Fix isinstance check for Generic classes

[dolfinus]

After upgrading to typing-extensions==4.6.1 this code example started failing on Python 3.7:

fromtyping_extensionsimportProtocol, runtime_checkablefromtypingimportGeneric, TypeVarfrompathlibimportPathfromdataclassesimportdataclass@runtime_checkableclassContainsException(Protocol):
""" Protocol for objects containing ``.exception`` attribute """@propertydefexception(self) ->Exception:
""" Exception object with traceback """T=TypeVar("T")
@dataclass(eq=False, frozen=True)classPathContainer(Generic[T]):
path: Texception: Exception@dataclass(eq=False, frozen=True)classPathWithFailure(PathContainer[Path]):
passfile=PathWithFailure(path=Path('/some'), exception=RuntimeError("some"))
isinstance(file, ContainsException)

Traceback (most recent call last):
File "/home/maxim/Repo/typing_extensions/1.py", line 33, in <module>
isinstance(file, ContainsException)
File "/home/maxim/Repo/typing_extensions/src/typing_extensions.py", line 599, in __instancecheck__
if super().__instancecheck__(instance):
File "/home/maxim/.pyenv/versions/3.7.8/lib/python3.7/abc.py", line 139, in __instancecheck__
return _abc_instancecheck(cls, instance)
File "/home/maxim/Repo/typing_extensions/src/typing_extensions.py", line 583, in __subclasscheck__
return super().__subclasscheck__(other)
File "/home/maxim/.pyenv/versions/3.7.8/lib/python3.7/abc.py", line 143, in __subclasscheck__
return _abc_subclasscheck(cls, subclass)
File "/home/maxim/Repo/typing_extensions/src/typing_extensions.py", line 661, in _proto_hook
and other._is_protocol
AttributeError: type object 'PathWithFailure' has no attribute '_is_protocol'

Who said that objects of generic class must have _is_protocol property?

Caused by #184

[ghost]

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[AlexWaygood]

Thank you! I think this probably also needs a changelog entry (under a new "Unreleased" heading at the top of the file)

[AlexWaygood]

(A test would also be great, to ensure that this doesn't regress again)

[dolfinus]

Added test and changelog item

[AlexWaygood]

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[AlexWaygood]

LGTM. I confirmed that the new test fails on main, and passes with this PR. Sorry again for the breakage!

[JelleZijlstra]

This fix was released as part of version 4.6.2 just now.