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bpo-44524: Fix cryptic TypeError message when trying to subclass special forms in typing (GH-27710)

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This was a Python 3.9 regression.
(cherry picked from commit a3a4d20d67)

Co-authored-by: Yurii Karabas <1998uriyyo@gmail.com>
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Miss Islington (bot) 2021-08-28 11:09:45 -07:00 • committed by GitHub
parent cd986e9031
commit 81fa08c5ea
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Lib/typing.py
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@ -1081,6 +1081,9 @@ class _GenericAlias(_BaseGenericAlias, _root=True):
return operator.getitem, (origin, args)
def __mro_entries__(self, bases):
if isinstance(self.__origin__, _SpecialForm):
raise TypeError(f"Cannot subclass {self!r}")
if self._name: # generic version of an ABC or built-in class
return super().__mro_entries__(bases)
if self.__origin__ is Generic: