I'm planning on multi-classing in Warlock as a Sorcerer in 5e. I'd like to be able to use my Warlock spell slots from Pact Magic (that I regain on a short rest) to gain sorcery points.
I'm inclined to believe the answer to this question is "Yes", because the Font of Magic feature does not specify that the slots expended need be Sorcerer spell slots or even Spellcasting spell slots. Although it does seem to be RAW, I find it hard to believe that this was RAI, because it essentially copies the Sorcerer capstone with three levels in Warlock. Does anyone know if there's an official ruling as to whether or not this is allowed?
Here's the relevant rule:
Converting a Spell Slot to Sorcery Points. As a bonus action on your turn, you can expend one spell slot and gain a number of sorcery points equal to the slot’s level.
Best Answer
Yes, Flexible Casting doesn't specify where the spell slot comes from, just that you expend one to gain points. This means that you can expend a spell slot that you have from Pact Magic to satisfy its requirement, and thus gain sorcery points from it.
And yes, this has been confirmed as the correct reading by Jeremy Crawford, the lead rules designer for D&D 5e, for exactly the use you're asking about (though Crawford's tweets are no longer considered official rulings):