Partner deals and opens 1NT showing 15-17 hcp, a balanced hand that could have a 5 card major, 5422 with 5 minor possible with some values in the short suits.
RHO passes and for the time being let's assume that you bid 2NT which a xfer to diamonds. Your LHO asks what 2NT means and then bids 3C which is explained as natural. Partner now bids 3D which means that he likes diamonds.
RHO now bids 3♠, explained as natural. Opponents are playing as partners for the first time and are not weak. Your call?
1NT - (p) - 2NT* - (3♣)
3♦ - (3♠) - ?