[m-rev.] for post-commit review: document requirement for non-abstract instances in interfaces

[Julien Fischer]

On Thu, 3 Feb 2022, Zoltan Somogyi wrote:

>>> +        words("is to move this instance declaration"),
>>> +        words("to the implementation section,"),
>>> +        words("replacing it in the interface section"),
>>> +        words("with its abstract version, which contains"),
>>> +        words("just the class name and the instance type vector."), nl],
>>
>> In the case where there are type class constraints on the instance declaration,
>> they also need to be replicated in the abstract declaration.
>
> I will reword what I added to take that into account, but section 10.4 does NOT
> mention this point (neither does it deny it), and does not give an example
> either.

That's because type class constraints on instances have not been
discussed at all at that point.  However, section 10.7 says:

     For abstract instance declarations, the type class constraints on an
     abstract instance declaration must exactly match the type class
     constraints on the corresponding non-abstract instance declaration that
     defines that instance.

Julien.