A private keyword makes sense, it would just extend the existing class facade. This just... Yeah too much.
— I don't see how instanceA would be able to access private vars of instanceB
— I don't see why it's needed, or useful to anyone writing sane classes in the first place.
— It's not
— BUT I HAVE made private vars in one way or another at some point for a class-like thingy
— Just neat to have supporting syntax
— Like await
— If you define classes either in global scope or some userspace scope, and have concerns over families/friend classes talking to each other privately, then they shouldn't be in that scope to begin with IMO