Cannot derive Send bound when a value of associated type with lifetime bound where clause across await point.

[zirconium-n]

I tried this code:

#![feature(generic_associated_types)]

trait Foo<'ctx, 'env> {
    type X: Send + Sync + 'env
    where
        'env: 'ctx;
}

fn foo<'ctx, 'env: 'ctx, T: Foo<'ctx, 'env>>(x: T::X) -> impl Send + 'env + 'ctx {
    async move {
        let x = x;
        std::future::ready(()).await;
    }
}

I expected to see this happen: compile success

Instead, this happened: compile failed with error message:

error[E0478]: lifetime bound not satisfied
 --> src/main.rs:9:58
  |
9 | fn foo<'ctx, 'env: 'ctx, T: Foo<'ctx, 'env>>(x: T::X) -> impl Send + 'env + 'ctx {
  |   

The error message says it's a lifetime bound but it's not. If I change impl Send to impl Sized then it compiles.

Meta

rustc --version --verbose:

rustc 1.60.0-nightly (51126be1b 2022-01-24)
binary: rustc
commit-hash: 51126be1b260216b41143469086e6e6ee567647e
commit-date: 2022-01-24
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

Backtrace

<backtrace>

[zirconium-n]

The code itself does not use GAT, but where bound on associated types is feature-gated by GAT.

[zirconium-n]

Seems a duplicate of #64552,