What would happen if someone were to use a wish spell to request something that is literally impossible? For example, the resurrection of someone who is not dead. Would the spell simply be wasted, or should there be some odd side effect?
It is stated in the spell's description that the DM always decides if the desired effect is fulfilled or not, or if it works only partially. In this case, the spell would probably just be wasted*.
*If the DM decides the Wish will have some odd side-effect, then that would ofcourse happen instead. Could make for some awesome plot, too.
The Wish spell has various "safe" uses, like replicating the effects of a lower-level spell. When you would use a Wish to replicate Resurrection, it would play out after the rules of the Resurrection spell. When you cast Resurrection on someone who is still alive, nothing happens.
However, when you want to do something with Wish which diverges from the safe uses, like resurrecting someone without access to their remains, you subject yourself to DM fiat. When the DM considers the wish reasonable, they can make it happen. When the wish is impossible, either intentionally or because the players lack information, the wish is obviously not reasonable, and the DM can decide what happens. The options the DM can decide freely among are:
- They simply decide that nothing happens.
- They could try to follow the intention of the wish as best as they can. In this example, the reason why the player-character wants to resurrect someone could be because they want to talk to them. The DM could make that possible by teleporting the still living target to them.
- They have the option to twist the wish and follow it literally to the wording but not the intention. When it is "Resurrecting someone who is not dead", they could for example have the Wish spell slay them just to immediately resurrect them. Another option would be to resurrect the target in the future as soon as it actually dies.
- They can invoke the "unforseeable consequences" clause and have it do literally whatever they want without any relation at all to the wording or intention of the initial wish spell.
This wish can be fulfilled by killing the person and then resurrecting it.
An impossible wish would be "I wish for wishes to not come true."
My solution would be to have the group travel back in time, maybe just seconds before the wish was uttered. Then they can choose how long they want to play Groundhog Day.
Just brainstorming a bit on Joninean's answer, one approach would be to let the wish try to take effect, but have to amend its effects over time. The world would get stranger and stranger, as the wish built itself up around an impossibility, trying to make itself possible. Perhaps eventually the wish might even have to distort language to make itself a reality.
Personally, I'd play with the idea of trying to make the wish get bigger and bigger until the players are convinced to do something nefarious to stop the onslaught. Then, have the wish kindly retract, bundling itself up until it disappears with a poof of logic, leaving the party with nothing except their nefarious act and its consequences to deal with.
I'd expect you'd have to do this exactly once before the party elects to be more careful with their wish wording in the future.