Standard library header <span> (C++20)

#include <initializer_list>
// 主要独立实现
namespace std {
  // 常量
  inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
  template<class T>
  concept /*integral-constant-like*/ = // 仅用于说明
    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; // 仅用于说明
  template</*integral-constant-like*/ T>
  constexpr size_t /*maybe-static-ext*/<T> = { T::value };
  // 类模板 span
  template<class ElementType, size_t Extent = dynamic_extent>
  class span; // 部分独立实现
  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;
  // 对象表示的视图
  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:
    // 常量和类型
    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                    = /* 实现定义 */;
    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;
    // 构造函数、复制和赋值
    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(/* 参见描述 */)
      span(const span<OtherElementType, OtherExtent>& s) noexcept;
    constexpr span& operator=(const span& other) noexcept = default;
    // 子视图
    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, /* 参见描述 */> 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;
    // 观察器
    constexpr size_type size() const noexcept;
    constexpr size_type size_bytes() const noexcept;
    constexpr bool empty() const noexcept;
    // 元素访问
    constexpr reference operator[](size_type idx) const;
    constexpr reference at(size_type idx) const; // 独立环境已删除
    constexpr reference front() const;
    constexpr reference back() const;
    constexpr pointer data() const noexcept;
    // 迭代器支持
    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()<span class="sy