1) and check(fn_4) fails to type check, and although the error message is not related to type inference, annotating the type parameter T fixes the problem. the same thing happens when removing the bound F: Fn(T), but then for all check(fn_*).
Seems to be related to #79207. cc @jackh726
Error message
Compiling playground v0.0.1 (/playground)
error[E0631]: type mismatch in function arguments
--> src/lib.rs:23:11
|
18 | fn fn_1(_: ()) {}
| -------------- found signature of `fn(()) -> _`
...
23 | check(fn_1); // Error
| ----- ^^^^ expected signature of `for<'a> fn(<() as Variable<'a>>::Type) -> _`
| |
| required by a bound introduced by this call
|
note: required by a bound in `check`
--> src/lib.rs:12:8
|
9 | fn check<F, T>(_: F)
| ----- required by a bound in this
...
12 | F: for<'a> Fn(<T as Variable<'a>>::Type),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `check`
error[E0277]: expected a `Fn<(<() as Variable<'a>>::Type,)>` closure, found `impl Fn(())`
--> src/lib.rs:26:11
|
26 | check(fn_4); // Error
| ----- ^^^^ expected an `Fn<(<() as Variable<'a>>::Type,)>` closure, found `impl Fn(())`
| |
| required by a bound introduced by this call
|
note: required by a bound in `check`
--> src/lib.rs:12:8
|
9 | fn check<F, T>(_: F)
| ----- required by a bound in this
...
12 | F: for<'a> Fn(<T as Variable<'a>>::Type),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `check`
help: consider further restricting this bound
|
17 | fn test(arg: impl Fn(()) + for<'a> std::ops::Fn<(<() as Variable<'a>>::Type,)>) {
| +++++++++++++++++++++++++++++++++++++++++++++++++++++
Some errors have detailed explanations: E0277, E0631.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `playground` due to 2 previous errors
Context
Original thread in users forum
Meta
Tested on stable and nightly branches of playground:
Stable version: 1.56.1- Nightly version: 1.58.0-nightly (2021-11-09 8b09ba6)