constexpr array<index_type, rank_> strides() const noexcept;
constexpr index_type required_span_size() const noexcept;
template<class. Indices>
constexpr size_t operator()(Indices.. idxs) const noexcept;
static constexpr bool is_always_exhaustive() noexcept;
constexpr bool is_exhaustive() const noexcept;
constexpr index_type stride(rank_type r) const noexcept;
template<class LayoutLeftPaddedMapping>
friend constexpr bool operator=(const mapping& x, const LayoutLeftPaddedMapping& y) noexcept;