[m-rev.] for-post-commit review: add an XXX about possible improvement

[Julien Fischer]

On Tue, 28 May 2024 at 16:59, Peter Wang <novalazy at gmail.com> wrote:
>
>
> I'm still unsure what problem the restriction avoids.
> (Or tries to avoid: as check_typeclass.m mentions, the type equivalence
> could be hidden in a different module from the instance declaration.)

I think it was an attempt to minimise the possibility of inadvertent
overlapping instances
involving them.  (For example, we had such a problem when stack(T) == list(T).)

Julien.