I'm currently playing as an artificer. I know that when an artificer imbues one of their infusions into an item, it makes a non-magic item into a semi-magic item.
Say I imbue the Enhanced Weapon artificer infusion into a staff, and then my party's druid casts the shillelagh cantrip on the same staff.
Would the effects of Enhanced Weapon stack with those of shillelagh? Or would this negate the infusion, or otherwise not work as intended?
The description of infusing an item says:
Whenever you finish a long rest, you can touch a non-magical object and imbue it with one of your artificer infusions, turning it into a magic item.
It clearly points out that the object must be non-magical to start and becomes magical after the process. So the Artificer makes a magical staff.
Now the Druid comes along and casts Shillelagh:
For the duration, you can use your spellcasting ability instead of Strength for the attack and damage rolls of melee attacks using that weapon, and the weapon's damage die becomes a d8. The weapon also becomes magical, if it isn't already. The spell ends if you cast it again or if you let go of the weapon.
The spell calls out that the weapon can be magical. No magical "plus" bonuses to worry about stacking.
The spell changes the attack and damage rolls from Strength to Wisdom and updating the damage die. Nothing in the description would indicate it would cancel out the infusion; it doesn't care.
It also shows that the spell only lasts while the Druid is holding the weapon. Your phrasing didn't make it clear that the Druid would maintain control of the staff after infusion, so I wanted to point this out. I don't want you to assume that the Artificer could hold the staff and the Druid cast Shillelagh and it would still work.
Infusions make a regular item into magic item:
Artificer infusions are extraordinary processes that rapidly turn a nonmagical object into a magic item.
The class lists several ways an infusion can end but none of them involve casting spells on the item.
Shillelagh can affect a magic weapon, given the last sentence:
The wood of a club or quarterstaff you are holding is imbued with nature's power. [...] The weapon also becomes magical, if it isn't already.
As an infusion is not the same as the spell, it would stack.
That last sentence also means it doesn't work when Shillelagh is done first, but that's mostly irrelevant. You can release the staff and re-cast the spell after infusing without a hassle.
The clause that says the spell ends when the caster lets go of the staff means that the Druid has to be the one using the weapon, but there is no such restriction on artificer infusions (though some will require attunement).
