gh-105237: Allow calling issubclass(X, typing.Protocol) again (#105239)

Lib/test/test_typing.py

@@ -2758,6 +2758,65 @@ class ProtocolTests(BaseTestCase):
         with self.assertRaisesRegex(TypeError, only_classes_allowed):
             issubclass(1, BadPG)
 
+    def test_issubclass_and_isinstance_on_Protocol_itself(self):
+        class C:
+            def x(self): pass
+
+        self.assertNotIsSubclass(object, Protocol)
+        self.assertNotIsInstance(object(), Protocol)
+
+        self.assertNotIsSubclass(str, Protocol)
+        self.assertNotIsInstance('foo', Protocol)
+
+        self.assertNotIsSubclass(C, Protocol)
+        self.assertNotIsInstance(C(), Protocol)
+
+        only_classes_allowed = r"issubclass\(\) arg 1 must be a class"
+
+        with self.assertRaisesRegex(TypeError, only_classes_allowed):
+            issubclass(1, Protocol)
+        with self.assertRaisesRegex(TypeError, only_classes_allowed):
+            issubclass('foo', Protocol)
+        with self.assertRaisesRegex(TypeError, only_classes_allowed):
+            issubclass(C(), Protocol)
+
+        T = TypeVar('T')
+
+        @runtime_checkable
+        class EmptyProtocol(Protocol): pass
+
+        @runtime_checkable
+        class SupportsStartsWith(Protocol):
+            def startswith(self, x: str) -> bool: ...
+
+        @runtime_checkable
+        class SupportsX(Protocol[T]):
+            def x(self): ...
+
+        for proto in EmptyProtocol, SupportsStartsWith, SupportsX:
+            with self.subTest(proto=proto.__name__):
+                self.assertIsSubclass(proto, Protocol)
+
+        # gh-105237 / PR #105239:
+        # check that the presence of Protocol subclasses
+        # where `issubclass(X, <subclass>)` evaluates to True
+        # doesn't influence the result of `issubclass(X, Protocol)`
+
+        self.assertIsSubclass(object, EmptyProtocol)
+        self.assertIsInstance(object(), EmptyProtocol)
+        self.assertNotIsSubclass(object, Protocol)
+        self.assertNotIsInstance(object(), Protocol)
+
+        self.assertIsSubclass(str, SupportsStartsWith)
+        self.assertIsInstance('foo', SupportsStartsWith)
+        self.assertNotIsSubclass(str, Protocol)
+        self.assertNotIsInstance('foo', Protocol)
+
+        self.assertIsSubclass(C, SupportsX)
+        self.assertIsInstance(C(), SupportsX)
+        self.assertNotIsSubclass(C, Protocol)
+        self.assertNotIsInstance(C(), Protocol)
+
     def test_protocols_issubclass_non_callable(self):
         class C:
             x = 1