ICE: opaque type with non-universal region args

[lcnr]

all of the following examples result in the same ICE and should imo all be added as test cases once this is fixed:

this one should pass

fn foo<'a: 'a>() -> impl Sized {
    let _: () = foo::<'a>();
    loop {}
}

this one should error afaik

fn foo<'a: 'a>() -> impl Sized {
    let _: *mut &'a () = foo::<'a>();
    loop {}
}

this one should pass

fn foo<'a: 'a>() -> impl Sized + 'a {
    let _: *mut &'a () = foo::<'a>();
    loop {}
}

this one should error

trait Trait<'a, 'b> {}
impl<'a, 'b, T> Trait<'a, 'b> for T {}

fn foo<'a: 'a, 'b: 'b>() -> impl Trait<'a, 'b> {
    let _: *mut &'a () = foo::<'a, 'b>();
    let _: *mut &'b () = foo::<'a, 'b>();
    loop {}
}

they all ICE with

rror: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: opaque type with non-universal region substs
 --> src/lib.rs:2:12
  |
2 |     let _: () = foo::<'a>();
  |            ^^
  |
  = note: delayed at compiler/rustc_borrowck/src/region_infer/opaque_types.rs:94:35

[matthiaskrgr]

Will this be fixed by #110844 ?

[aliemjay]

This is a regression of #103491. Cc @cjgillot.

The error variants should be changed to make the lifetime invariant. Otherwise we would infer 'static:

fn foo<'a: 'a>() -> impl Sized {
-    let _: &'a () = foo::<'a>();
+    let _: *mut &'a () = foo::<'a>();
    loop {}
}

[matthiaskrgr]

Right now ( rustc 1.73.0-nightly (6ef7d16 2023-08-19) ) it seems that 1 and 3 still ICE and 2 and 4 error.
@rustbot prioritize since this affects stable

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high