TAIT: it's possible to impl a trait for a tait by using projections

[lcnr]

#![feature(type_alias_impl_trait)]
type Alias = impl Sized;

fn constrain() -> Alias {
    1i32
}

trait HideIt {
    type Assoc;
}

impl HideIt for () {
    type Assoc = Alias;
}

pub trait Yay {}

impl Yay for <() as HideIt>::Assoc {}
// impl Yay for i32 {} // this errors
// impl Yay for u32 {} // this also errors

As coherence checking keeps opaque types opaque this should not be unsound. Considering that a direct impl for opaque types is forbidden, this is an inconsistency we should resolve though.

cc @oli-obk

[oli-obk]

The two commented out lines do not cause any errors (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8366951aa5afd218745dd0ebc4564387) So I'd say this is unsound or at least causes ICEs: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=07fbbfb3b0ecc1dc59ffc71b18b06f1e

[lcnr]

what, is that a recent regression? :o these do error with rustc 1.63.0-nightly (5435ed691 2022-06-07)

[lqd]

It looks to be from last week's #99506, and out of that rollup #99345 looks likely.

[oli-obk]

Hmm... that one got reverted in #99079

cc @compiler-errors

[lqd]

Was that a revert, or a targeted fix for closures ?

[oli-obk]

revert + actual fix of the original issue

[lqd]

The behavior is the same on master, no errors + ICEs. I'll try to manually bisect within the rollup to confirm cargo-bisect-rustc's findings.

[compiler-errors]

Isn't this just another form of #99554?

[lqd]

(I wish we had try builds for each rolled up PR 😅) It was indeed introduced in the same rollup: by eecfdfb / #99383.

Reverting that does indeed bring the conflicting implementation errors back, and fixes the ICE.