`Fn`s with different arguments should be different types

[jethrogb]

Currently you can't write something like this:

pub trait MyTrait {
}

impl<F> MyTrait for F where F: FnMut() {
}

impl<F> MyTrait for F where F: FnMut(usize) -> usize {
}

This results in an error about conflicting implementations. This is because there might exist a type that implements both FnMut() and FnMut(usize) -> usize. In reality such a type will of course never exist.

Potential dupe of #19032 but I'm not sure

[TimNN]

Actually, I'm pretty sure that it is easily possible for a type to implement both, since the Fn traits can be implememted for a type like any other traits, ie. you can do struct A; impl Fn() for A { ... }.

[nagisa]

Once implementing Fn-traits is made stable, what @TimNN said is correct. Such case could also occur once we get something like default arguments, resulting in one value having multiple ways to call it – and thus multiple FnOnce implementations.

This issue is likely inactionable.

[arielb1]

Yea. Not a bug. Implementing FnOnce with different input types is intentionally allowed.