def testProto3ParserDropDefaultScalar(self):
    message_proto2 = unittest_pb2.TestAllTypes()
    message_proto2.optional_int32 = 0
    message_proto2.optional_string = ''
    message_proto2.optional_bytes = b''
    self.assertEqual(len(message_proto2.ListFields()), 3)

    message_proto3 = unittest_proto3_arena_pb2.TestAllTypes()
    message_proto3.ParseFromString(message_proto2.SerializeToString())
    self.assertEqual(len(message_proto3.ListFields()), 0)