If you put a small blanket or towel or something on a part of your bed you don't use (like a bottom corner) the cat will ALWAYS lie on that and not on a bit you need. Problem solved. But if it's already there? You're sleeping in the floor.

I'd ask if she/he wouldn't mind to share bed

Yay!! The universe sent me a cat! 😻

spoon it

This known as, 'My Life'.

As long as I have enough space I do not worry. When kitty is in the middle of my bed I'd likely shift it to the side a bit. But I prefer to sleep without sharing my bed with a pet.

I wouldn't, that happens to me all the time. Well, sort of. The question is a bit problematic. If you went to sleep and then found out, then you found out while you were asleep, which is impossible. Or if you found out after you woke up, then how would you possibly know that the cat was already on your bed when you went to sleep?

Take it outside I'm allergic

it would scooch over or be scooched over