What if you sold those call options without owning any shares at all? When you get assigned over the weekend, you can only buy those shares on Monday when trading resumes, and then deliver them.

This is not a covered call, this is being short a naked call.

Well, sure. But this whole idea is to keep your old shares that have a low basis, so you don't get hit with capital gain tax. If you don't own any shares then you don't have any low basic shares to protect.

Google answer is that you would be short the delivered shares. AFAIK everything happens the instant the market opens. The broker doesn't have to buy 100 shares and then sell them again to deliver them. They just sell 100 shares and leave you with a short position.

This question came up in a IBKR forum:
"If your short call gets assigned and you don't own shares, you automatically enter a short position. There is no need to buy shares to deliver them. However, the question is what happens when the underlying shares are unavailable to short, as designated by the broker due to limited borrow. I confirmed with IBKR that they are likely to immediately close the short position in this scenario."

One comment summed it up: "Why would you ever allow yourself to be in this position?"

The original question was "What happens when you get assigned on short calls while the underlying is not available to short?"

Interesting topic to think about. There are a lot of non-obvious nuances.