default-body `async fn` in trait with `impl Trait` return fails to compile

[compiler-errors]

This code fails to compile after #101679:

#![feature(async_fn_in_trait, return_position_impl_trait_in_trait)]
#![allow(incomplete_features)]

use std::fmt::Debug;

trait Foo {
    async fn baz(&self) -> impl Debug {
        ""
    }
}

struct Bar;

impl Foo for Bar {}

fn main() {
    let _ = Bar.baz();
}

This is the same RPIT issue as #101968 (comment). Filing as a known-bug.

[compiler-errors]

I have a hacky solution to fix this and to fix #101968 (comment), but it's disgusting, so I want to separate it from #101679.