Add test for RPIT defined with different hidden types with different substs #111943
Description
The following code:
trait Trait {}
impl Trait for () {}
fn foo<T: Trait, U: Trait>() -> impl Trait {
let a: T = foo::<T, U>();
loop {}
let _: T = foo::<U, T>();
}used to incorrectly compile but now correctly errors, we should add a test to ensure this doesn't regress.