Min TAIT + GAT lifetime issue

[ivan770]

I tried this code:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=71e56b49edea6de440eed3a417322308

Code that is an equivalent to the one above, but that compiles: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f071b171eda8921c1d4902999417594d

I'm not sure about the exact reason for this to happen, but I can suspect the presence of an additional lifetime in impl block that is causing this bug.

I expected to see this happen: It compiles

Instead, this happened: Lifetime error

Meta

rustc --version --verbose: Latest playground version

[ivan770]

@rustbot label +T-compiler +F-type_alias_impl_trait +F-generic_associated_types +A-lifetimes +requires-nightly

[b-naber]

@rustbot claim

[jackh726]

Updated and minimized example:

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

pub trait Stream {
    type Item;
}

impl Stream for () {
    type Item = i32;
}

trait Yay<AdditionalValue> {
    type InnerStream<'s>: Stream<Item = i32> + 's;
    fn foo<'s>() -> Self::InnerStream<'s>;
}

impl<'a> Yay<&'a ()> for () {
    type InnerStream<'s> = impl Stream<Item = i32> + 's;
    fn foo<'s>() -> Self::InnerStream<'s> { todo!() }
}

[jackh726]

Yeah, so somehow 'a is ending up in the substs for the opaque type impl Stream<Item = i32> + 's, and that isn't able to be proven.

[jackh726]

GATs issue triage: not blocking. This requires the type_alias_impl_trait feature and seems to revolve around the substs for them, rather than a GATs issue.