Closure type mismatch on higher-ranked bounds

[cramertj]

In the following code, identical impls for a concrete type, a fn type, and a closure exist yet the only the closure fails to meet the higher-ranked bound:

trait FnLt<'a> {
    fn apply(self, input: &'a u8) -> &'a u8;
}

// Struct impl
struct Foo;
impl<'a> FnLt<'a> for Foo {
    fn apply(self, input: &'a u8) -> &'a u8 {
        input
    }
}

// Closure impl
impl<'a, T> FnLt<'a> for T
where
    T: FnOnce(&'a u8) -> &'a u8,
{
    fn apply(self, input: &'a u8) -> &'a u8 {
        (self)(input)
    }
}

fn take_fn_lt(_: impl for<'a> FnLt<'a>) {}

fn main() {
    take_fn_lt(Foo); // Works
    
    fn foo(x: &u8) -> &u8 { x }
    take_fn_lt(foo); // Works
    
    take_fn_lt(|x: &u8| -> &u8 { x }); // Doesn't work
}

The error is this:

error[E0271]: type mismatch resolving `for<'a> <[closure@src/main.rs:31:16: 31:37] as std::ops::FnOnce<(&'a u8,)>>::Output == &'a u8`
  --> src/main.rs:31:5
   |
31 |     take_fn_lt(|x: &u8| -> &u8 { x }); // Doesn't work
   |     ^^^^^^^^^^ expected bound lifetime parameter 'a, found concrete lifetime
   |
   = note: required because of the requirements on the impl of `for<'a> FnLt<'a>` for `[closure@src/main.rs:31:16: 31:37]`
note: required by `take_fn_lt`
  --> src/main.rs:23:1
   |
23 | fn take_fn_lt(_: impl for<'a> FnLt<'a>) {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Edit: the closure can be made to work through coercion to a fn pointer (which is unsurprising, since it's then the same as foo). take_fn_lt({ |x: &u8| -> &u8 { x } } as fn(&u8) -> &u8); compiles.

[Mark-Simulacrum]

I believe this is the same issue as #41078.

[cramertj]

@Mark-Simulacrum I'm not sure this is a proper duplicate-- when I've hit that other issue, I've always been able to get it to go away by explicitly writing out the signature of the closure types. I can't do that here-- you'll notice that the type of the closure is specified manually, yet it still fails.

cc @nikomatsakis

[cramertj]

I'm going to reopen this, as I think it at least needs further investigation in order to determine if it is the same as #41076 (which I doubt).

[aturon]

cc @eddyb @nikomatsakis, this is a pretty persistent and important issue for async/await work. It might be most expedient to address it by adding AsyncFn/AsyncFnMut/AsyncFnOnce unstable traits -- does that seem plausible?

[eddyb]

@nikomatsakis Looks like this works fine if the bound in take_fn_lt is replaced with FnOnce(&u8) -> &u8, which means that either the "closure signature deduction" hack is kicking in there and making it work (unlike FnLt, which doesn't get the deduction) or going through trait selection imposes additional constraints.

@aturon Not entirely opposed to AsyncFn* - if unstable, they're easy to add, but I worry that blanket impls won't cut it (as they don't in this issue). And trait aliases aren't implemented yet AFAIK.