Can a Shapechanger be affected by Polymorph?

@AtomicDekeCan a willing shapechanger (i.e. a changeling) be affected by Polymorph, or does the auto-success saving throw come into play?
i'd let a shapechanger forgo the save – but remember that polymorph does change mental stats, so I'd rule NPCs opt to save

— Mike Mearls (@mikemearls) August 11, 2015