How do you handle Wish?

[JLandan]

Aren't spell slots and extra attacks an in-game phenomenon, recognized in game and referenced as an in game phenomenon?

Not additional spell slots beyond those granted by caster levels. Nor are additional extra attacks beyond those granted by class levels. Those would be meta-game, not in-game. The spell description is explicit about what it can do. None of it involves meta-game knowledge or core rules changes. Suppose the wish was to roll a d30 instead of a d20? Should that be allowed? Or automatic success for all saving throws? Or to instantly advance ten levels? What if the wish was for ownership of the DM's car?

Wish alters reality in-game, not meta-game.

It can sometimes be difficult to tell the difference between meta-game and in-game. A good question to ask is who the wish is for, the PC or the player? You may say spell slots and extra attacks are in-game and allow such, perhaps with some crazy rationalization. I would not.