Unexpected trait bound not satisfied in GAT & HRTB

[Millione]

I tried this code:

fn main() {
    let mut log_service = LogService { inner: Inner };
    log_service.call(());
}

pub trait Service<Request> {
    type Response;

    fn call(&mut self, req: Request) -> Self::Response;
}

pub struct LogService<S> {
    inner: S,
}

impl<T, U, S> Service<T> for LogService<S>
where
    S: Service<T, Response = U>,
    U: Extension + 'static,
    for<'a> U::Item<'a>: std::fmt::Debug,
{
    type Response = S::Response;

    fn call(&mut self, req: T) -> Self::Response {
        self.inner.call(req)
    }
}

pub struct Inner;

impl Service<()> for Inner {
    type Response = Resp;

    fn call(&mut self, req: ()) -> Self::Response {
        Resp::A(req)
    }
}

pub trait Extension {
    type Item<'a>;

    fn touch<F>(self, f: F) -> Self
    where
        for<'a> F: Fn(Self::Item<'a>);
}

pub enum Resp {
    A(()),
}

impl Extension for Resp {
    type Item<'a> = RespItem<'a>;
    fn touch<F>(self, _f: F) -> Self
    where
        for<'a> F: Fn(Self::Item<'a>),
    {
        match self {
            Self::A(a) => Self::A(a),
        }
    }
}

pub enum RespItem<'a> {
    A(&'a ()),
}

impl<'a> std::fmt::Debug for RespItem<'a> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::A(arg0) => f.debug_tuple("A").field(arg0).finish(),
        }
    }
}

Meta

rustc --version --verbose:

rustc 1.66.0-nightly (bed4ad65b 2022-10-25)
binary: rustc
commit-hash: bed4ad65bf7a1cef39e3d66b3670189581b3b073
commit-date: 2022-10-25
host: x86_64-unknown-linux-gnu
release: 1.66.0-nightly
LLVM version: 15.0.2

Backtrace

error[E0599]: the method `call` exists for struct `LogService<Inner>`, but its trait bounds were not satisfied
  --> src/main.rs:3:17
   |
3  |     log_service.call(());
   |                 ^^^^ method cannot be called on `LogService<Inner>` due to unsatisfied trait bounds
...
12 | pub struct LogService<S> {
   | ------------------------
   | |
   | method `call` not found for this struct
   | doesn't satisfy `LogService<Inner>: Service<_>`
   |
note: trait bound `<_ as Extension>::Item<'a>: Debug` was not satisfied
  --> src/main.rs:20:26
   |
16 | impl<T, U, S> Service<T> for LogService<S>
   |               ----------     -------------
...
20 |     for<'a> U::Item<'a>: std::fmt::Debug,
   |                          ^^^^^^^^^^^^^^^ unsatisfied trait bound introduced here

For more information about this error, try `rustc --explain E0599`.

[Millione]

It doesn't seem to be related to GAT, only influenced by TAIT.

[Millione]

for<'a> U::Item<'a>: std::fmt::Debug,

I found rustc type system have inferred type U to Resp correctly, but still refused to compile. However, there is a workaround without knowing why.

for<'a> <S::Response as Extension>::Item<'a>: std::fmt::Debug,

[LYF1999]

seems like another version of #94207

[LYF1999]

@rustbot claim