Wish is a 9th level spell, one of the uses written into it is:
The basic use of this spell is to duplicate any other spell of 8th level or lower. You don’t need to meet any requirements in that spell, including costly components. The spell simply takes effect.
The question is what level is that copy of the spell cast at? I can see three possibilities here:
- Wish was cast from a 9th level spell slot, so the copied spell is cast at 9th level.
- Wish can cast up to an 8th level spell as a copy, so the copy cast is cast as if it were using an 8th level slot.
- The spell being copied is of Nth level, so the copy is cast as if it were using an Nth level slot.
There are good reasons to cast low level spells at higher levels, the extra die per level of healing for cure wounds, the extra missile per level for magic missile, etc, so using a 9th level slot to cast a low level spell copy at the low level would generally be undesirable, but the wish spell isn't clear.
The first two possibilities both make some sense and are useful, in the 9th level example you just get the spell as if you had cast it normally in the slot, in the 8th level example you're sacrificing one level of casting bonuses for the ability to cast possibly unknown, unprepared or cross-class spells (no restriction against copying say cleric spells as a wizard with wish) - and that utility seems like a fair trade off.
But which is 'officially' correct?