GAT cannot infer implicit lifetime bounds.

[oxalica]

I'm trying to simulate closures returning mutable captured variable with GAT-version FnMut.

It cannot infer implicit bound 'a: 'r here and emit an error. But there is self: &'r mut Self alive while Self contains &'a mut i32.

#![feature(generic_associated_types)]

trait MyFnMut<Arg> {
    type Output<'a>;
    fn call_mut(&mut self, arg: Arg) -> Self::Output;
}

struct Closure1<'a>(&'a mut i32);

impl<'a> MyFnMut<()> for Closure1<'a> {
    type Output<'r> = &'r mut i32;
    fn call_mut(&mut self, _: ()) -> Self::Output {
        self.0 // error
    }
}

The same type signature without GAT works well:

// This works
impl<'a> Closure1<'a> {
    fn call_mut(&mut self, _: ()) -> &mut i32 {
        self.0
    }
}

Playground

BTW: Returning unimplemented!() in call_mut (the first one) will cause an ICE. Don't know if it is expected.

[hellow554]

BTW: Returning unimplemented!() in call_mut (the first one) will cause an ICE. Don't know if it is expected.

an ICE is never expected. If you haven't, please search for similar issues and then if none is applicable, open an issue and report the ICE (with a small example).

[csmoe]

ICE Duplicate of #56445 (comment)
cc @pnkfelix

[JohnTitor]

Cannot see the ICE with the above playground link on latest nightly.