I had similar doubts about this patch and needed opinions of other core developers.

> Maybe, if __len__() raises an OverflowError: call again the len(), but using the "__len__" method instead of the slot?

Following patch implements this idea. I don't like it because it is too complicated.

I think that we should either document that raising an OverflowError by __len__() is normal and interpreted as true in Boolean context, or document that __len__() should return a value not larger than sys.maxsize, otherwise len() and bool() can raise an OverflowError (see issue15718).