gh-105834: Add tests for calling issubclass() between two protocols

Lib/test/test_typing.py

@@ -2759,6 +2759,80 @@ def x(self): ...
         with self.assertRaisesRegex(TypeError, only_classes_allowed):
             issubclass(1, BadPG)
 
+    def test_implicit_issubclass_between_two_protocols(self):
+        @runtime_checkable
+        class CallableMembersProto(Protocol):
+            def meth(self): ...
+
+        # All the below protocols should be considered "subclasses"
+        # of CallableMembersProto at runtime,
+        # even though none of them explicitly subclass CallableMembersProto
+
+        class IdenticalProto(Protocol):
+            def meth(self): ...
+
+        class SupersetProto(Protocol):
+            def meth(self): ...
+            def meth2(self): ...
+
+        class NonCallableMembersProto(Protocol):
+            meth: Callable[[], None]
+
+        class NonCallableMembersSupersetProto(Protocol):
+            meth: Callable[[], None]
+            meth2: Callable[[str, int], bool]
+
+        class MixedMembersProto1(Protocol):
+            meth: Callable[[], None]
+            def meth2(self): ...
+
+        class MixedMembersProto2(Protocol):
+            def meth(self): ...
+            meth2: Callable[[str, int], bool]
+
+        for proto in (
+            IdenticalProto, SupersetProto, NonCallableMembersProto,
+            NonCallableMembersSupersetProto, MixedMembersProto1, MixedMembersProto2
+        ):
+            with self.subTest(proto=proto.__name__):
+                self.assertIsSubclass(proto, CallableMembersProto)
+
+        # These two shouldn't be considered subclasses of CallableMembersProto, however,
+        # since they don't have the `meth` protocol member
+
+        class EmptyProtocol(Protocol): ...
+        class UnrelatedProtocol(Protocol):
+            def wut(self): ...
+
+        self.assertNotIsSubclass(EmptyProtocol, CallableMembersProto)
+        self.assertNotIsSubclass(UnrelatedProtocol, CallableMembersProto)
+
+        # These aren't protocols at all (despite having annotations),
+        # so they should only be considered subclasses of CallableMembersProto
+        # if they *actually have an attribute* matching the `meth` member
+        # (just having an annotation is insufficient)
+
+        class AnnotatedButNotAProtocol:
+            meth: Callable[[], None]
+
+        class NotAProtocolButAnImplicitSubclass:
+            def meth(self): pass
+
+        class NotAProtocolButAnImplicitSubclass2:
+            meth: Callable[[], None]
+            def meth(self): pass
+
+        class NotAProtocolButAnImplicitSubclass3:
+            meth: Callable[[], None]
+            meth2: Callable[[int, str], bool]
+            def meth(self): pass
+            def meth(self, x, y): return True
+
+        self.assertNotIsSubclass(AnnotatedButNotAProtocol, CallableMembersProto)
+        self.assertIsSubclass(NotAProtocolButAnImplicitSubclass, CallableMembersProto)
+        self.assertIsSubclass(NotAProtocolButAnImplicitSubclass2, CallableMembersProto)
+        self.assertIsSubclass(NotAProtocolButAnImplicitSubclass3, CallableMembersProto)
+
     def test_isinstance_checks_not_at_whim_of_gc(self):
         self.addCleanup(gc.enable)
         gc.disable()