I think partner has 6 diamonds, 4 hearts, and a game force here. If not, I'd fully expect a 3S call. In fact, I really don't understand this auction; how can partner be game-forcing and shapely enough to bid like this, but have no interest in slam. Hmmm. I'm real close -- real close -- to bidding 6D here. What's not to love about 3 aces AND the top of your range? I guess you're not ruffing hearts in hand, but really?
I have no idea, but maybe he has 7 or 8 diamonds and invented a bid. Or maybe he has QJxx hearts. I'm passing and having a conversation with partner later about fast arrival if 6 rolls.
5H out of the question. If he had 5 hearts, he would have bid 3H over 3D to pattern out. We're already in a game force.
Looking fwd to the spoiler!