[foobarkey]

Never is a good starting point as a guide but of course there are cases where is-a makes sense too.

Composition generally is not good enough to model all cases without resulting to some AOP or meta programming also, but in that case the is-a or a base class would arguably be the simpler approach, at least it can be reasoned about and debugged, as opposed to some AOP/Meta spaghetti that can only probably be understood from docs