template<class... OtherIndexTypes>
constexpr reference operator[](OtherIndexTypes... indices) const;
template<class OtherIndexType>
constexpr reference operator[](span<OtherIndexType, rank()> indices) const;
template<class OtherIndexType>
constexpr reference operator[](const array<OtherIndexType, rank()>& indices) const;
constexpr size_type size() const noexcept;
[[nodiscard]] constexpr bool empty() const noexcept;
friend constexpr void swap(mdspan& x, mdspan& y) noexcept;