Fix using typing_extensions.runtime_checkable in combination with typing.Protocol on 3.12.2+

CHANGELOG.md

@@ -4,6 +4,9 @@
   `__static_attributes__` attribute to all classes in Python,
   which broke some assumptions made by the implementation of
   `typing_extensions.Protocol`.
+- Fix `AttributeError` when using `typing_extensions.runtime_checkable`
+  in combination with `typing.Protocol` on Python 3.12.2 or newer.
+  Patch by Alex Waygood.
 - At runtime, `assert_never` now includes the repr of the argument
   in the `AssertionError`. Patch by Hashem, backporting of the original
   fix https://github.com/python/cpython/pull/91720 by Jelle Zijlstra.

src/test_typing_extensions.py

@@ -3536,6 +3536,18 @@ class Commentable(Protocol):
         )
         self.assertIs(type(exc.__cause__), CustomError)
 
+    def test_extensions_runtimecheckable_on_typing_Protocol(self):
+        @runtime_checkable
+        class Functor(typing.Protocol):
+            def foo(self) -> None: ...
+
+        self.assertNotIsSubclass(object, Functor)
+
+        class Bar:
+            def foo(self): pass
+
+        self.assertIsSubclass(Bar, Functor)
+
 
 class Point2DGeneric(Generic[T], TypedDict):
     a: T

src/typing_extensions.py

@@ -676,9 +676,14 @@ def close(self): ...
                             ' got %r' % cls)
         cls._is_runtime_protocol = True
 
-        # Only execute the following block if it's a typing_extensions.Protocol class.
-        # typing.Protocol classes don't need it.
-        if isinstance(cls, _ProtocolMeta):
+        # typing.Protocol classes on <=3.11 break if we execute this block,
+        # because typing.Protocol classes on <=3.11 don't have a
+        # `__protocol_attrs__` attribute, and this block relies on the
+        # `__protocol_attrs__` attribute. Meanwhile, typing.Protocol classes on 3.12.2+
+        # break if we *don't* execute this block, because *they* assume that all
+        # protocol classes have a `__non_callable_proto_members__` attribute
+        # (which this block sets)
+        if isinstance(cls, _ProtocolMeta) or sys.version_info >= (3, 12, 2):
             # PEP 544 prohibits using issubclass()
             # with protocols that have non-method members.
             # See gh-113320 for why we compute this attribute here,