Example only did this multiple times against yi. I go to polymorph him slightly before he uses his Q ability. it shows me it on cooldown as if I used it however he goes into his ability and comes out un morphed before and after.
There is a momentary cast time on polymorph. This is just enough time for yi to enter Alpha strike and avoid being turned.
Its not even cast time. Its travel time. Whimsy becomes a projectile when casted on an enemy. Going untargetable breaks homing projectiles so if Yi alpha strikes at any time after Whimsy is casted and before it polymorphs him, it won't affect him at all.
Whimsy goes on cooldown on-cast. As an analogy, what you are pretty much asking for is if Ezreal's Q (this could be any champion/skill) missed it would reset simply because it didn't hit a target.
