But only if it can get bug-type moves, which Greninja does not (with the exception of U-turn, but that doesn't count for obvious reasons).
It counts the same as any other type he can become. The type change occurs before the attack, so he gets STAB on the U-turn. He may never be defensively bug-type, but he can be and is often offensively bug-type (since U-turn is a fairly common move for Protean Greninja).
Actually, thanks to Hidden Power, Greninja can become any type except Fairy (there is no Fairy Hidden Power.) Obviously it will only ever want to run a few of those Hidden Powers, but if you really want a Greninja for every color of the rainbow...
Actually, thanks to Hidden Power, Greninja can become any type except Fairy (there is no Fairy Hidden Power.) Obviously it will only ever want to run a few of those Hidden Powers, but if you really want a Greninja for every color of the rainbow...