Bid 3 ♠

This shows either
(a) a hand with a probable 9 tricks in NT if partner can stop the opener’s suit
or
(b) a very strong one suited hand with at least game values.

Partner’s first priority is to bid 3 NT with a stop in opener’s suit.

Without a stop and with poor values, he makes the cheapest bid available, which is not 3 NT.

Any other bid shows primary values (not necessarily length) in the suit bid.

If overcaller removes 3 NT, he will have the very strong hand type.