Yes. But the problem is there is no rule that you absolutely must take a long rest. If a PC uses a week of downtime to take nothing but successive short rests they'll generate a minimum of 112 spell slots to use on their next adventure. And that can't possibly be RAI.
— Brianna_Heine (@Call_Me_Be_Be) August 14, 2018
Warlock/sorcerer using their lock slots to make sorc points, use sorc points to make generic slots, short rest, repeat. That's the combo I'm guessing they mean, but I don't like to assume.
— Jeremy Crawford (@JeremyECrawford) August 14, 2018
DMs, if you allow multiclassing in your game and someone is tempted to abuse the combination of Flexible Casting and Pact Magic, remember this: one way to read the multiclass rules is that your Pact Magic slots are useless for any non-warlock thing besides casting spells. #DnD https://t.co/bfEt5Zp2UF
— Jeremy Crawford (@JeremyECrawford) August 14, 2018
If someone’s trying to abuse a “coffeelock” as in that tweet, seems simpler to enforce the exhaustion penalties if someone isn’t taking long rests. Or even better, explain that it’s a problem out of game. Yes, I did, indeed, write that in 2015. Pointing out that you can read the multiclass rule in a new way doesn’t counter what I said then.
— Jeremy Crawford (@JeremyECrawford) August 14, 2018