constexpr mapping() noexcept;
template<class OtherIndexType>
constexpr mapping(const extents_type& e, span<OtherIndexType, rank_> s) noexcept;
template<class OtherIndexType>
constexpr mapping(const extents_type& e, const array<OtherIndexType, rank_>& s) noexcept;
template<class StridedLayoutMapping>
constexpr explicit(see below)
mapping(const StridedLayoutMapping& other) noexcept;