bpo-44524: Fix cryptic TypeError message when trying to subclass special forms in `typing` (GH-27710)
This was a Python 3.9 regression.
(cherry picked from commit a3a4d20d6798aa2975428d51f3a4f890248810cb)
Co-authored-by: Yurii Karabas <1998uriyyo@gmail.com>
diff --git a/Lib/typing.py b/Lib/typing.py
index 423329a..4c2d9fc 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1081,6 +1081,9 @@ def __reduce__(self):
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: