std::ranges::possibly-const-range

namespace std::ranges {
  template<input_range R>
  constexpr auto& possibly-const-range(R& r) noexcept {
    if constexpr (constant_range<const R> && !constant_range<R>) {
      return const_cast<const R&>(r);
    } else {
      return r;
    }
  }
}