The July 2017 issue of the ACBL Bridge Bulletin recommends that 5 of a major is used as a stopper for an unbid suit.  In the given example, South bids 5S.  "A raise to the five level of the last bid major in a sequence where three suits have been bid asks partner to bid slam with a control in the unbid suit, diamonds in this case."

North should respond:

P: lacking a first or second-round diamond control

5NT: with the guarded DK, allowing the hand to be played in 6NT from the right side

6D: with the DA, and

6S: with a singleton diamond.

Fixed typo, hand was missing cA and I managed to mistype the bidding too!

Sorry to misattribute a hand provided by a winner of 2 Bermuda Bowls, 15 NABCs, 3 USBCs :)

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