Recursive RPIT incorrectly errors: "concrete type differs from previous defining opaque type use"

The following code fails to compile, both in the old solver and next-solver. I think it should compile.

#[expect(unconditional_recursion)]
pub fn foo<'a: 'a>() -> impl Sized + use<'a> {
    let x: Cov<'static> = foo::<'a>();
    x
}

struct Cov<'a>(&'a ());

error: concrete type differs from previous defining opaque type use
 --> src/lib.rs:4:5
  |
4 |     x
  |     ^ expected `Cov<'static>`, got `Cov<'a>`
  |
note: previous use here
 --> src/lib.rs:3:27
  |
3 |     let x: Cov<'static> = foo::<'a>();
  |                           ^^^^^^^^^^^

This code compiles in both solvers if Covis changed to be invariant. But it still doesn't compile if it's contravariant.

This code also compiles if use<'a> is changed to use<>

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Reproducible on the playground with version 1.96.0-nightly (2026-03-08 b41f22de2a13a0babd28)

Start with the Rust reproduction in src/lib.rs and compile it with both the old solver and next-solver. Compare the behavior when Cov is invariant or contravariant and when use<'a> becomes use<>; done means the original covariant recursive RPIT example compiles without the concrete-type error.