constexpr explicit basic_mask(value_type x) noexcept;
template<size_t UBytes, class UAbi>
constexpr explicit basic_mask(const basic_mask<UBytes, UAbi>& x) noexcept;
template<class G> constexpr explicit basic_mask(G&& gen);
constexpr basic_mask(const bitset<size()>& b) noexcept;
constexpr explicit basic_mask(unsigned_integral auto val) noexcept;