Normally there is a Python 3 compatibility warning emitted when a class is based on object, implements __eq__(), but does not define __hash__():

$ python -3 -c 'class C(object): __eq__ = lambda self, other: True'
-c:1: DeprecationWarning: Overriding __eq__ blocks inheritance of __hash__ in 3.x

But when warnings are raised as exceptions, this warning seems to be suppressed:

$ python -3 -Werror -c 'class C(object): __eq__ = lambda self, other: True'

Perhaps there is bad exception handling at the warn() call site.