This seems to be an issue with typing than mock since mock just uses the output from dir() . I am not able to bisect the relevant change but below is the output of dir(Foo[int]) in Python 3.6 and master.

Python 3.6.9

['__abstractmethods__', '__args__', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__extra__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__next_in_mro__', '__orig_bases__', '__origin__', '__parameters__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__slots__', '__str__', '__subclasshook__', '__tree_hash__', '__weakref__', '_abc_cache', '_abc_generic_negative_cache', '_abc_generic_negative_cache_version', '_abc_registry', '_gorg', 'bar']


master branch : 

['__args__', '__call__', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattr__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__instancecheck__', '__le__', '__lt__', '__module__', '__mro_entries__', '__ne__', '__new__', '__or__', '__origin__', '__parameters__', '__reduce__', '__reduce_ex__', '__repr__', '__ror__', '__setattr__', '__sizeof__', '__slots__', '__str__', '__subclasscheck__', '__subclasshook__', '__weakref__', '_inst', '_name', '_paramspec_tvars', '_typevar_types', 'copy_with']