include/span

// -*- C++ -*-
//===------------------------------ span ---------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
//
//===---------------------------------------------------------------------===//

#ifndef _LIBCPP_SPAN
#define _LIBCPP_SPAN

/*
    span synopsis

namespace std {

// constants
inline constexpr ptrdiff_t dynamic_extent = -1;

// [views.span], class template span
template <class ElementType, ptrdiff_t Extent = dynamic_extent>
    class span;

// [span.comparison], span comparison operators
template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
    constexpr bool operator==(span<T, X> l, span<U, Y> r);
template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
    constexpr bool operator!=(span<T, X> l, span<U, Y> r);
template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
    constexpr bool operator<(span<T, X> l, span<U, Y> r);
template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
    constexpr bool operator<=(span<T, X> l, span<U, Y> r);
template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
    constexpr bool operator>(span<T, X> l, span<U, Y> r);
template <class T, ptrdiff_t X, class U, ptrdiff_t Y>
    constexpr bool operator>=(span<T, X> l, span<U, Y> r);

// [span.objectrep], views of object representation
template <class ElementType, ptrdiff_t Extent>
    span<const byte, ((Extent == dynamic_extent) ? dynamic_extent : 
        (static_cast<ptrdiff_t>(sizeof(ElementType)) * Extent))> as_bytes(span<ElementType, Extent> s) noexcept;

template <class ElementType, ptrdiff_t Extent>
    span<      byte, ((Extent == dynamic_extent) ? dynamic_extent : 
        (static_cast<ptrdiff_t>(sizeof(ElementType)) * Extent))> as_writable_bytes(span<ElementType, Extent> s) noexcept;


namespace std {
template <class ElementType, ptrdiff_t Extent = dynamic_extent>
class span {
public:
    // constants and types
    using element_type = ElementType;
    using value_type = remove_cv_t<ElementType>;
    using index_type = ptrdiff_t;
    using difference_type = ptrdiff_t;
    using pointer = element_type*;
    using reference = element_type&;
    using iterator = implementation-defined;
    using const_iterator = implementation-defined;
    using reverse_iterator = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    static constexpr index_type extent = Extent;

    // [span.cons], span constructors, copy, assignment, and destructor
    constexpr span() noexcept;
    constexpr span(pointer ptr, index_type count);
    constexpr span(pointer firstElem, pointer lastElem);
    template <size_t N>
        constexpr span(element_type (&arr)[N]) noexcept;
    template <size_t N>
        constexpr span(array<value_type, N>& arr) noexcept;
    template <size_t N>
        constexpr span(const array<value_type, N>& arr) noexcept;
    template <class Container>
        constexpr span(Container& cont);
    template <class Container>
        constexpr span(const Container& cont);
    constexpr span(const span& other) noexcept = default;
    template <class OtherElementType, ptrdiff_t OtherExtent>
        constexpr span(const span<OtherElementType, OtherExtent>& s) noexcept;
    ~span() noexcept = default;
    constexpr span& operator=(const span& other) noexcept = default;

    // [span.sub], span subviews
    template <ptrdiff_t Count>
        constexpr span<element_type, Count> first() const;
    template <ptrdiff_t Count>
        constexpr span<element_type, Count> last() const;
    template <ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent>
        constexpr span<element_type, see below> subspan() const;

    constexpr span<element_type, dynamic_extent> first(index_type count) const;
    constexpr span<element_type, dynamic_extent> last(index_type count) const;
    constexpr span<element_type, dynamic_extent> subspan(index_type offset, index_type count = dynamic_extent) const;

    // [span.obs], span observers
    constexpr index_type size() const noexcept;
    constexpr index_type size_bytes() const noexcept;
    constexpr bool empty() const noexcept;

    // [span.elem], span element access
    constexpr reference operator[](index_type idx) const;
    constexpr reference operator()(index_type idx) const;
    constexpr pointer data() const noexcept;

    // [span.iterators], span iterator support
    constexpr iterator begin() const noexcept;
    constexpr iterator end() const noexcept;
    constexpr const_iterator cbegin() const noexcept;
    constexpr const_iterator cend() const noexcept;
    constexpr reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator rend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

private:
    pointer data_;     // exposition only
    index_type size_;  // exposition only
};

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 Container>
    span(Container&) -> span<typename Container::value_type>;

template<class Container>
    span(const Container&) -> span<const typename Container::value_type>;

} // namespace std

*/

#include <cstddef>      // for ptrdiff_t
#include <iterator>     // for iterators
#include <array>        // for array
#include <type_traits>  // for remove_cv, etc
#include <cstddef>      // for byte

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER > 17

inline constexpr ptrdiff_t dynamic_extent = -1;
template <typename _Tp, ptrdiff_t _Extent = dynamic_extent> class span;


template <class _Tp>
struct __is_span_impl : public false_type {};

template <class _Tp, ptrdiff_t _Extent>
struct __is_span_impl<span<_Tp, _Extent>> : public true_type {};

template <class _Tp>
struct __is_span : public __is_span_impl<remove_cv_t<_Tp>> {};

template <class _Tp>
struct __is_std_array_impl : public false_type {};

template <class _Tp, size_t _Sz>
struct __is_std_array_impl<array<_Tp, _Sz>> : public true_type {};

template <class _Tp>
struct __is_std_array : public __is_std_array_impl<remove_cv_t<_Tp>> {};

template <class _Tp, class _ElementType, class = void>
struct __is_span_compatible_container : public false_type {};

template <class _Tp, class _ElementType>
struct __is_span_compatible_container<_Tp, _ElementType,
        void_t<
        // is not a specialization of span
            typename enable_if<!__is_span<_Tp>::value, nullptr_t>::type,
        // is not a specialization of array
            typename enable_if<!__is_std_array<_Tp>::value, nullptr_t>::type,
        // is_array_v<Container> is false,
            typename enable_if<!is_array_v<_Tp>, nullptr_t>::type,
        // data(cont) and size(cont) are well formed
            decltype(data(declval<_Tp>())),
            decltype(size(declval<_Tp>())),
        // remove_pointer_t<decltype(data(cont))>(*)[] is convertible to ElementType(*)[]
            typename enable_if<
				is_convertible_v<remove_pointer_t<decltype(data(declval<_Tp &>()))>(*)[],
				                 _ElementType(*)[]>,
                nullptr_t>::type
        >>
    : public true_type {};


template <typename _Tp, ptrdiff_t _Extent>
class _LIBCPP_TEMPLATE_VIS span {
public:
//  constants and types
    using element_type           = _Tp;
    using value_type             = remove_cv_t<_Tp>;
    using index_type             = ptrdiff_t;
    using difference_type        = ptrdiff_t;
    using pointer                = _Tp *;
    using const_pointer          = const _Tp *; // not in standard
    using reference              = _Tp &;
    using const_reference        = const _Tp &; // not in standard
    using iterator               =  __wrap_iter<pointer>;
    using const_iterator         =  __wrap_iter<const_pointer>;
    using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
    using const_reverse_iterator = _VSTD::reverse_iterator<const_iterator>;

    static constexpr index_type extent = _Extent;
    static_assert (_Extent >= 0, "Can't have a span with an extent < 0");

// [span.cons], span constructors, copy, assignment, and destructor 
    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}
    { static_assert(_Extent == 0, "Can't default construct a statically sized span with size > 0"); }

    constexpr span           (const span&) noexcept = default;
    constexpr span& operator=(const span&) noexcept = default;

    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __ptr, index_type __count) : __data{__ptr}
        { (void)__count; _LIBCPP_ASSERT(_Extent == __count, "size mismatch in span's constructor (ptr, len)"); }
    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __f, pointer __l) : __data{__f}
        { (void)__l;     _LIBCPP_ASSERT(_Extent == distance(__f, __l), "size mismatch in span's constructor (ptr, ptr)"); }

    _LIBCPP_INLINE_VISIBILITY constexpr span(element_type (&__arr)[_Extent])          noexcept : __data{__arr} {}
    _LIBCPP_INLINE_VISIBILITY constexpr span(      array<value_type, _Extent>& __arr) noexcept : __data{__arr.data()} {}
    _LIBCPP_INLINE_VISIBILITY constexpr span(const array<value_type, _Extent>& __arr) noexcept : __data{__arr.data()} {}

    template <class _Container>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(      _Container& __c,
            enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
        : __data{_VSTD::data(__c)}
        { _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (container))"); }

    template <class _Container>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(const _Container& __c,
            enable_if_t<__is_span_compatible_container<const _Container, _Tp>::value, nullptr_t> = nullptr)
        : __data{_VSTD::data(__c)}
        { _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (const container)"); }

    template <class _OtherElementType>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(const span<_OtherElementType, _Extent>& __other,
                       enable_if_t<
                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                          nullptr_t> = nullptr)
        : __data{__other.data()} {}

    template <class _OtherElementType>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(const span<_OtherElementType, dynamic_extent>& __other,
                       enable_if_t<
                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                          nullptr_t> = nullptr) noexcept
        : __data{__other.data()} { _LIBCPP_ASSERT(_Extent == __other.size(), "size mismatch in span's constructor (other span)"); }


//  ~span() noexcept = default;

    template <ptrdiff_t _Count>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span<element_type, _Count> first() const noexcept
    {
        static_assert(_Count >= 0, "Count must be >= 0 in span::first()");
        static_assert(_Count <= _Extent, "Count out of range in span::first()");
        return {data(), _Count};
    }

    template <ptrdiff_t _Count>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span<element_type, _Count> last() const noexcept
    {
        static_assert(_Count >= 0, "Count must be >= 0 in span::last()");
        static_assert(_Count <= _Extent, "Count out of range in span::last()");
        return {data() + size() - _Count, _Count};
    }
    
    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> first(index_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count >= 0 && __count <= size(), "Count out of range in span::first(count)");
        return {data(), __count};
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> last(index_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count >= 0 && __count <= size(), "Count out of range in span::last(count)");
        return {data() + size() - __count, __count};
    }

    template <ptrdiff_t _Offset, ptrdiff_t _Count = dynamic_extent>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr auto subspan() const noexcept
        -> span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>
    {
        _LIBCPP_ASSERT(_Offset >= 0 && _Offset <= size(), "Offset out of range in span::subspan()");
        return {data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
    }


    inline _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent>
       subspan(index_type __offset, index_type __count = dynamic_extent) const noexcept
    { 
        _LIBCPP_ASSERT( __offset >= 0 && __offset <= size(), "Offset out of range in span::subspan(offset, count)");
        _LIBCPP_ASSERT((__count  >= 0 && __count  <= size()) || __count == dynamic_extent, "Count out of range in span::subspan(offset, count)");
        if (__count == dynamic_extent)
            return {data() + __offset, size() - __offset};
        _LIBCPP_ASSERT(__offset + __count <= size(), "count + offset out of range in span::subspan(offset, count)");
        return {data() + __offset, __count};
    }

    _LIBCPP_INLINE_VISIBILITY constexpr index_type size()       const noexcept { return _Extent; }
    _LIBCPP_INLINE_VISIBILITY constexpr index_type size_bytes() const noexcept { return _Extent * sizeof(element_type); }
    _LIBCPP_INLINE_VISIBILITY constexpr bool empty()            const noexcept { return _Extent == 0; }

    _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](index_type __idx) const noexcept
    {
        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T,N>[] index out of bounds");
		return __data[__idx];
	}

    _LIBCPP_INLINE_VISIBILITY constexpr reference operator()(index_type __idx) const noexcept
    {
        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T,N>() index out of bounds");
		return __data[__idx];
	}

    _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }

// [span.iter], span iterator support
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_iterator          cbegin() const noexcept { return const_iterator(data()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_iterator            cend() const noexcept { return const_iterator(data() + size()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_reverse_iterator crbegin() const noexcept { return const_reverse_iterator(cend()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_reverse_iterator   crend() const noexcept { return const_reverse_iterator(cbegin()); }

    _LIBCPP_INLINE_VISIBILITY constexpr void swap(span &__other) noexcept
    {
        pointer __p = __data;
        __data = __other.__data;
        __other.__data = __p;
    }
    
    _LIBCPP_INLINE_VISIBILITY span<const byte, _Extent * sizeof(element_type)> __as_bytes() const noexcept
    { return {reinterpret_cast<const byte *>(data()), size_bytes()}; }

    _LIBCPP_INLINE_VISIBILITY span<byte, _Extent * sizeof(element_type)> __as_writeable_bytes() const noexcept
    { return {reinterpret_cast<byte *>(data()), size_bytes()}; }

private:
    pointer    __data;

};


template <typename _Tp>
class _LIBCPP_TEMPLATE_VIS span<_Tp, dynamic_extent> {
private:

public:
//  constants and types
    using element_type           = _Tp;
    using value_type             = remove_cv_t<_Tp>;
    using index_type             = ptrdiff_t;
    using difference_type        = ptrdiff_t;
    using pointer                = _Tp *;
    using const_pointer          = const _Tp *; // not in standard
    using reference              = _Tp &;
    using const_reference        = const _Tp &; // not in standard
    using iterator               =  __wrap_iter<pointer>;
    using const_iterator         =  __wrap_iter<const_pointer>;
    using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
    using const_reverse_iterator = _VSTD::reverse_iterator<const_iterator>;

    static constexpr index_type extent = dynamic_extent;

// [span.cons], span constructors, copy, assignment, and destructor 
    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}, __size{0} {}

    constexpr span           (const span&) noexcept = default;
    constexpr span& operator=(const span&) noexcept = default;

    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __ptr, index_type __count) : __data{__ptr}, __size{__count} {}
    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __f, pointer __l) : __data{__f}, __size{distance(__f, __l)} {}

    template <size_t _Sz>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(element_type (&__arr)[_Sz]) noexcept : __data{__arr}, __size{_Sz} {}

    template <size_t _Sz>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(array<value_type, _Sz>& __arr)       noexcept : __data{__arr.data()}, __size{_Sz} {}
    
    template <size_t _Sz>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(const array<value_type, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}

    template <class _Container>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(      _Container& __c,
            enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
        : __data{_VSTD::data(__c)}, __size{(index_type) _VSTD::size(__c)} {}

    template <class _Container>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(const _Container& __c,
            enable_if_t<__is_span_compatible_container<const _Container, _Tp>::value, nullptr_t> = nullptr)
        : __data{_VSTD::data(__c)}, __size{(index_type) _VSTD::size(__c)} {}


    template <class _OtherElementType, ptrdiff_t _OtherExtent>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span(const span<_OtherElementType, _OtherExtent>& __other,
                       enable_if_t<
                          is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                          nullptr_t> = nullptr) noexcept
        : __data{__other.data()}, __size{__other.size()} {}

//    ~span() noexcept = default;

    template <ptrdiff_t _Count>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span<element_type, _Count> first() const noexcept
    {
        static_assert(_Count >= 0);
        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::first()");
        return {data(), _Count};
    }

    template <ptrdiff_t _Count>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span<element_type, _Count> last() const noexcept
    {
        static_assert(_Count >= 0);
        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::last()");
        return {data() + size() - _Count, _Count};
    }

    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> first(index_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count >= 0 && __count <= size(), "Count out of range in span::first(count)");
        return {data(), __count};
    }
        
    _LIBCPP_INLINE_VISIBILITY
    constexpr span<element_type, dynamic_extent> last (index_type __count) const noexcept
    {
        _LIBCPP_ASSERT(__count >= 0 && __count <= size(), "Count out of range in span::last(count)");
        return {data() + size() - __count, __count};
    }

    template <ptrdiff_t _Offset, ptrdiff_t _Count = dynamic_extent>
    inline _LIBCPP_INLINE_VISIBILITY
        constexpr span<_Tp, dynamic_extent> subspan() const noexcept
    {
        _LIBCPP_ASSERT(_Offset >= 0 && _Offset <= size(), "Offset out of range in span::subspan()");
        _LIBCPP_ASSERT(_Count == dynamic_extent || _Offset + _Count <= size(), "Count out of range in span::subspan()");
        return {data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
    }

    constexpr span<element_type, dynamic_extent>
    inline _LIBCPP_INLINE_VISIBILITY
       subspan(index_type __offset, index_type __count = dynamic_extent) const noexcept
    { 
        _LIBCPP_ASSERT( __offset >= 0 && __offset <= size(), "Offset out of range in span::subspan(offset, count)");
        _LIBCPP_ASSERT((__count  >= 0 && __count  <= size()) || __count == dynamic_extent, "count out of range in span::subspan(offset, count)");
        if (__count == dynamic_extent)
            return {data() + __offset, size() - __offset};
        _LIBCPP_ASSERT(__offset + __count <= size(), "Offset + count out of range in span::subspan(offset, count)");
        return {data() + __offset, __count};
    }

    _LIBCPP_INLINE_VISIBILITY constexpr index_type size()       const noexcept { return __size; }
    _LIBCPP_INLINE_VISIBILITY constexpr index_type size_bytes() const noexcept { return __size * sizeof(element_type); }
    _LIBCPP_INLINE_VISIBILITY constexpr bool empty()            const noexcept { return __size == 0; }

    _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](index_type __idx) const noexcept
    {
        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T>[] index out of bounds");
		return __data[__idx];
	}

    _LIBCPP_INLINE_VISIBILITY constexpr reference operator()(index_type __idx) const noexcept
    {
        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T>() index out of bounds");
		return __data[__idx];
	}

    _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }

// [span.iter], span iterator support
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                 begin() const noexcept { return iterator(data()); }
    _LIBCPP_INLINE_VISIBILITY constexpr iterator                   end() const noexcept { return iterator(data() + size()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_iterator          cbegin() const noexcept { return const_iterator(data()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_iterator            cend() const noexcept { return const_iterator(data() + size()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
    _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_reverse_iterator crbegin() const noexcept { return const_reverse_iterator(cend()); }
    _LIBCPP_INLINE_VISIBILITY constexpr const_reverse_iterator   crend() const noexcept { return const_reverse_iterator(cbegin()); }

    _LIBCPP_INLINE_VISIBILITY constexpr void swap(span &__other) noexcept
    {
        pointer __p = __data;
        __data = __other.__data;
        __other.__data = __p;

        index_type __sz = __size;
        __size = __other.__size;
        __other.__size = __sz;
    }

    _LIBCPP_INLINE_VISIBILITY span<const byte, dynamic_extent> __as_bytes() const noexcept
    { return {reinterpret_cast<const byte *>(data()), size_bytes()}; }

    _LIBCPP_INLINE_VISIBILITY span<byte, dynamic_extent> __as_writeable_bytes() const noexcept
    { return {reinterpret_cast<byte *>(data()), size_bytes()}; }

private:
    pointer    __data;
    index_type __size;
};

template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
    constexpr bool
    operator==(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
    { return equal(__lhs.begin(), __lhs.end(), __rhs.begin(), __rhs.end()); }

template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
    constexpr bool
    operator!=(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
    { return !(__rhs == __lhs); }

template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
    constexpr bool
    operator< (const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
    { return lexicographical_compare (__lhs.begin(), __lhs.end(), __rhs.begin(), __rhs.end()); }
    
template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
    constexpr bool
    operator<=(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
    { return !(__rhs < __lhs); }

template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
    constexpr bool
    operator> (const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
    { return __rhs < __lhs; }
    
template <class _Tp1, ptrdiff_t _Extent1, class _Tp2, ptrdiff_t _Extent2>
    constexpr bool
    operator>=(const span<_Tp1, _Extent1>& __lhs, const span<_Tp2, _Extent2>& __rhs)
    { return !(__lhs < __rhs); }

//  as_bytes & as_writeable_bytes
template <class _Tp, ptrdiff_t _Extent>
    auto as_bytes(span<_Tp, _Extent> __s) noexcept 
    -> decltype(__s.__as_bytes())
    { return __s.__as_bytes(); }

template <class _Tp, ptrdiff_t _Extent>
    auto as_writeable_bytes(span<_Tp, _Extent> __s) noexcept
    -> typename enable_if<!is_const_v<_Tp>, decltype(__s.__as_writeable_bytes())>::type
    { return __s.__as_writeable_bytes(); }

template <class _Tp, ptrdiff_t _Extent>
    constexpr void swap(span<_Tp, _Extent> &__lhs, span<_Tp, _Extent> &__rhs) noexcept
    { __lhs.swap(__rhs); }


//  Deduction guides
template<class _Tp, size_t _Sz>
    span(_Tp (&)[_Sz]) -> span<_Tp, _Sz>;
  
template<class _Tp, size_t _Sz>
    span(array<_Tp, _Sz>&) -> span<_Tp, _Sz>;

template<class _Tp, size_t _Sz>
    span(const array<_Tp, _Sz>&) -> span<const _Tp, _Sz>;

template<class _Container>
    span(_Container&) -> span<typename _Container::value_type>;

template<class _Container>
    span(const _Container&) -> span<const typename _Container::value_type>;

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP_STD_VER > 17
#endif // _LIBCPP_SPAN