Unexpected higher-ranked lifetime error in GAT usage

[ethe]

I tried this code:

#![feature(generic_associated_types)]
#![feature(type_alias_impl_trait)]

use std::future::Future;

pub trait FutureIterator: 'static {
    type Iterator;

    type Future<'s, 'cx>: Future<Output = Self::Iterator> + Send + 'cx
    where
        's: 'cx;

    fn get_iter<'s, 'cx>(&'s self, info: &'cx ()) -> Self::Future<'s, 'cx>;
}

trait IterCaller: 'static {
    type Future1<'cx>: Future<Output = ()> + Send + 'cx;
    type Future2<'cx>: Future<Output = ()> + Send + 'cx;

    fn call_1<'s, 'cx>(&'s self, cx: &'cx ()) -> Self::Future1<'cx>
    where
        's: 'cx;
    fn call_2<'s, 'cx>(&'s self, cx: &'cx ()) -> Self::Future2<'cx>
    where
        's: 'cx;
}

struct UseIter<FI1, FI2> {
    fi_1: FI1,
    fi_2: FI2,
}

impl<FI1, FI2> IterCaller for UseIter<FI1, FI2>
where
    FI1: FutureIterator + 'static + Send + Sync,
    for<'s, 'cx> FI1::Future<'s, 'cx>: Send,
    FI2: FutureIterator + 'static + Send + Sync,
{
    type Future1<'cx> = impl Future<Output = ()> + Send + 'cx
    where
        Self: 'cx;

    type Future2<'cx> = impl Future<Output = ()> + Send + 'cx
    where
        Self: 'cx;

    fn call_1<'s, 'cx>(&'s self, cx: &'cx ()) -> Self::Future1<'cx>
    where
        's: 'cx,
    {
        async {
            self.fi_1.get_iter(cx).await;
        }
    }

    fn call_2<'s, 'cx>(&'s self, cx: &'cx ()) -> Self::Future2<'cx>
    where
        's: 'cx,
    {
        async {
            self.fi_2.get_iter(cx).await;
        }
    }
}

fn main() {}

I expected to see this happen: both type declaration of IterCaller::call_1 and IterCaller::call_2 pass the compiling

Instead, this happened:

error: higher-ranked lifetime error
  --> src/main.rs:60:9
   |
60 | /         async {
61 | |             self.fi_2.get_iter(cx).await;
62 | |         }
   | |_________^

Meta

rustc --version --verbose:

rustc 1.64.0-nightly (f9cba6374 2022-07-31)
binary: rustc
commit-hash: f9cba63746d0fff816250b2ba7b706b5d4dcf000
commit-date: 2022-07-31
host: x86_64-unknown-linux-gnu
release: 1.64.0-nightly
LLVM version: 14.0.6

[aliemjay]

This is another manifestation of #92096.

Minimized:

#![feature(generic_associated_types)]

pub trait FutureIterator {
    type Future<'s, 'cx>: Send
    where
        's: 'cx;
}

fn call_2<I: FutureIterator>() -> impl Send {
    async { // a generator checked for autotrait impl `Send`
        let x = None::<I::Future<'_, '_>>; // a type referencing GAT
        async {}.await; // a yield point
    }
}

@rustbot label F-generic_associated_types A-async-await A-lifetimes A-generators T-types

[tmandry]

This is an unfortunate error message. Since GATs is close to stabilization, does it make sense to nominate for types team discussion?

@rustbot label +I-types-nominated

[eholk]

Yeah, I'd like to hear from the types team on this. @nikomatsakis or @jackh726, is adding I-types-nominated the way to do that?

@rustbot label +AsyncAwait-Triaged

[jackh726]

This error message sucks, but is likely related to TAITs. Nevertheless, I'm going to take some time this upcoming week (I think) to take a look at least to improve the error message.