TLDR: S&W has a new 9mm PCC called the Response with interchangeable mag wells. It ships with mag wells for both M&P and Glock mags.
So now we know why S&W is doing factory extended mags for the M&P.
https://www.smith-wesson.com/response
Adaptability is the advantage: our first MSR-style carbine with an interchangeable magwell system.
Chambered in 9MM, the RESPONSE features our new FLEXMAG™ magwell technology, allowing you to change out the magwell to fit double-stack 9MM magazines from S&W and other manufacturers. The RESPONSE will ship with an M&P magwell and a second magwell capable of receiving Glock® G17® and G19® magazines.
That's right. It takes Glock® mags.
RESPONSE features familiar MSR-style controls and accessories compatible with many popular MSR-style stocks, triggers, grips, and charging handles. This PCC also features a 16" 1/2x28 threaded barrel, and full-length handguard with three, six, and nine o'clock M-LOK® slot positions. Lastly, RESPONSE features a simple & reliable blowback design with a standard MSR-style buffer & spring for minimal recoil.