constexpr index_type required_span_size() const noexcept;
template<class... Indices>
constexpr index_type operator()(Indices... i) const noexcept;
constexpr index_type stride(rank_type i) const;
template<class OtherExtents>
friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;