Fix isinstance check for Generic classes

CHANGELOG.md

@@ -1,3 +1,9 @@
+# Unreleased
+
+- Fix regression in version 4.6.1 where comparing a generic class against a 
+  runtime-checkable protocol using `isinstance()` would cause `AttributeError`
+  to be raised if using Python 3.7
+
 # Release 4.6.1 (May 23, 2023)
 
 - Change deprecated `@runtime` to formal API `@runtime_checkable` in the error

src/test_typing_extensions.py

@@ -2246,6 +2246,28 @@ class Foo: ...
         del f.x
         self.assertNotIsInstance(f, HasX)
 
+    def test_protocols_isinstance_generic_classes(self):
+        T = TypeVar("T")
+
+        class Foo(Generic[T]):
+            x: T
+
+            def __init__(self, x):
+                self.x = x
+
+        class Bar(Foo[int]):
+            ...
+
+        @runtime_checkable
+        class HasX(Protocol):
+            x: int
+
+        foo = Foo(1)
+        self.assertIsInstance(foo, HasX)
+
+        bar = Bar(2)
+        self.assertIsInstance(bar, HasX)
+
     def test_protocols_support_register(self):
         @runtime_checkable
         class P(Protocol):
@@ -4330,6 +4352,7 @@ class Y(Generic[T], NamedTuple):
 
                 a = A(3)
                 self.assertIs(type(a), G)
+                self.assertIsInstance(a, G)
                 self.assertEqual(a.x, 3)
 
                 things = "arguments" if sys.version_info >= (3, 11) else "parameters"

src/typing_extensions.py

@@ -9,7 +9,6 @@
 import typing
 import warnings
 
-
 __all__ = [
     # Super-special typing primitives.
     'Any',
@@ -659,7 +658,9 @@ def _proto_hook(cls, other):
                     isinstance(annotations, collections.abc.Mapping)
                     and attr in annotations
                     and issubclass(other, (typing.Generic, _ProtocolMeta))
-                    and other._is_protocol
+                    # All subclasses of Generic have an _is_proto attribute on 3.8+
+                    # But not on 3.7
+                    and getattr(other, "_is_protocol", False)
                 ):
                     break
             else: