-Znext-solver overflow evaluating the requirement

[cynecx]

I tried this code (with -Znext-solver):

use std::future::Future;

trait AsyncFn<I, O>: Fn(I) -> Self::Fut {
    type Fut: Future<Output = O>;
}

impl<F, Fut, I, O> AsyncFn<I, O> for F
where
    F: Fn(I) -> Fut,
    Fut: Future<Output = O>
{
    type Fut = Fut;
}

fn okay<F: for<'a> AsyncFn<&'a mut Vec<u8>, ()>>(f: F) {
    let mut data = vec![];

    let fut = f(&mut data);
}

I expected to see this happen: Compiles without errors.

Instead, this happened:

With -Znext-solver:

error[E0275]: overflow evaluating the requirement `<F as FnOnce<(&mut Vec<u8>,)>>::Output == _`
  --> a.rs:18:15
   |
18 |     let fut = f(&mut data);
   |               ^^^^^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`a`)

error[E0275]: overflow evaluating the requirement `<F as AsyncFn<&mut Vec<u8>, ()>>::Fut normalizes-to _`
  --> a.rs:18:15
   |
18 |     let fut = f(&mut data);
   |               ^^^^^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`a`)

error: aborting due to 2 previous errors

rustc --version --verbose:

rustc 1.77.0-nightly (e51e98dde 2023-12-31)
binary: rustc
commit-hash: e51e98dde6a60637b6a71b8105245b629ac3fe77
commit-date: 2023-12-31
host: aarch64-apple-darwin
release: 1.77.0-nightly
LLVM version: 17.0.6

[compiler-errors]

This is tracked via rust-lang/trait-system-refactor-initiative#84