Does the Martial Adept feat give an additional D8 superiority dice to battlemaster fighters?

@TrippyGameDoes the Martial Adept feat give an additional D8 superiority dice to battlemaster fighters, or is it just an extra D6? it's one of whatever type the fighter now has; also upgrades with the fighter's other ones

— (((Mike Mearls))) (@mikemearls) July 15, 2016