A good solution might be to add a new license clause stipulating whether the owner is okay with their code being used to train AI models.

Part of the clause would explain that if you are okay with your code being trained on, then you're also accepting being okay with it being copied verbatim at some point down the line during code completion.

You do get a bit of tragedy of the commons where everybody wants to use the AI model but nobody wants their own code trained on.

I don't like the idea of a world where licensing and copyright law prevents us from enjoying the progress of AI. Caveat: I am not an expert on open source.