Semantics of an empty PermittedSubtypes attribute for the VM

[Brian Goetz]

> You might think I’m arguing here for allowing S to be
> an empty set, and I might in similar cases, but there are two
> other reasons to outlaw the edge case, rather than ask
> the spec. to account for it (either by extending the general
> rule, or adding a special rule).  First, if C wants to permit
> exactly zero subclasses, there’s already a notation for that.
> 


… unless C is an interface.