This skinny slam occurred recently and proved too difficult for most pairs to bid. South’s opening bid is clear but I wouldn’t recommend West’s overcall. Two level overcalls should be 6 card suits unless holding a good one level opener and this is not one. At North’s second turn, 3S would be very invitational and 4C would be natural and FG. 4D is clearly a shortage control in diamonds and is showing a hand which is either holding very strong spades and wants to make a slam try or has heart support and wants to make a slam try. North has such values because clubs can be set up with two losers at most, spades with one loser. North also has heart support, albeit, rather poor for slam bidding. The hand has 6, or less, losers which, opposite a 7 loser opener, is worth issuing a slam invite. From a point count view, North has 10 hcp and 5 for the void so clearly has values beyond those required for game.
When South hears the slam invitation, they can revalue the three losers in diamonds to be only one loser opposite the singleton ( and none if it is a void). South revalues from six losers to four losers. This makes it clear that South also should be moving towards a slam so they make a cue bid in clubs. Alternatively, using a point count evaluation method, South has 12hcp ( the JD should be ignored), 3 points for the singleton and 1 for the doubleton, giving 16 points which is sufficient to accept the slam invite. Despite the void, North has poor trumps and cannot use RKC so they just jump to 6H since there is no clear way to investigate 7H and it requires some specific cards in the South hand.
The lead at the table was the AD. Declarer ruffed, cashed the AC, ruffed another diamond, cashed the KC, ruffed a club and cashed the AH. When the QH came down, 13 tricks were easy. While this seems like a very good score, any slam is above average and a difficult- to- bid slam will be a near top without any need to play for an overtrick.