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#include <initializer_list>
 
// mostly freestanding
namespace std {
 // constants
 inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
 
 template<class T>
 concept /*integral-constant-like*/ = // exposition only
 is_integral_v<decltype(T::value)> &&
 !is_same_v<bool, remove_const_t<decltype(T::value)>> &&
 convertible_to<T, decltype(T::value)> &&
 equality_comparable_with<T, decltype(T::value)> &&
 bool_constant<T() == T::value>::value &&
 bool_constant<static_cast<decltype(T::value)>(T()) == T::value>::value;
 
 template<class T>
 constexpr size_t /*maybe-static-ext*/ = dynamic_extent; // exposition only
 template</*integral-constant-like*/ T>
 constexpr size_t /*maybe-static-ext*/<T> = { T::value };
 
 // class template span
 template<class ElementType, size_t Extent = dynamic_extent>
 class span; // partially freestanding
 
 template<class ElementType, size_t Extent>
 constexpr bool ranges::enable_view <span<ElementType, Extent>> = true;
 template<class ElementType, size_t Extent>
 constexpr bool ranges::enable_borrowed_range <span<ElementType, Extent>> = true;
 
 // views of object representation
 template<class ElementType, size_t Extent>
 span<const byte,
 Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
 as_bytes(span<ElementType, Extent> s) noexcept;
 
 template<class ElementType, size_t Extent>
 span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
 as_writable_bytes(span<ElementType, Extent> s) noexcept;
}

namespace std {
 template<class ElementType, size_t Extent = dynamic_extent>
 class span
 {
 public:
 // constants and types
 using element_type = ElementType;
 using value_type = remove_cv_t<ElementType>;
 using size_type = size_t;
 using difference_type = ptrdiff_t;
 using pointer = element_type*;
 using const_pointer = const element_type*;
 using reference = element_type&;
 using const_reference = const element_type&;
 using iterator = /* implementation-defined */;
 using const_iterator = std::const_iterator <iterator>;
 using reverse_iterator = std::reverse_iterator <iterator>;
 using const_reverse_iterator = std::const_iterator <reverse_iterator>;
 static constexpr size_type extent = Extent;
 
 // constructors, copy, and assignment
 constexpr span() noexcept;
 template<class It>
 constexpr explicit(extent != dynamic_extent) span(It first, size_type count);
 template<class It, class End>
 constexpr explicit(extent != dynamic_extent) span(It first, End last);
 template<size_t N>
 constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
 template<class T, size_t N>
 constexpr span(array<T, N>& arr) noexcept;
 template<class T, size_t N>
 constexpr span(const array<T, N>& arr) noexcept;
 template<class R>
 constexpr explicit(extent != dynamic_extent) span(R&& r);
 constexpr explicit(extent != dynamic_extent)
 span(std::initializer_list <value_type> il);
 constexpr span(const span& other) noexcept = default;
 template<class OtherElementType, size_t OtherExtent>
 constexpr explicit(/* see description */)
 span(const span<OtherElementType, OtherExtent>& s) noexcept;
 
 constexpr span& operator=(const span& other) noexcept = default;
 
 // subviews
 template<size_t Count>
 constexpr span<element_type, Count> first() const;
 template<size_t Count>
 constexpr span<element_type, Count> last() const;
 template<size_t Offset, size_t Count = dynamic_extent>
 constexpr span<element_type, /* see description */> subspan() const;
 
 constexpr span<element_type, dynamic_extent> first(size_type count) const;
 constexpr span<element_type, dynamic_extent> last(size_type count) const;
 constexpr span<element_type, dynamic_extent> subspan(
 size_type offset,
 size_type count = dynamic_extent) const;
 
 // observers
 constexpr size_type size() const noexcept;
 constexpr size_type size_bytes() const noexcept;
 constexpr bool empty() const noexcept;
 
 // element access
 constexpr reference operator[](size_type idx) const;
 constexpr reference at(size_type idx) const; // freestanding-deleted
 constexpr reference front() const;
 constexpr reference back() const;
 constexpr pointer data() const noexcept;
 
 // iterator support
 constexpr iterator begin() const noexcept;
 constexpr iterator end() const noexcept;
 constexpr const_iterator cbegin() const noexcept { return begin(); }
 constexpr const_iterator cend() const noexcept { return end(); }
 constexpr reverse_iterator rbegin() const noexcept;
 constexpr reverse_iterator rend() const noexcept;
 constexpr const_reverse_iterator crbegin() const noexcept { return rbegin(); }
 constexpr const_reverse_iterator crend() const noexcept { return rend(); }
 
 private:
 pointer /*data_*/; // exposition only
 size_type /*size_*/; // exposition only
 };
 
 template<class It, class EndOrSize>
 span(It, EndOrSize)
 -> span<remove_reference_t<iter_reference_t<It>>, /*maybe-static-ext*/<EndOrSize>>;
 template<class T, size_t N>
 span(T (&)[N]) -> span<T, N>;
 template<class T, size_t N>
 span(array<T, N>&) -> span<T, N>;
 template<class T, size_t N>
 span(const array<T, N>&) -> span<const T, N>;
 template<class R>
 span(R&&) -> span<remove_reference_t<ranges::range_reference_t <R>>>;
}