Simply removing the restriction isn't actually appropriate, as the variants that allow addition of new members should *not* allow addition of new descriptors.

That's why I'm wondering if the current subclassing restriction is wrong: if you subclass an Enum derivative that already has defined members, then adding new members is OK, but adding new behaviour is not. If you subclass an Enum derivative with no members, then adding either members or behaviours is fine.

If we relaxed the "no subclassing" rule to "no new non-members", then this Enums would be natively extensible and this customisation hack wouldn't be necessary at all.

We have plenty of time before 3.4 - let's get the existing implementation in before worrying further about this.