Function emplace

src/include/expected.hpp:1762–1770

  template <class... Args, detail::enable_if_t<std::is_nothrow_constructible<
                               T, Args &&...>::value> * = nullptr>
  void emplace(Args &&...args) {
    if (has_value()) {
      val().~T();
    } else {
      err().~unexpected<E>();
      this->m_has_val = true;
    }
    ::new (valptr()) T(std::forward<Args>(args)...);
  }

  template <class... Args, detail::enable_if_t<!std::is_nothrow_constructible<
                               T, Args &&...>::value> * = nullptr>