gh-104935: typing: Fix interactions between @runtime_checkable and Generic

Lib/test/test_typing.py

@@ -2472,6 +2472,46 @@ def f():
         self.assertNotIsSubclass(types.FunctionType, P)
         self.assertNotIsInstance(f, P)
 
+    def test_runtime_checkable_generic_non_protocol(self):
+        # Make sure this doesn't raise AttributeError
+        with self.assertRaisesRegex(
+            TypeError,
+            "@runtime_checkable can be only applied to protocol classes",
+        ):
+            @runtime_checkable
+            class Foo[T]: ...
+
+    def test_runtime_checkable_generic(self):
+        @runtime_checkable
+        class Foo[T](Protocol):
+            def meth(self) -> T: ...
+
+        class Impl:
+            def meth(self) -> int: ...
+
+        self.assertIsSubclass(Impl, Foo)
+
+        class NotImpl:
+            def method(self) -> int: ...
+
+        self.assertNotIsSubclass(NotImpl, Foo)
+
+    def test_pep695_generics_can_be_runtime_checkable(self):
+        @runtime_checkable
+        class HasX(Protocol):
+            x: int
+        class Bar[T]:
+            x: T
+            def __init__(self, x):
+                self.x = x
+        class Capybara[T]:
+            y: str
+            def __init__(self, y):
+                self.y = y
+
+        self.assertIsInstance(Bar(1), HasX)
+        self.assertNotIsInstance(Capybara('a'), HasX)
+
     def test_everything_implements_empty_protocol(self):
         @runtime_checkable
         class Empty(Protocol):

Misc/NEWS.d/next/Library/2023-05-25-08-50-47.gh-issue-104935.-rm1BR.rst

@@ -0,0 +1,4 @@
+Fix bugs with the interaction between :func:`typing.runtime_checkable` and
+:class:`typing.Generic` that were introduced by the :pep:`695`
+implementation. All generic classes now again have an ``_is_protocol``
+attribute. Patch by Jelle Zijlstra.

Objects/typevarobject.c

@@ -1640,6 +1640,11 @@ int _Py_initialize_generic(PyInterpreterState *interp)
     MAKE_TYPE(paramspecargs);
     MAKE_TYPE(paramspeckwargs);
 #undef MAKE_TYPE
+    if (PyDict_SetItemString(interp->cached_objects.generic_type->tp_dict,
+                             "_is_protocol", Py_False) < 0) {
+        return -1;
+    }
+    PyType_Modified(interp->cached_objects.generic_type);
     return 0;
 }