This raises an interesting question. Several interesting questions, actually. Let's say that the wizard in your party is polymorphed into, I dunno, a Tyrannosaur.
He fails his save throw against a medusa's gaze by 5 or more, and is instantly turned to stone. He never took damage...so is he a statue of a wizard, or a statue of a t-rex now? And if t-rex, does he revert back to his normal form when the polymorph spell ends (his "normal form" being an unpetrified flesh-and-blood wizard)?
First off, I would say that the spell ends immediately when the wizard is petrified because I don't think you can concentrate on a spell when petrified. But the wizard is petrified and remains so despite the spell ending. Nevertheless, I'd have the statue be in the form of a t-rex because that's a more amusing thing to have to deal with. If the statue is depetrified, it immediately becomes a wizard.
EDIT: Oh, that all assumes it was the wizard that cast
Polymorph on themselves. If it was someone else that cast
Polymorph on the wizard, then the spell could continue. If it's still in effect and the statue is depetrified, then you still have a t-rex.
He is attacked by a handful of shadows, and his new-and-improved strength score is reduced to zero long before his new-and-improved hit points are. The t-rex dies, since its hit points drop to zero...and since no damage carries over, so the wizard still has his hit points (and doesn't die). But does the Strength damage carry over, since it's supposed to last until the wizard's next Short or Long Rest?
According to the shadow stat block, the target just dies; it does not mention HP falling to zero as the cause. So the t-rex dies and immediately reverts to wizard form because the dead cannot concentrate on a spell. The issue of the STR loss carryover can incite a whole new argument about whether the wizard can even be raised or not by anything short of
True Resurrection. Since the STR loss can be removed by a short rest, it seems like it is intended to be fairly weak, so I would play it that if the wizard is raised, even with
Revivify, the STR loss is removed.
If you were sticking closer to the wording of the various raising spells, though, I think there's a case to be made that anything short of
True Resurrection would require an accompanying
Greater Restoration to remove the STR loss. Whether the
Greater Restoration has to be done before
True Resurrection, or has to be done after, or whether it doesn't matter is another debate which has been had, but which I think is just terribly uninteresting because answers other than "it doesn't matter" don't make the game any more fun.
He is targeted by a rival wizard's Disintegrate spell. He fails its save throw, and the T-rex is dropped to exactly 0 hit points as a result. Per the Disintegrate spell, the t-rex immediately turns to dust...but per the Polymorph spell, he immediately turns back into a wizard. Which one triggers? or do they both trigger? and in what order?
Per Crawford, first the damage is applied
completely; that is, if it takes the t-rex to 0, then
Polymorph ends and the carryover damage applies to the wizard. If the carryover damage takes the wizard to 0, then the wizard is disintegrated.