gh-104555: Runtime-checkable protocols: Don't let previous calls to isinstance() influence whether issubclass() raises an exception

Lib/test/test_typing.py

@@ -2654,6 +2654,42 @@ class D(PNonCall): ...
         with self.assertRaises(TypeError):
             issubclass(D, PNonCall)
 
+    def test_no_weird_caching_with_issubclass_after_isinstance(self):
+        @runtime_checkable
+        class Spam(Protocol):
+            x: int
+
+        class Eggs:
+            def __init__(self) -> None:
+                self.x = 42
+
+        # gh-104555: ABCMeta might cache the result of this isinstance check
+        # if we called super().__instancecheck__ in the wrong place
+        # in _ProtocolMeta.__instancheck__...
+        self.assertIsInstance(Eggs(), Spam)
+
+        # ...and if it did, then TypeError wouldn't be raised here!
+        with self.assertRaises(TypeError):
+            issubclass(Eggs, Spam)
+
+    def test_no_weird_caching_with_issubclass_after_isinstance_pep695(self):
+        @runtime_checkable
+        class Spam[T](Protocol):
+            x: T
+
+        class Eggs[T]:
+            def __init__(self, x: T) -> None:
+                self.x = x
+
+        # gh-104555: ABCMeta might cache the result of this isinstance check
+        # if we called super().__instancecheck__ in the wrong place
+        # in _ProtocolMeta.__instancheck__...
+        self.assertIsInstance(Eggs(42), Spam)
+
+        # ...and if it did, then TypeError wouldn't be raised here!
+        with self.assertRaises(TypeError):
+            issubclass(Eggs, Spam)
+
     def test_protocols_isinstance(self):
         T = TypeVar('T')
 

Lib/typing.py

@@ -1798,7 +1798,10 @@ def __instancecheck__(cls, instance):
             raise TypeError("Instance and class checks can only be used with"
                             " @runtime_checkable protocols")
 
-        if super().__instancecheck__(instance):
+        # gh-104555: Don't call super().__instancecheck__ here,
+        # ABCMeta.__instancecheck__ would erroneously use it to populate the cache,
+        # which would cause incorrect results for *issubclass()* calls
+        if type.__instancecheck__(cls, instance):
             return True
 
         if is_protocol_cls:
@@ -1813,7 +1816,7 @@ def __instancecheck__(cls, instance):
             else:
                 return True
 
-        return False
+        return super().__instancecheck__(instance)
 
 
 class Protocol(Generic, metaclass=_ProtocolMeta):