+ /// Given an existential predicate like `?Self: PartialEq<u32>` (e.g., derived from `dyn PartialEq<u32>`),
+ /// and a concrete type `self_ty`, returns a full predicate where the existentially quantified variable `?Self`
+ /// has been replaced with `self_ty` (e.g., `self_ty: PartialEq<u32>`, in our example).