std::ranges::in_value_result(C++23)

namespace std::ranges {
  // (1)
  template<class I, class T>
  struct in_value_result {
    [[no_unique_address]] I in;
    [[no_unique_address]] T value;

    template<class I2, class T2>
      requires convertible_to<const I&, I2> &&
               convertible_to<const T&, T2>
    constexpr operator in_value_result<I2, T2>() const & {
      return {in, value};
    }

    template<class I2, class T2>
      requires convertible_to<I, I2> &&
               convertible_to<T, T2>
    constexpr operator in_value_result<I2, T2>() && {
      return {std::move(in), std::move(value)};
    }
  };

  // (2)
  template<class I, class T>
  using fold_left_with_iter_result = in_value_result<I, T>;

  // (3)
  template<class I, class T>
  using fold_left_first_with_iter_result = in_value_result<I, T>;   
}