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Is your patch complete?  There seem to be a lot of places in object.c that call tp_compare slots.  Shouldn't we try to make sure that none of these places can ever be called with two different types?

It also looks like we have to do something special about a value of &_PyObject_SlotCompare in tp_compare, because this function from typeobject.c checks itself the type of its arguments and does specific things with them.  This should be thought about.

Moreover there is the issue of the user subclassing built-in types, e.g. PyInt_Type.tp_compare is capable of comparing different user subclasses of "int" for each other.