Method MakeVector

tests/gtest/gtest.h:11134–11136

 private:
  template <typename T, size_t... I>
  std::vector<T> MakeVector(IndexSequence<I...>) const {
    return std::vector<T>{static_cast<T>(v_.template Get<I>())...};
  }

  FlatTuple<Ts...> v_;
};