  void expectTrailers(
      std::function<void()> callback = std::function<void()>()) {
    if (callback) {
      EXPECT_CALL(*this, onTrailers(testing::_))
          .WillOnce(testing::InvokeWithoutArgs(callback))
          .RetiresOnSaturation();
    } else {
      EXPECT_CALL(*this, onTrailers(testing::_));
    }
  }
The product receives input or data, but it does
        not validate or incorrectly validates that the input has the
        properties that are required to process the data safely and
        correctly.