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

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This header is part of the ranges library.

Contents

[edit] Namespace aliases

namespace std {

    namespace views = ranges::views;

}

The namespace alias std::views is provided as a shorthand for std::ranges::views.

Concepts

Range concepts
Defined in namespace std::ranges
specifies that a type is a range, that is, it provides a begin iterator and an end sentinel
(concept) [edit]
specifies that a type is a range and iterators obtained from an expression of it can be safely returned without danger of dangling
(concept) [edit]
specifies that a range knows its size in constant time
(concept) [edit]
specifies that a range is a view, that is, it has constant time copy/move/assignment
(concept) [edit]
specifies a range whose iterator type satisfies input_iterator
(concept) [edit]
specifies a range whose iterator type satisfies output_iterator
(concept) [edit]
specifies a range whose iterator type satisfies forward_iterator
(concept) [edit]
specifies a range whose iterator type satisfies bidirectional_iterator
(concept) [edit]
specifies a range whose iterator type satisfies random_access_iterator
(concept) [edit]
specifies a range whose iterator type satisfies contiguous_iterator
(concept) [edit]
specifies that a range has identical iterator and sentinel types
(concept) [edit]
specifies the requirements for a range to be safely convertible to a view
(concept) [edit]
specifies that a range has read-only elements
(concept) [edit]

Functions

Range conversions
Defined in namespace std::ranges
constructs a non-view range from another range
(function template) [edit]

Classes

Range primitives
Defined in namespace std::ranges
obtains associated types of a range
(alias template) [edit]
Views
Defined in namespace std::ranges
helper class template for defining a view, using the curiously recurring template pattern
(class template) [edit]
combines an iterator-sentinel pair into a view
(class template) [edit]
Dangling iterator handling
Defined in namespace std::ranges
a placeholder type indicating that an iterator or a subrange should not be returned since it would be dangling
(class) [edit]
obtains iterator type or subrange type of a borrowed_range
(alias template) [edit]
Range adaptor objects utility
Defined in namespace std::ranges
helper base class template for defining a range adaptor closure object
(class template) [edit]
Factories
Defined in namespace std::ranges
an empty view with no elements
(class template) (variable template) [edit]
a view that contains a single element of a specified value
(class template) (customization point object) [edit]
a view consisting of a sequence generated by repeatedly incrementing an initial value
(class template) (customization point object) [edit]
a view consisting of the elements obtained by successive application of operator>> on the associated input stream
(class template) (customization point object) [edit]
a view consisting of a generated sequence by repeatedly producing the same value
(class template) (customization point object) [edit]
a view consisting of tuples of results calculated by the n-ary cartesian product of the adapted views
(class template) (customization point object) [edit]
Adaptors
Defined in namespace std::ranges
a view that includes all elements of a range
(alias template) (range adaptor object) [edit]
a view of the elements of some other range
(class template) [edit]
a view with unique ownership of some range
(class template) [edit]
a view that consists of the elements of a range that satisfies a predicate
(class template) (range adaptor object) [edit]
a view of a sequence that applies a transformation function to each element
(class template) (range adaptor object) [edit]
a view consisting of the first N elements of another view
(class template) (range adaptor object) [edit]
a view consisting of the initial elements of another view, until the first element on which a predicate returns false
(class template) (range adaptor object) [edit]
a view consisting of elements of another view, skipping the first N elements
(class template) (range adaptor object) [edit]
a view consisting of the elements of another view, skipping the initial subsequence of elements until the first element where the predicate returns false
(class template) (range adaptor object) [edit]
a view consisting of the sequence obtained from flattening a view of ranges
(class template) (range adaptor object) [edit]
a view over the subranges obtained from splitting another view using a delimiter
(class template) (range adaptor object) [edit]
a view over the subranges obtained from splitting another view using a delimiter
(class template) (range adaptor object) [edit]
creates a subrange from an iterator and a count
(customization point object) [edit]
converts a view into a common_range
(class template) (range adaptor object) [edit]
a view that iterates over the elements of another bidirectional view in reverse order
(class template) (range adaptor object) [edit]
converts a view into a constant_range
(class template) (range adaptor object) [edit]
a view of a sequence that casts each element to an rvalue
(class template) (range adaptor object) [edit]
takes a view consisting of tuple-like values and a number N and produces a view of N'th element of each tuple
(class template) (range adaptor object) [edit]
takes a view consisting of pair-like values and produces a view of the first elements of each pair
(class template) (range adaptor object) [edit]
takes a view consisting of pair-like values and produces a view of the second elements of each pair
(class template) (range adaptor object) [edit]
a view consisting of tuples of references to corresponding elements of the adapted views
(class template) (customization point object) [edit]
a view consisting of tuples of results of application of a transformation function to corresponding elements of the adapted views
(class template) (customization point object) [edit]
a view consisting of tuples of references to adjacent elements of the adapted view
(class template) (range adaptor object) [edit]
a view consisting of tuples of results of application of a transformation function to adjacent elements of the adapted view
(class template) (range adaptor object) [edit]
a view consisting of the sequence obtained from flattening a view of ranges, with the delimiter in between elements
(class template) (range adaptor object) [edit]
a view consisting of elements of another view, advancing over N elements at a time
(class template) (range adaptor object) [edit]
a view whose Mth element is a view over the Mth through (M + N - 1)th elements of another view
(class template) (range adaptor object) [edit]
a range of views that are N-sized non-overlapping successive chunks of the elements of another view
(class template) (range adaptor object) [edit]
splits the view into subranges between each pair of adjacent elements for which the given predicate returns false
(class template) (range adaptor object) [edit]

Customization point objects

Range access
Defined in namespace std::ranges
returns an iterator to the beginning of a range
(customization point object) [edit]
returns a sentinel indicating the end of a range
(customization point object) [edit]
returns an iterator to the beginning of a read-only range
(customization point object) [edit]
returns a sentinel indicating the end of a read-only range
(customization point object) [edit]
returns a reverse iterator to a range
(customization point object) [edit]
returns a reverse end iterator to a range
(customization point object) [edit]
returns a reverse iterator to a read-only range
(customization point object) [edit]
returns a reverse end iterator to a read-only range
(customization point object) [edit]
returns an integer equal to the size of a range
(customization point object) [edit]
returns a signed integer equal to the size of a range
(customization point object) [edit]
checks whether a range is empty
(customization point object) [edit]
obtains a pointer to the beginning of a contiguous range
(customization point object) [edit]
obtains a pointer to the beginning of a read-only contiguous range
(customization point object) [edit]

Enumerations

Defined in namespace std::ranges
specifies whether a std::ranges::subrange models std::ranges::sized_range
(enum) [edit]

Helpers

obtains the number of components of a std::ranges::subrange
(class template specialization) [edit]
obtains the type of the iterator or the sentinel of a std::ranges::subrange
(class template specialization) [edit]
from-range construction tag
(class) (constant) [edit]

[edit] Synopsis

#include <compare>
#include <initializer_list>
#include <iterator>
 
namespace std::ranges {
  inline namespace /* unspecified */ {
    // range access
    inline constexpr /* unspecified */ begin   = /* unspecified */;
    inline constexpr /* unspecified */ end     = /* unspecified */;
    inline constexpr /* unspecified */ cbegin  = /* unspecified */;
    inline constexpr /* unspecified */ cend    = /* unspecified */;
    inline constexpr /* unspecified */ rbegin  = /* unspecified */;
    inline constexpr /* unspecified */ rend    = /* unspecified */;
    inline constexpr /* unspecified */ crbegin = /* unspecified */;
    inline constexpr /* unspecified */ crend   = /* unspecified */;
 
    inline constexpr /* unspecified */ size    = /* unspecified */;
    inline constexpr /* unspecified */ ssize   = /* unspecified */;
    inline constexpr /* unspecified */ empty   = /* unspecified */;
    inline constexpr /* unspecified */ data    = /* unspecified */;
    inline constexpr /* unspecified */ cdata   = /* unspecified */;
  }
 
  // ranges
  template<class T>
    concept range = /* see description */;
 
  template<class T>
    inline constexpr bool enable_borrowed_range = false;
 
  template<class T>
    concept borrowed_range = /* see description */;
 
  template<class T>
    using iterator_t = decltype(ranges::begin(declval<T&>()));
  template<range R>
    using sentinel_t = decltype(ranges::end(declval<R&>()));
  template<range R>
    using const_iterator_t = const_iterator<iterator_t<R>>;
  template<range R>
    using range_difference_t = iter_difference_t<iterator_t<R>>;
  template<sized_range R>
    using range_size_t = decltype(ranges::size(declval<R&>()));
  template<range R>
    using range_value_t = iter_value_t<iterator_t<R>>;
  template<range R>
    using range_reference_t = iter_reference_t<iterator_t<R>>;
  template<range R>
    using range_const_reference_t = iter_const_reference_t<iterator_t<R>>;
  template<range R>
    using range_rvalue_reference_t = iter_rvalue_reference_t<iterator_t<R>>;
 
  // sized ranges
  template<class>
    inline constexpr bool disable_sized_range = false;
 
  template<class T>
    concept sized_range = /* see description */;
 
  // views
  template<class T>
    inline constexpr bool enable_view = /* see description */;
 
  struct view_base {};
 
  template<class T>
    concept view = /* see description */;
 
  // other range refinements
  template<class R, class T>
    concept output_range = /* see description */;
 
  template<class T>
    concept input_range = /* see description */;
 
  template<class T>
    concept forward_range = /* see description */;
 
  template<class T>
    concept bidirectional_range = /* see description */;
 
  template<class T>
    concept random_access_range = /* see description */;
 
  template<class T>
    concept contiguous_range = /* see description */;
 
  template<class T>
    concept common_range = /* see description */;
 
  template<class T>
    concept viewable_range = /* see description */;
 
  template<class T>
    concept constant_range = /* see description */;
 
  // class template view_interface
  template<class D>
    requires is_class_v<D> && same_as<D, remove_cv_t<D>>
  class view_interface;
 
  // sub-ranges
  enum class subrange_kind : bool { unsized, sized };
 
  template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K =
    /* see description */>
    requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
  class subrange;
 
  template<class I, class S, subrange_kind K>
    inline constexpr bool enable_borrowed_range<subrange<I, S, K>> = true;
 
  // dangling iterator handling
  struct dangling;
 
  // class template elements_of
  template<range R, class Allocator = allocator<byte>>
    struct elements_of;
 
  template<range R>
    using borrowed_iterator_t = /* see description */;
 
  template<range R>
    using borrowed_subrange_t = /* see description */;
 
  // range conversions
  template<class C, input_range R, class... Args> requires (!view<C>)
    constexpr C to(R&& r, Args&&... args);
  template<template<class...> class C, input_range R, class... Args>
    constexpr auto to(R&& r, Args&&... args);
  template<class C, class... Args> requires (!view<C>)
    constexpr auto to(Args&&... args);
  template<template<class...> class C, class... Args>
    constexpr auto to(Args&&... args);
 
  // empty view
  template<class T>
    requires is_object_v<T>
  class empty_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<empty_view<T>> = true;
 
  namespace views {
    template<class T>
      inline constexpr empty_view<T> empty{};
  }
 
  // single view
  template<move_constructible T>
    requires is_object_v<T>
  class single_view;
 
  namespace views { inline constexpr /* unspecified */ single = /* unspecified */; }
 
  template<bool Const, class T>
    using __maybe_const = conditional_t<Const, const T, T>;   // exposition only
 
  // iota view
  template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t>
    requires __weakly_equality_comparable_with<W, Bound> && copyable<W>
  class iota_view;
 
  template<class W, class Bound>
    inline constexpr bool enable_borrowed_range<iota_view<W, Bound>> = true;
 
  namespace views { inline constexpr /* unspecified */ iota = /* unspecified */; }
 
  // repeat view
  template<move_constructible W, semiregular Bound = unreachable_sentinel_t>
    requires (is_object_v<W> && same_as<W, remove_cv_t<W>>
      && (__is_integer_like<Bound> || same_as<Bound, unreachable_sentinel_t>))
  class repeat_view;
 
  namespace views { inline constexpr /* unspecified */ repeat = /* unspecified */; }
 
  // istream view
  template<movable Val, class CharT, class Traits = char_traits<CharT>>
    requires /* see description */
  class basic_istream_view;
  template<class Val>
    using istream_view = basic_istream_view<Val, char>;
  template<class Val>
    using wistream_view = basic_istream_view<Val, wchar_t>;
 
  namespace views { template<class T> inline constexpr /* unspecified */ istream =
      /* unspecified */; }
 
  // range adaptor objects
  template<class D>
    requires is_class_v<D> && same_as<D, remove_cv_t<D>>
  class range_adaptor_closure { };
 
  // all view
  namespace views {
    inline constexpr /* unspecified */ all = /* unspecified */;
 
    template<viewable_range R>
      using all_t = decltype(all(declval<R>()));
  }
 
  // ref view
  template<range R>
    requires is_object_v<R>
  class ref_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<ref_view<T>> = true;
 
  // owning view
  template<range R>
    requires /* see description */
  class owning_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<owning_view<T>> =
      enable_borrowed_range<T>;
 
  // as rvalue view
  template<view V>
    requires input_range<V>
  class as_rvalue_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<as_rvalue_view<T>> =
      enable_borrowed_range<T>;
 
  namespace views { inline constexpr /* unspecified */ as_rvalue = /* unspecified */; }
 
  // filter view
  template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class filter_view;
 
  namespace views { inline constexpr /* unspecified */ filter = /* unspecified */; }
 
  // transform view
  template<input_range V, move_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             __can_reference<invoke_result_t<F&, range_reference_t<V>>>
  class transform_view;
 
  namespace views { inline constexpr /* unspecified */ transform = /* unspecified */; }
 
  // take view
  template<view> class take_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<take_view<T>> =
      enable_borrowed_range<T>;
 
  namespace views { inline constexpr /* unspecified */ take = /* unspecified */; }
 
  // take while view
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
    class take_while_view;
 
  namespace views { inline constexpr /* unspecified */ take_while = /* unspecified */; }
 
  // drop view
  template<view V>
    class drop_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<drop_view<T>> =
      enable_borrowed_range<T>;
 
  namespace views { inline constexpr /* unspecified */ drop = /* unspecified */; }
 
  // drop while view
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
    class drop_while_view;
 
  template<class T, class Pred>
    inline constexpr bool enable_borrowed_range<drop_while_view<T, Pred>> =
      enable_borrowed_range<T>;
 
  namespace views { inline constexpr /* unspecified */ drop_while = /* unspecified */; }
 
  // join view
  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>>
  class join_view;
 
  namespace views { inline constexpr /* unspecified */ join = /* unspecified */; }
 
  // join with view
  template<class R, class P>
    concept __compatible_joinable_ranges = /* see description */; // exposition only
 
  template<input_range V, forward_range Pattern>
    requires view<V> && input_range<range_reference_t<V>>
          && view<Pattern>
          && __compatible_joinable_ranges<range_reference_t<V>, Pattern>
  class join_with_view;
 
  namespace views { inline constexpr /* unspecified */ join_with = /* unspecified */; }
 
  // lazy split view
  template<class R>
    concept __tiny_range = /* see description */;   // exposition only
 
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to> &&
             (forward_range<V> || __tiny_range<Pattern>)
  class lazy_split_view;
 
  // split view
 template<forward_range V, forward_range Pattern>
   requires view<V> && view<Pattern> &&
            indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
  class split_view;
 
  namespace views {
    inline constexpr /* unspecified */ lazy_split = /* unspecified */;
    inline constexpr /* unspecified */ split = /* unspecified */;
  }
 
  // counted view
  namespace views { inline constexpr /* unspecified */ counted = /* unspecified */; }
 
  // common view
  template<view V>
    requires (!common_range<V> && copyable<iterator_t<V>>)
  class common_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<common_view<T>> =
      enable_borrowed_range<T>;
 
  namespace views { inline constexpr /* unspecified */ common = /* unspecified */; }
 
  // reverse view
  template<view V>
    requires bidirectional_range<V>
  class reverse_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<reverse_view<T>> =
      enable_borrowed_range<T>;
 
  namespace views { inline constexpr /* unspecified */ reverse = /* unspecified */; }
 
  // as const view
  template<input_range R>
    constexpr auto& __possibly_const_range(R& r) {          // exposition only
      if constexpr (constant_range<const R> && !constant_range<R>) {
        return const_cast<const R&>(r);
      } else {
        return r;
      }
    }
 
  template<view V>
    requires input_range<V>
  class as_const_view;
 
  template<class T>
    inline constexpr bool enable_borrowed_range<as_const_view<T>> =
      enable_borrowed_range<T>;
 
  namespace views { inline constexpr /* unspecified */ as_const = /* unspecified */; }
 
  // elements view
  template<input_range V, size_t N>
    requires /* see description */
  class elements_view;
 
  template<class T, size_t N>
    inline constexpr bool enable_borrowed_range<elements_view<T, N>> =
      enable_borrowed_range<T>;
 
  template<class R>
    using keys_view = elements_view<R, 0>;
  template<class R>
    using values_view = elements_view<R, 1>;
 
  namespace views {
    template<size_t N>
      inline constexpr /* unspecified */ elements = /* unspecified */;
    inline constexpr auto keys = elements<0>;
    inline constexpr auto values = elements<1>;
  }
 
  // zip view
  template<input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0)
  class zip_view;
 
  template<class... Views>
    inline constexpr bool enable_borrowed_range<zip_view<Views...>> =
      (enable_borrowed_range<Views> && ...);
 
  namespace views { inline constexpr /* unspecified */ zip = /* unspecified */; }
 
  // zip transform view
  template<move_constructible F, input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<Views>...> &&
             __can_reference<invoke_result_t<F&, range_reference_t<Views>...>>
  class zip_transform_view;
 
  namespace views { inline constexpr /* unspecified */ zip_transform =
      /* unspecified */; }
 
  // adjacent view
  template<forward_range V, size_t N>
    requires view<V> && (N > 0)
  class adjacent_view;
 
  template<class V, size_t N>
    inline constexpr bool enable_borrowed_range<adjacent_view<V, N>> =
      enable_borrowed_range<V>;
 
  namespace views {
    template<size_t N>
      inline constexpr /* unspecified */ adjacent = /* unspecified */;
    inline constexpr auto pairwise = adjacent<2>;
  }
 
  // adjacent transform view
  template<forward_range V, move_constructible F, size_t N>
    requires /* see description */
  class adjacent_transform_view;
 
  namespace views {
    template<size_t N>
      inline constexpr /* unspecified */ adjacent_transform = /* unspecified */;
    inline constexpr auto pairwise_transform = adjacent_transform<2>;
  }
 
  // chunk view
  template<view V>
    requires input_range<V>
  class chunk_view;
 
  template<view V>
    requires forward_range<V>
  class chunk_view<V>;
 
  template<class V>
    inline constexpr bool enable_borrowed_range<chunk_view<V>> =
      forward_range<V> && enable_borrowed_range<V>;
 
  namespace views { inline constexpr /* unspecified */ chunk = /* unspecified */; }
 
  // slide view
  template<forward_range V>
    requires view<V>
  class slide_view;
 
  template<class V>
    inline constexpr bool enable_borrowed_range<slide_view<V>> =
      enable_borrowed_range<V>;
 
  namespace views { inline constexpr /* unspecified */ slide = /* unspecified */; }
 
  // chunk by view
  template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class chunk_by_view;
 
  namespace views { inline constexpr /* unspecified */ chunk_by = /* unspecified */; }
 
  // stride view
  template<input_range V>
    requires view<V>
  class stride_view;
 
  template<class V>
    inline constexpr bool enable_borrowed_range<stride_view<V>> =
        enable_borrowed_range<V>;
 
  namespace views { inline constexpr /* unspecified */ stride = /* unspecified */; }
 
  // cartesian product view
  template<input_range First, forward_range... Vs>
    requires (view<First> && ... && view<Vs>)
  class cartesian_product_view;
 
  namespace views { inline constexpr /* unspecified */ cartesian_product =
      /* unspecified */; }
}
 
namespace std {
  namespace views = ranges::views;
 
  template<class T> struct tuple_size;
  template<size_t I, class T> struct tuple_element;
 
  template<class I, class S, ranges::subrange_kind K>
  struct tuple_size<ranges::subrange<I, S, K>>
    : integral_constant<size_t, 2> {};
  template<class I, class S, ranges::subrange_kind K>
  struct tuple_element<0, ranges::subrange<I, S, K>> {
    using type = I;
  };
  template<class I, class S, ranges::subrange_kind K>
  struct tuple_element<1, ranges::subrange<I, S, K>> {
    using type = S;
  };
  template<class I, class S, ranges::subrange_kind K>
  struct tuple_element<0, const ranges::subrange<I, S, K>> {
    using type = I;
  };
  template<class I, class S, ranges::subrange_kind K>
  struct tuple_element<1, const ranges::subrange<I, S, K>> {
    using type = S;
  };
 
  struct from_range_t { explicit from_range_t() = default; };
  inline constexpr from_range_t from_range{};
}

[edit] Concept range

namespace std::ranges {
  template< class T >
    concept range = requires(T& t) {
      ranges::begin(t); // equality-preserving for forward iterators
      ranges::end(t);
  };
}

[edit] Concept borrowed_range

namespace std::ranges {
  template<class T>
    concept borrowed_range =
      range<T> && (is_lvalue_reference_v<T> ||
        enable_borrowed_range<remove_cvref_t<T>>);
}

[edit] Concept sized_range

namespace std::ranges {
  template< class T >
    concept sized_range = range<T> &&
      requires(T& t) {
        ranges::size(t);
      };
}

[edit] Concept view

namespace std::ranges {
  template<class T>
    inline constexpr bool enable_view = derived_from<T, view_base>;
 
  template<class T>
    concept view = range<T>
                && movable<T>
                && enable_view<T>;
}

[edit] Concept output_range

namespace std::ranges {
  template<class R, class T>
    concept output_range =
      range<R> && output_iterator<iterator_t<R>, T>;
}

[edit] Concept input_range

namespace std::ranges {
  template<class T>
    concept input_range =
      range<T> && input_iterator<iterator_t<T>>;
}

[edit] Concept forward_range

namespace std::ranges {
  template<class T>
    concept forward_range =
      input_range<T> && forward_iterator<iterator_t<T>>;
}

[edit] Concept bidirectional_range

namespace std::ranges {
  template<class T>
    concept bidirectional_range =
      forward_range<T> && bidirectional_iterator<iterator_t<T>>;
}

[edit] Concept random_access_range

namespace std::ranges {
  template<class T>
    concept random_access_range =
      bidirectional_range<T> && random_access_iterator<iterator_t<T>>;
}

[edit] Concept contiguous_range

namespace std::ranges {
  template<class T>
    concept contiguous_range =
      random_access_range<T> && contiguous_iterator<iterator_t<T>> &&
        requires(T& t) {
          { ranges::data(t) } -> same_as<add_pointer_t<range_reference_t<T>>>;
        };
}

[edit] Concept common_range

namespace std::ranges {
  template<class T>
    concept common_range =
      range<T> && same_as<iterator_t<T>, sentinel_t<T>>;
}

[edit] Concept viewable_range

namespace std::ranges {
  template<class T>
    concept viewable_range =
      range<T> && (borrowed_range<T> || view<remove_cvref_t<T>>);
}

[edit] Helper concepts

Note: The following names are only for exposition, they are not part of the interface.

namespace std::ranges { // unspecified, for name lookup only
  template<class R>
    concept /*simple-view*/ =                                     // exposition only
      view<R> && range<const R> &&
      same_as<iterator_t<R>, iterator_t<const R>> &&
      same_as<sentinel_t<R>, sentinel_t<const R>>;
 
  template<class I>
    concept /*has-arrow*/ =                                       // exposition only
      input_iterator<I> &&
      (is_pointer_v<I> || requires(I i) { i.operator->(); });
 
  template<class T, class U>
    concept /*different-from*/ =                                  // exposition only
      !same_as<remove_cvref_t<T>, remove_cvref_t<U>>;
 
  template<class I>
    concept /*decrementable*/ =                                   // exposition only
      incrementable<I> && requires(I i) {
        { --i } -> same_as<I&>;
        { i-- } -> same_as<I>;
      };
 
  template<class I>
    concept /*advanceable*/ =                                     // exposition only
      /*decrementable*/<I> && totally_ordered<I> &&
      requires(I i, const I j, const iter_difference_t<I> n) {
          { i += n } -> same_as<I&>;
          { i -= n } -> same_as<I&>;
        I { j +  n };
        I { n +  j };
        I { j -  n };
          { j -  j } -> convertible_to<iter_difference_t<I>>;
      };
}

[edit] Class template std::ranges::view_interface

namespace std::ranges {
  template<class D>
    requires is_class_v<D> && same_as<D, remove_cv_t<D>>
  class view_interface {
  private:
    constexpr D& derived() noexcept {               // exposition only
      return static_cast<D&>(*this);
    }
    constexpr const D& derived() const noexcept {   // exposition only
      return static_cast<const D&>(*this);
    }
 
  public:
    constexpr bool empty() requires forward_range<D> {
      return ranges::begin(derived()) == ranges::end(derived());
    }
    constexpr bool empty() const requires forward_range<const D> {
      return ranges::begin(derived()) == ranges::end(derived());
    }
 
    constexpr explicit operator bool()
      requires requires { ranges::empty(derived()); } {
        return !ranges::empty(derived());
      }
    constexpr explicit operator bool() const
      requires requires { ranges::empty(derived()); } {
        return !ranges::empty(derived());
      }
 
    constexpr auto data() requires contiguous_iterator<iterator_t<D>> {
      return to_address(ranges::begin(derived()));
    }
    constexpr auto data() const
      requires range<const D> && contiguous_iterator<iterator_t<const D>> {
        return to_address(ranges::begin(derived()));
      }
 
    constexpr auto size() requires forward_range<D> &&
      sized_sentinel_for<sentinel_t<D>, iterator_t<D>> {
        return ranges::end(derived()) - ranges::begin(derived());
      }
    constexpr auto size() const requires forward_range<const D> &&
      sized_sentinel_for<sentinel_t<const D>, iterator_t<const D>> {
        return ranges::end(derived()) - ranges::begin(derived());
      }
 
    constexpr decltype(auto) front() requires forward_range<D>;
    constexpr decltype(auto) front() const requires forward_range<const D>;
 
    constexpr decltype(auto) back() requires bidirectional_range<D> && common_range<D>;
    constexpr decltype(auto) back() const
      requires bidirectional_range<const D> && common_range<const D>;
 
    template<random_access_range R = D>
      constexpr decltype(auto) operator[](range_difference_t<R> n) {
        return ranges::begin(derived())[n];
      }
    template<random_access_range R = const D>
      constexpr decltype(auto) operator[](range_difference_t<R> n) const {
        return ranges::begin(derived())[n];
      }
  };
}

[edit] Class template std::ranges::subrange

namespace std::ranges {
  template<class From, class To>
    concept /*uses-nonqualification-pointer-conversion*/ =      // exposition only
      is_pointer_v<From> && is_pointer_v<To> &&
      !convertible_to<remove_pointer_t<From>(*)[], remove_pointer_t<To>(*)[]>;
 
  template<class From, class To>
    concept /*convertible-to-non-slicing*/ =                    // exposition only
      convertible_to<From, To> &&
      !/*uses-nonqualification-pointer-conversion*/<decay_t<From>, decay_t<To>>;
 
  template<class T>
    concept /*pair-like*/ =                                     // exposition only
      !is_reference_v<T> && requires(T t) {
        typename tuple_size<T>::type;                       // ensures tuple_size<T>
                                                            // is complete
        requires derived_from<tuple_size<T>, integral_constant<size_t, 2>>;
        typename tuple_element_t<0, remove_const_t<T>>;
        typename tuple_element_t<1, remove_const_t<T>>;
        { std::get<0>(t) } -> convertible_to<const tuple_element_t<0, T>&>;
        { std::get<1>(t) } -> convertible_to<const tuple_element_t<1, T>&>;
      };
 
  template<class T, class U, class V>
    concept /*pair-like-convertible-from*/ =                    // exposition only
      !range<T> && /*pair-like*/<T> &&
      constructible_from<T, U, V> &&
      /*convertible-to-non-slicing*/<U, tuple_element_t<0, T>> &&
      convertible_to<V, tuple_element_t<1, T>>;
 
  template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K =
      sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized>
    requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
  class subrange : public view_interface<subrange<I, S, K>> {
  private:
    static constexpr bool StoreSize =                       // exposition only
      K == subrange_kind::sized && !sized_sentinel_for<S, I>;
    I begin_ = I();                                         // exposition only
    S end_ = S();                                           // exposition only
    /*make-unsigned-like-t*/<iter_difference_t<I>> size_ = 0;   // exposition only;
                                                                // present only
                                                                // when StoreSize is true
  public:
    subrange() requires default_initializable<I> = default;
 
    constexpr subrange(/*convertible-to-non-slicing*/<I> auto i, S s)
      requires (!StoreSize);
 
    constexpr subrange(/*convertible-to-non-slicing*/<I> auto i, S s,
                       /*make-unsigned-like-t*/<iter_difference_t<I>> n)
      requires (K == subrange_kind::sized);
 
    template</*different-from*/<subrange> R>
      requires borrowed_range<R> &&
               /*convertible-to-non-slicing*/<iterator_t<R>, I> &&
               convertible_to<sentinel_t<R>, S>
    constexpr subrange(R&& r) requires (!StoreSize || sized_range<R>);
 
    template<borrowed_range R>
      requires /*convertible-to-non-slicing*/<iterator_t<R>, I> &&
               convertible_to<sentinel_t<R>, S>
    constexpr subrange(R&& r, /*make-unsigned-like-t*/<iter_difference_t<I>> n)
      requires (K == subrange_kind::sized)
        : subrange{ranges::begin(r), ranges::end(r), n} {}
 
    template</*different-from*/<subrange> PairLike>
      requires /*pair-like-convertible-from*/<PairLike, const I&, const S&>
    constexpr operator PairLike() const;
 
    constexpr I begin() const requires copyable<I>;
    [[nodiscard]] constexpr I begin() requires (!copyable<I>);
    constexpr S end() const;
 
    constexpr bool empty() const;
    constexpr /*make-unsigned-like-t*/<iter_difference_t<I>> size() const
      requires (K == subrange_kind::sized);
 
    [[nodiscard]] constexpr subrange next(iter_difference_t<I> n = 1) const &
      requires forward_iterator<I>;
    [[nodiscard]] constexpr subrange next(iter_difference_t<I> n = 1) &&;
    [[nodiscard]] constexpr subrange prev(iter_difference_t<I> n = 1) const
      requires bidirectional_iterator<I>;
    constexpr subrange& advance(iter_difference_t<I> n);
  };
 
  template<input_or_output_iterator I, sentinel_for<I> S>
    subrange(I, S) -> subrange<I, S>;
 
  template<input_or_output_iterator I, sentinel_for<I> S>
    subrange(I, S, /*make-unsigned-like-t*/<iter_difference_t<I>>) ->
      subrange<I, S, subrange_kind::sized>;
 
  template<borrowed_range R>
    subrange(R&&) ->
      subrange<iterator_t<R>, sentinel_t<R>,
               (sized_range<R> || sized_sentinel_for<sentinel_t<R>, iterator_t<R>>)
                 ? subrange_kind::sized : subrange_kind::unsized>;
 
  template<borrowed_range R>
    subrange(R&&, /*make-unsigned-like-t*/<range_difference_t<R>>) ->
      subrange<iterator_t<R>, sentinel_t<R>, subrange_kind::sized>;
 
  template<size_t N, class I, class S, subrange_kind K>
    requires ((N == 0 && copyable<I>) || N == 1)
    constexpr auto get(const subrange<I, S, K>& r);
 
  template<size_t N, class I, class S, subrange_kind K>
    requires (N < 2)
    constexpr auto get(subrange<I, S, K>&& r);
}
 
namespace std {
  using ranges::get;
}

[edit] Class std::ranges::dangling

namespace std::ranges {
  struct dangling {
    constexpr dangling() noexcept = default;
    constexpr dangling(auto&&...) noexcept {}
  };
}

[edit] Class template std::ranges::empty_view

namespace std::ranges {
  template<class T>
    requires is_object_v<T>
  class empty_view : public view_interface<empty_view<T>> {
  public:
    static constexpr T* begin() noexcept { return nullptr; }
    static constexpr T* end() noexcept { return nullptr; }
    static constexpr T* data() noexcept { return nullptr; }
    static constexpr size_t size() noexcept { return 0; }
    static constexpr bool empty() noexcept { return true; }
  };
}

[edit] Class template std::ranges::single_view

namespace std::ranges {
  template<copy_constructible T>
    requires is_object_v<T>
  class single_view : public view_interface<single_view<T>> {
  private:
    /*copyable-box*/<T> value_;             // exposition only
 
  public:
    single_view() requires default_initializable<T> = default;
    constexpr explicit single_view(const T& t);
    constexpr explicit single_view(T&& t);
    template<class... Args>
      requires constructible_from<T, Args...>
    constexpr explicit single_view(in_place_t, Args&&... args);
 
    constexpr T* begin() noexcept;
    constexpr const T* begin() const noexcept;
    constexpr T* end() noexcept;
    constexpr const T* end() const noexcept;
    static constexpr size_t size() noexcept;
    constexpr T* data() noexcept;
    constexpr const T* data() const noexcept;
  };
 
  template<class T>
    single_view(T) -> single_view<T>;
}

[edit] Class template std::ranges::iota_view

namespace std::ranges {
  template<class I>
    concept decrementable = /* see description */;  // exposition only
 
  template<class I>
    concept advanceable = /* see description */;    // exposition only
 
  template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t>
    requires /*weakly-equality-comparable-with*/<W, Bound> && copyable<W>
  class iota_view : public view_interface<iota_view<W, Bound>> {
  private:
    // class iota_view::iterator
    struct iterator;                    // exposition only
 
    // class iota_view::sentinel
    struct sentinel;                    // exposition only
 
    W value_ = W();                     // exposition only
    Bound bound_ = Bound();             // exposition only
 
  public:
    iota_view() requires default_initializable<W> = default;
    constexpr explicit iota_view(W value);
    constexpr iota_view(type_identity_t<W> value, type_identity_t<Bound> bound);
    constexpr iota_view(iterator first, /* see description */ last);
 
    constexpr iterator begin() const;
    constexpr auto end() const;
    constexpr iterator end() const requires same_as<W, Bound>;
 
    constexpr auto size() const requires /* see description */;
  };
 
  template<class W, class Bound>
    requires (!/*is-integer-like*/<W> || !/*is-integer-like*/<Bound> ||
              (/*is-signed-integer-like*/<W> == /*is-signed-integer-like*/<Bound>))
    iota_view(W, Bound) -> iota_view<W, Bound>;
}

[edit] Class template std::ranges::iota_view::iterator

namespace std::ranges {
  template<weakly_incrementable W, semiregular Bound>
    requires /*weakly-equality-comparable-with*/<W, Bound> && copyable<W>
  struct iota_view<W, Bound>::iterator {
  private:
    W value_ = W();             // exposition only
  public:
    using iterator_concept = /* see description */;
    using iterator_category = input_iterator_tag; // present only if W models incrementable
    using value_type = W;
    using difference_type = /*IOTA-DIFF-T*/(W);
 
    iterator() requires default_initializable<W> = default;
    constexpr explicit iterator(W value);
 
    constexpr W operator*() const noexcept(is_nothrow_copy_constructible_v<W>);
 
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires incrementable<W>;
 
    constexpr iterator& operator--() requires /*decrementable*/<W>;
    constexpr iterator operator--(int) requires /*decrementable*/<W>;
 
    constexpr iterator& operator+=(difference_type n)
      requires /*advanceable*/<W>;
    constexpr iterator& operator-=(difference_type n)
      requires /*advanceable*/<W>;
    constexpr W operator[](difference_type n) const
      requires /*advanceable*/<W>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<W>;
 
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires totally_ordered<W>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires totally_ordered<W>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires totally_ordered<W>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires totally_ordered<W>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires totally_ordered<W> && three_way_comparable<W>;
 
    friend constexpr iterator operator+(iterator i, difference_type n)
      requires /*advanceable*/<W>;
    friend constexpr iterator operator+(difference_type n, iterator i)
      requires /*advanceable*/<W>;
 
    friend constexpr iterator operator-(iterator i, difference_type n)
      requires /*advanceable*/<W>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires /*advanceable*/<W>;
  };
}

[edit] Class template std::ranges::iota_view::sentinel

namespace std::ranges {
  template<weakly_incrementable W, semiregular Bound>
    requires /*weakly-equality-comparable-with*/<W, Bound> && copyable<W>
  struct iota_view<W, Bound>::sentinel {
  private:
    Bound bound_ = Bound();     // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(Bound bound);
 
    friend constexpr bool operator==(const iterator& x, const sentinel& y);
 
    friend constexpr iter_difference_t<W> operator-(const iterator& x, const sentinel& y)
      requires sized_sentinel_for<Bound, W>;
    friend constexpr iter_difference_t<W> operator-(const sentinel& x, const iterator& y)
      requires sized_sentinel_for<Bound, W>;
  };
}

[edit] Class template std::ranges::repeat_view

namespace std::ranges {
  template<copy_constructible W, semiregular Bound = unreachable_sentinel_t>
    requires (is_object_v<W> && same_as<W, remove_cv_t<W>>
      && (/*is-integer-like*/<Bound> || same_as<Bound, unreachable_sentinel_t>))
  class repeat_view : public view_interface<repeat_view<W, Bound>> {
  private:
    // class range_view::iterator
    struct iterator;
 
    /*copyable-box*/<W> value_ = W(); // exposition only
    Bound bound_ = Bound(); // exposition only
 
  public:
    repeat_view() requires default_initializable<W> = default;
 
    constexpr explicit repeat_view(const W& value, Bound bound = Bound());
    constexpr explicit repeat_view(W&& value, Bound bound = Bound());
    template<class... WArgs, class... BoundArgs>
      requires constructible_from<W, WArgs...>
        && constructible_from<Bound, BoundArgs...>
    constexpr explicit repeat_view(piecewise_construct_t,
      tuple<WArgs...> value_args, tuple<BoundArgs...> bound_args = tuple<>{});
 
    constexpr iterator begin() const;
    constexpr iterator end() const requires (!same_as<Bound, unreachable_sentinel_t>);
    constexpr unreachable_sentinel_t end() const noexcept;
 
    constexpr auto size() const requires (!same_as<Bound, unreachable_sentinel_t>);
  };
 
  template<class W, class Bound>
    repeat_view(W, Bound) -> repeat_view<W, Bound>;
}

[edit] Class template std::ranges::repeat_view::iterator

namespace std::ranges {
  template<copy_constructible W, semiregular Bound = unreachable_sentinel_t>
    requires /*is-integer-like*/<Bound> || same_as<Bound, unreachable_sentinel_t>
  class repeat_view<W, Bound>::iterator {
  private:
    using /*index-type*/ = // exposition only
      conditional_t<same_as<Bound, unreachable_sentinel_t>,
        ptrdiff_t,
        Bound
      >;
    const W* value_ = nullptr; // exposition only
    /*index-type*/ current_ = /*index-type*/(); // exposition only
 
    // exposition only
    constexpr explicit iterator(const W * value, /*index-type*/ b = /*index-type*/());
 
  public:
    using iterator_concept = random_access_iterator_tag;
    using iterator_category = random_access_iterator_tag;
    using value_type = W;
    using difference_type = conditional_t</*is-signed-like*/</*index-type*/>,
        /*index-type*/,
        /*IOTA-DIFF-T*/(/*index-type*/)
      >;
 
    iterator() = default;
 
    constexpr const W& operator*() const noexcept;
 
    constexpr iterator& operator++();
    constexpr iterator operator++(int);
 
    constexpr iterator& operator--();
    constexpr iterator operator--(int);
 
    constexpr iterator& operator+=(difference_type n);
    constexpr iterator& operator-=(difference_type n);
    constexpr const W& operator[](difference_type n) const noexcept;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y);
    friend constexpr auto operator<=>(const iterator& x, const iterator& y);
 
    friend constexpr iterator operator+(iterator i, difference_type n);
    friend constexpr iterator operator+(difference_type n, iterator i);
 
    friend constexpr iterator operator-(iterator i, difference_type n);
    friend constexpr difference_type operator-(const iterator& x, const iterator& y);
  };
}

[edit] Class template std::ranges::basic_istream_view

namespace std::ranges {
  template<class Val, class CharT, class Traits>
    concept /*stream-extractable*/ =                // exposition only
      requires(basic_istream<CharT, Traits>& is, Val& t) {
         is >> t;
      };
 
  template<movable Val, class CharT, class Traits = char_traits<CharT>>
    requires default_initializable<Val> &&
             /*stream-extractable*/<Val, CharT, Traits>
  class basic_istream_view :
    public view_interface<basic_istream_view<Val, CharT, Traits>> {
  public:
    constexpr explicit basic_istream_view(basic_istream<CharT, Traits>& stream);
 
    constexpr auto begin() {
      *stream_ >> value_;
      return iterator{*this};
    }
 
    constexpr default_sentinel_t end() const noexcept;
 
  private:
    struct iterator;                            // exposition only
    basic_istream<CharT, Traits>* stream_;      // exposition only
    Val value_ = Val();                         // exposition only
  };
}

[edit] Class template std::ranges::basic_istream_view::iterator

namespace std::ranges {
  template<movable Val, class CharT, class Traits>
    requires default_initializable<Val> &&
             /*stream-extractable*/<Val, CharT, Traits>
  class basic_istream_view<Val, CharT, Traits>::iterator {
  public:
    using iterator_concept = input_iterator_tag;
    using difference_type = ptrdiff_t;
    using value_type = Val;
 
    constexpr explicit iterator(basic_istream_view& parent) noexcept;
 
    iterator(const iterator&) = delete;
    iterator(iterator&&) = default;
 
    iterator& operator=(const iterator&) = delete;
    iterator& operator=(iterator&&) = default;
 
    iterator& operator++();
    void operator++(int);
 
    Val& operator*() const;
 
    friend bool operator==(const iterator& x, default_sentinel_t);
 
  private:
    basic_istream_view* parent_;                                // exposition only
  };
}

[edit] Class template std::ranges::ref_view

namespace std::ranges {
  template<range R>
    requires is_object_v<R>
  class ref_view : public view_interface<ref_view<R>> {
  private:
    R* r_;                      // exposition only
  public:
    template</*different-from*/<ref_view> T>
      requires /* see description */
    constexpr ref_view(T&& t);
 
    constexpr R& base() const { return *r_; }
 
    constexpr iterator_t<R> begin() const { return ranges::begin(*r_); }
    constexpr sentinel_t<R> end() const { return ranges::end(*r_); }
 
    constexpr bool empty() const
      requires requires { ranges::empty(*r_); }
    { return ranges::empty(*r_); }
 
    constexpr auto size() const requires sized_range<R>
    { return ranges::size(*r_); }
 
    constexpr auto data() const requires contiguous_range<R>
    { return ranges::data(*r_); }
  };
 
  template<class R>
    ref_view(R&) -> ref_view<R>;
}

[edit] Class template std::ranges::owning_view

namespace std::ranges {
  template<range R>
    requires movable<R> && (!/*is-initializer-list*/<R>)
  class owning_view : public view_interface<owning_view<R>> {
  private:
    R r_ = R();         // exposition only
  public:
    owning_view() requires default_initializable<R> = default;
    constexpr owning_view(R&& t);
 
    owning_view(owning_view&&) = default;
    owning_view& operator=(owning_view&&) = default;
 
    constexpr R& base() & noexcept { return r_; }
    constexpr const R& base() const& noexcept { return r_; }
    constexpr R&& base() && noexcept { return std::move(r_); }
    constexpr const R&& base() const&& noexcept { return std::move(r_); }
 
    constexpr iterator_t<R> begin() { return ranges::begin(r_); }
    constexpr sentinel_t<R> end() { return ranges::end(r_); }
 
    constexpr auto begin() const requires range<const R>
    { return ranges::begin(r_); }
    constexpr auto end() const requires range<const R>
    { return ranges::end(r_); }
 
    constexpr bool empty() requires requires { ranges::empty(r_); }
    { return ranges::empty(r_); }
    constexpr bool empty() const requires requires { ranges::empty(r_); }
    { return ranges::empty(r_); }
 
    constexpr auto size() requires sized_range<R>
    { return ranges::size(r_); }
    constexpr auto size() const requires sized_range<const R>
    { return ranges::size(r_); }
 
    constexpr auto data() requires contiguous_range<R>
    { return ranges::data(r_); }
    constexpr auto data() const requires contiguous_range<const R>
    { return ranges::data(r_); }
  };
}

[edit] Class template std::ranges::filter_view

namespace std::ranges {
  template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class filter_view : public view_interface<filter_view<V, Pred>> {
  private:
    V base_ = V();                              // exposition only
    /*copyable-box*/<Pred> pred_;               // exposition only
 
    // class filter_view::iterator
    class iterator;                             // exposition only
 
    // class filter_view::sentinel
    class sentinel;                             // exposition only
 
  public:
    filter_view() requires default_initializable<V> &&
      default_initializable<Pred> = default;
    constexpr filter_view(V base, Pred pred);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr const Pred& pred() const;
 
    constexpr iterator begin();
    constexpr auto end() {
      if constexpr (common_range<V>)
        return iterator{*this, ranges::end(base_)};
      else
        return sentinel{*this};
    }
  };
 
  template<class R, class Pred>
    filter_view(R&&, Pred) -> filter_view<views::all_t<R>, Pred>;
}

[edit] Class template std::ranges::filter_view::iterator

namespace std::ranges {
  template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class filter_view<V, Pred>::iterator {
  private:
    iterator_t<V> current_ = iterator_t<V>();   // exposition only
    filter_view* parent_ = nullptr;             // exposition only
 
  public:
    using iterator_concept  = /* see description */;
    using iterator_category = /* see description */;        // not always present
    using value_type        = range_value_t<V>;
    using difference_type   = range_difference_t<V>;
 
    iterator() requires default_initializable<iterator_t<V>> = default;
    constexpr iterator(filter_view& parent, iterator_t<V> current);
 
    constexpr const iterator_t<V>& base() const & noexcept;
    constexpr iterator_t<V> base() &&;
    constexpr range_reference_t<V> operator*() const;
    constexpr iterator_t<V> operator->() const
      requires /*has-arrow*/<iterator_t<V>> && copyable<iterator_t<V>>;
 
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires forward_range<V>;
 
    constexpr iterator& operator--() requires bidirectional_range<V>;
    constexpr iterator operator--(int) requires bidirectional_range<V>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<iterator_t<V>>;
 
    friend constexpr range_rvalue_reference_t<V> iter_move(const iterator& i)
      noexcept(noexcept(ranges::iter_move(i.current_)));
 
    friend constexpr void iter_swap(const iterator& x, const iterator& y)
      noexcept(noexcept(ranges::iter_swap(x.current_, y.current_)))
      requires indirectly_swappable<iterator_t<V>>;
  };
}

[edit] Class template std::ranges::filter_view::sentinel

namespace std::ranges {
  template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class filter_view<V, Pred>::sentinel {
  private:
    sentinel_t<V> end_ = sentinel_t<V>();       // exposition only
 
  public:
    sentinel() = default;
    constexpr explicit sentinel(filter_view& parent);
 
    constexpr sentinel_t<V> base() const;
 
    friend constexpr bool operator==(const iterator& x, const sentinel& y);
  };
}

[edit] Class template std::ranges::transform_view

namespace std::ranges {
  template<input_range V, copy_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             /*can-reference*/<invoke_result_t<F&, range_reference_t<V>>>
  class transform_view : public view_interface<transform_view<V, F>> {
  private:
    // class template transform_view::iterator
    template<bool> struct iterator;             // exposition only
 
    // class template transform_view::sentinel
    template<bool> struct sentinel;             // exposition only
 
    V base_ = V();                              // exposition only
    /*copyable-box*/<F> fun_;                   // exposition only
 
  public:
    transform_view() requires default_initializable<V> &&
      default_initializable<F> = default;
    constexpr transform_view(V base, F fun);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr iterator<false> begin();
    constexpr iterator<true> begin() const
      requires range<const V> &&
               regular_invocable<const F&, range_reference_t<const V>>;
 
    constexpr sentinel<false> end();
    constexpr iterator<false> end() requires common_range<V>;
    constexpr sentinel<true> end() const
      requires range<const V> &&
               regular_invocable<const F&, range_reference_t<const V>>;
    constexpr iterator<true> end() const
      requires common_range<const V> &&
               regular_invocable<const F&, range_reference_t<const V>>;
 
    constexpr auto size() requires sized_range<V> { return ranges::size(base_); }
    constexpr auto size() const requires sized_range<const V>
    { return ranges::size(base_); }
  };
 
  template<class R, class F>
    transform_view(R&&, F) -> transform_view<views::all_t<R>, F>;
}

[edit] Class template std::ranges::transform_view::iterator

namespace std::ranges {
  template<input_range V, copy_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             /*can-reference*/<invoke_result_t<F&, range_reference_t<V>>>
  template<bool Const>
  class transform_view<V, F>::iterator {
  private:
    using Parent = /*maybe-const*/<Const, transform_view>;      // exposition only
    using Base = /*maybe-const*/<Const, V>;                     // exposition only
    iterator_t<Base> current_ = iterator_t<Base>();             // exposition only
    Parent* parent_ = nullptr;                                  // exposition only
 
  public:
    using iterator_concept  = /* see description */;
    using iterator_category = /* see description */;            // not always present
    using value_type        =
      remove_cvref_t<invoke_result_t<F&, range_reference_t<Base>>>;
    using difference_type   = range_difference_t<Base>;
 
    iterator() requires default_initializable<iterator_t<Base>> = default;
    constexpr iterator(Parent& parent, iterator_t<Base> current);
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
 
    constexpr const iterator_t<Base>& base() const & noexcept;
    constexpr iterator_t<Base> base() &&;
 
    constexpr decltype(auto) operator*() const {
      return invoke(*parent_->fun_, *current_);
    }
 
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires forward_range<Base>;
 
    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;
 
    constexpr iterator& operator+=(difference_type n)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type n)
      requires random_access_range<Base>;
 
    constexpr decltype(auto) operator[](difference_type n) const
      requires random_access_range<Base> {
      return invoke(*parent_->fun_, current_[n]);
    }
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<iterator_t<Base>>;
 
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
 
    friend constexpr iterator operator+(iterator i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, iterator i)
      requires random_access_range<Base>;
 
    friend constexpr iterator operator-(iterator i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
 
    friend constexpr decltype(auto) iter_move(const iterator& i)
      noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) {
      if constexpr (is_lvalue_reference_v<decltype(*i)>)
        return std::move(*i);
      else
        return *i;
    }
  };
}

[edit] Class template std::ranges::transform_view::sentinel

namespace std::ranges {
  template<input_range V, copy_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             /*can-reference*/<invoke_result_t<F&, range_reference_t<V>>>
  template<bool Const>
  class transform_view<V, F>::sentinel {
  private:
    using Parent = /*maybe-const*/<Const, transform_view>;  // exposition only
    using Base = /*maybe-const*/<Const, V>;                 // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();             // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end);
    constexpr sentinel(sentinel<!Const> i)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    constexpr sentinel_t<Base> base() const;
 
    template<bool OtherConst>
      requires sentinel_for<sentinel_t<Base>, iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<sentinel_t<Base>,
        iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, V>>
      operator-(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<sentinel_t<Base>,
        iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, V>>
      operator-(const sentinel& y, const iterator<OtherConst>& x);
  };
}

[edit] Class template std::ranges::take_view

namespace std::ranges {
  template<view V>
  class take_view : public view_interface<take_view<V>> {
  private:
    V base_ = V();                                      // exposition only
    range_difference_t<V> count_ = 0;                   // exposition only
 
    // class template take_view::sentinel
    template<bool> struct sentinel;                     // exposition only
 
  public:
    take_view() requires default_initializable<V> = default;
    constexpr take_view(V base, range_difference_t<V> count);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin() requires (!/*simple-view*/<V>) {
      if constexpr (sized_range<V>) {
        if constexpr (random_access_range<V>) {
          return ranges::begin(base_);
        } else {
          auto sz = range_difference_t<V>(size());
          return counted_iterator(ranges::begin(base_), sz);
        }
      } else {
        return counted_iterator(ranges::begin(base_), count_);
      }
    }
 
    constexpr auto begin() const requires range<const V> {
      if constexpr (sized_range<const V>) {
        if constexpr (random_access_range<const V>) {
          return ranges::begin(base_);
        } else {
          auto sz = range_difference_t<const V>(size());
          return counted_iterator(ranges::begin(base_), sz);
        }
      } else {
        return counted_iterator(ranges::begin(base_), count_);
      }
    }
 
    constexpr auto end() requires (!/*simple-view*/<V>) {
      if constexpr (sized_range<V>) {
        if constexpr (random_access_range<V>)
          return ranges::begin(base_) + range_difference_t<V>(size());
        else
          return default_sentinel;
      } else {
        return sentinel<false>{ranges::end(base_)};
      }
    }
 
    constexpr auto end() const requires range<const V> {
      if constexpr (sized_range<const V>) {
        if constexpr (random_access_range<const V>)
          return ranges::begin(base_) + range_difference_t<const V>(size());
        else
          return default_sentinel;
      } else {
        return sentinel<true>{ranges::end(base_)};
      }
    }
 
    constexpr auto size() requires sized_range<V> {
      auto n = ranges::size(base_);
      return ranges::min(n, static_cast<decltype(n)>(count_));
    }
 
    constexpr auto size() const requires sized_range<const V> {
      auto n = ranges::size(base_);
      return ranges::min(n, static_cast<decltype(n)>(count_));
    }
  };
 
  template<class R>
    take_view(R&&, range_difference_t<R>)
      -> take_view<views::all_t<R>>;
}

[edit] Class template std::ranges::take_view::sentinel

namespace std::ranges {
  template<view V>
  template<bool Const>
  class take_view<V>::sentinel {
  private:
    using Base = /*maybe-const*/<Const, V>;                            // exposition only
    template<bool OtherConst>
      using CI =
        counted_iterator<iterator_t</*maybe-const*/<OtherConst, V>>>;  // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();                        // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end);
    constexpr sentinel(sentinel<!Const> s)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    constexpr sentinel_t<Base> base() const;
 
    friend constexpr bool operator==(const CI<Const>& y, const sentinel& x);
 
    template<bool OtherConst = !Const>
      requires sentinel_for<sentinel_t<Base>, iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr bool operator==(const CI<OtherConst>& y, const sentinel& x);
  };
}

[edit] Class template std::ranges::take_while_view

namespace std::ranges {
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
  class take_while_view : public view_interface<take_while_view<V, Pred>> {
    // class template take_while_view::sentinel
    template<bool> class sentinel;                      // exposition only
 
    V base_ = V();                                      // exposition only
    /*copyable-box*/<Pred> pred_;                       // exposition only
 
  public:
    take_while_view() requires default_initializable<V> &&
      default_initializable<Pred> = default;
    constexpr take_while_view(V base, Pred pred);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr const Pred& pred() const;
 
    constexpr auto begin() requires (!/*simple-view*/<V>)
    { return ranges::begin(base_); }
 
    constexpr auto begin() const
      requires range<const V> &&
               indirect_unary_predicate<const Pred, iterator_t<const V>>
    { return ranges::begin(base_); }
 
    constexpr auto end() requires (!/*simple-view*/<V>)
    { return sentinel<false>(ranges::end(base_), addressof(*pred_)); }
 
    constexpr auto end() const
      requires range<const V> &&
               indirect_unary_predicate<const Pred, iterator_t<const V>>
    { return sentinel<true>(ranges::end(base_), addressof(*pred_)); }
  };
 
  template<class R, class Pred>
    take_while_view(R&&, Pred) -> take_while_view<views::all_t<R>, Pred>;
}

[edit] Class template std::ranges::take_while_view::sentinel

namespace std::ranges {
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
  template<bool Const>
  class take_while_view<V, Pred>::sentinel {
    using Base = /*maybe-const*/<Const, V>;             // exposition only
 
    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
    const Pred* pred_ = nullptr;                        // exposition only
 
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end, const Pred* pred);
    constexpr sentinel(sentinel<!Const> s)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    constexpr sentinel_t<Base> base() const { return end_; }
 
    friend constexpr bool operator==(const iterator_t<Base>& x, const sentinel& y);
 
    template<bool OtherConst = !Const>
      requires sentinel_for<sentinel_t<Base>, iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr bool operator==(const iterator_t</*maybe-const*/<OtherConst, V>>& x,
                                     const sentinel& y);
  };
}

[edit] Class template std::ranges::drop_view

namespace std::ranges {
  template<view V>
  class drop_view : public view_interface<drop_view<V>> {
  public:
    drop_view() requires default_initializable<V> = default;
    constexpr drop_view(V base, range_difference_t<V> count);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin()
      requires (!(/*simple-view*/<V> &&
                  random_access_range<const V> && sized_range<const V>));
    constexpr auto begin() const
      requires random_access_range<const V> && sized_range<const V>;
 
    constexpr auto end() requires (!/*simple-view*/<V>)
    { return ranges::end(base_); }
 
    constexpr auto end() const requires range<const V>
    { return ranges::end(base_); }
 
    constexpr auto size() requires sized_range<V> {
      const auto s = ranges::size(base_);
      const auto c = static_cast<decltype(s)>(count_);
      return s < c ? 0 : s - c;
    }
 
    constexpr auto size() const requires sized_range<const V> {
      const auto s = ranges::size(base_);
      const auto c = static_cast<decltype(s)>(count_);
      return s < c ? 0 : s - c;
    }
 
  private:
    V base_ = V();                              // exposition only
    range_difference_t<V> count_ = 0;           // exposition only
  };
 
  template<class R>
    drop_view(R&&, range_difference_t<R>) -> drop_view<views::all_t<R>>;
}

[edit] Class template std::ranges::drop_while_view

namespace std::ranges {
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
  class drop_while_view : public view_interface<drop_while_view<V, Pred>> {
  public:
    drop_while_view() requires default_initializable<V> &&
      default_initializable<Pred> = default;
    constexpr drop_while_view(V base, Pred pred);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr const Pred& pred() const;
 
    constexpr auto begin();
 
    constexpr auto end() { return ranges::end(base_); }
 
  private:
    V base_ = V();                                      // exposition only
    /*copyable-box*/<Pred> pred_;                       // exposition only
  };
 
  template<class R, class Pred>
    drop_while_view(R&&, Pred) -> drop_while_view<views::all_t<R>, Pred>;
}

[edit] Class template std::ranges::join_view

namespace std::ranges {
  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>>
  class join_view : public view_interface<join_view<V>> {
  private:
    using InnerRng = range_reference_t<V>;                  // exposition only
 
    // class template join_view::iterator
    template<bool Const>
      struct iterator;                                      // exposition only
 
    // class template join_view::sentinel
    template<bool Const>
      struct sentinel;                                      // exposition only
 
    V base_ = V();                                          // exposition only
 
    /*non-propagating-cache*/<remove_cv_t<InnerRng>>inner_;
      // exposition only, present only when !is_reference_v<InnerRng>
 
  public:
    join_view() requires default_initializable<V> = default;
    constexpr explicit join_view(V base);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin() {
      constexpr bool use_const = /*simple-view*/<V> &&
                                 is_reference_v<range_reference_t<V>>;
      return iterator<use_const>{*this, ranges::begin(base_)};
    }
 
    constexpr auto begin() const
      requires input_range<const V> &&
               is_reference_v<range_reference_t<const V>>
    { return iterator<true>{*this, ranges::begin(base_)}; }
 
    constexpr auto end() {
      if constexpr (forward_range<V> &&
                    is_reference_v<InnerRng> && forward_range<InnerRng> &&
                    common_range<V> && common_range<InnerRng>)
        return iterator</*simple-view*/<V>>{*this, ranges::end(base_)};
      else
        return sentinel</*simple-view*/<V>>{*this};
    }
 
    constexpr auto end() const
      requires input_range<const V> &&
               is_reference_v<range_reference_t<const V>> {
      if constexpr (forward_range<const V> &&
                    forward_range<range_reference_t<const V>> &&
                    common_range<const V> &&
                    common_range<range_reference_t<const V>>)
        return iterator<true>{*this, ranges::end(base_)};
      else
        return sentinel<true>{*this};
    }
  };
 
  template<class R>
    explicit join_view(R&&) -> join_view<views::all_t<R>>;
}

[edit] Class template std::ranges::join_view::iterator

namespace std::ranges {
  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>>
  template<bool Const>
  struct join_view<V>::iterator {
  private:
    using Parent    = /*maybe-const*/<Const, join_view>;        // exposition only
    using Base      = /*maybe-const*/<Const, V>;                // exposition only
    using OuterIter = iterator_t<Base>;                         // exposition only
    using InnerIter = iterator_t<range_reference_t<Base>>;      // exposition only
 
    static constexpr bool /*ref-is-glvalue*/ =                  // exposition only
      is_reference_v<range_reference_t<Base>>;
 
    OuterIter outer_ = OuterIter();                             // exposition only
    InnerIter inner_ = InnerIter();                             // exposition only
    Parent* parent_  = nullptr;                                 // exposition only
 
    constexpr void satisfy();                                   // exposition only
 
  public:
    using iterator_concept  = /* see description */;
    using iterator_category = /* see description */;            // not always present
    using value_type        = range_value_t<range_reference_t<Base>>;
    using difference_type   = /* see description */;
 
    iterator() requires default_initializable<OuterIter> &&
                         default_initializable<InnerIter> = default;
    constexpr iterator(Parent& parent, OuterIter outer);
    constexpr iterator(iterator<!Const> i)
      requires Const &&
               convertible_to<iterator_t<V>, OuterIter> &&
               convertible_to<iterator_t<InnerRng>, InnerIter>;
 
    constexpr decltype(auto) operator*() const { return *inner_; }
 
    constexpr InnerIter operator->() const
      requires /*has-arrow*/<InnerIter> && copyable<InnerIter>;
 
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int)
      requires /*ref-is-glvalue*/ && forward_range<Base> &&
               forward_range<range_reference_t<Base>>;
 
    constexpr iterator& operator--()
      requires /*ref-is-glvalue*/ && bidirectional_range<Base> &&
               bidirectional_range<range_reference_t<Base>> &&
               common_range<range_reference_t<Base>>;
 
    constexpr iterator operator--(int)
      requires /*ref-is-glvalue*/ && bidirectional_range<Base> &&
               bidirectional_range<range_reference_t<Base>> &&
               common_range<range_reference_t<Base>>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires /*ref-is-glvalue*/ && equality_comparable<iterator_t<Base>> &&
               equality_comparable<iterator_t<range_reference_t<Base>>>;
 
    friend constexpr decltype(auto) iter_move(const iterator& i)
    noexcept(noexcept(ranges::iter_move(i.inner_))) {
      return ranges::iter_move(i.inner_);
    }
 
    friend constexpr void iter_swap(const iterator& x, const iterator& y)
      noexcept(noexcept(ranges::iter_swap(x.inner_, y.inner_)))
      requires indirectly_swappable<InnerIter>;
  };
}

[edit] Class template std::ranges::join_view::sentinel

namespace std::ranges {
  template<input_range V>
    requires view<V> && input_range<range_reference_t<V>>
  template<bool Const>
  struct join_view<V>::sentinel {
  private:
    using Parent = /*maybe-const*/<Const, join_view>;       // exposition only
    using Base = /*maybe-const*/<Const, V>;                 // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();             // exposition only
 
  public:
    sentinel() = default;
 
    constexpr explicit sentinel(Parent& parent);
    constexpr sentinel(sentinel<!Const> s)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    template<bool OtherConst>
      requires sentinel_for<sentinel_t<Base>, iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
  };
}

[edit] Class template std::ranges::join_with_view

namespace std::ranges {
  template<class R, class P>
  concept /*compatible-joinable-ranges*/ =            // exposition only
      common_with<range_value_t<R>, range_value_t<P>> &&
      common_reference_with<range_reference_t<R>, range_reference_t<P>> &&
      common_reference_with<range_rvalue_reference_t<R>, range_rvalue_reference_t<P>>;
 
  template<class R>
  concept /*bidirectional-common*/ =                  // exposition only
        bidirectional_range<R> && common_range<R>;
 
  template<input_range V, forward_range Pattern>
    requires view<V> && input_range<range_reference_t<V>>
          && view<Pattern>
          && /*compatible-joinable-ranges*/<range_reference_t<V>, Pattern>
  class join_with_view : public view_interface<join_with_view<V, Pattern>> {
    using InnerRng = range_reference_t<V>;             // exposition only
 
    V base_ = V();                                          // exposition only
        // exposition only, present only when !is_reference_v<inner-rng>
    /*non-propagating-cache*/<remove_cv_t<InnerRng>> inner_;
    Pattern pattern_ = Pattern();                           // exposition only
 
    // class template join_with_view::iterator
    template<bool Const> struct iterator;                   // exposition only
 
    // class template join_with_view::sentinel
    template<bool Const> struct sentinel;                   // exposition only
 
  public:
    join_with_view()
      requires default_initializable<V> && default_initializable<Pattern> = default;
 
    constexpr join_with_view(V base, Pattern pattern);
 
    template<input_range R>
      requires constructible_from<V, views::all_t<R>> &&
               constructible_from<Pattern, single_view<range_value_t<InnerRng>>>
    constexpr join_with_view(R&& r, range_value_t<InnerRng> e);
 
    constexpr V base() const & requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin() {
      constexpr bool use_const =
          /*simple-view*/<V>
       && is_reference_v<InnerRng>
       && /*simple-view*/<Pattern>;
      return iterator<use_const>{*this, ranges::begin(base_)};
    }
    constexpr auto begin() const
      requires input_range<const V> &&
               forward_range<const Pattern> &&
               is_reference_v<range_reference_t<const V>> {
      return iterator<true>{*this, ranges::begin(base_)};
    }
 
    constexpr auto end() {
      if constexpr (forward_range<V> &&
                    is_reference_v<InnerRng> && forward_range<InnerRng> &&
                    common_range<V> && common_range<InnerRng>)
        return iterator</*simple-view*/<V> && /*simple-view*/<Pattern>>{
          *this, ranges::end(base_)
        };
      else
        return sentinel</*simple-view*/<V> && /*simple-view*/<Pattern>>{*this};
    }
    constexpr auto end() const
      requires input_range<const V> && forward_range<const Pattern> &&
               is_reference_v<range_reference_t<const V>> {
      using InnerConstRng = range_reference_t<const V>;
      if constexpr (forward_range<const V> && forward_range<InnerConstRng> &&
                    common_range<const V> && common_range<InnerConstRng>)
        return iterator<true>{*this, ranges::end(base_)};
      else
        return sentinel<true>{*this};
    }
  };
 
  template<class R, class P>
    join_with_view(R&&, P&&) -> join_with_view<views::all_t<R>, views::all_t<P>>;
 
  template<input_range R>
    join_with_view(R&&, range_value_t<range_reference_t<R>>)
      -> join_with_view<views::all_t<R>, single_view<range_value_t<range_reference_t<R>>>>;
}

[edit] Class template std::ranges::join_with_view::iterator

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && input_range<range_reference_t<V>>
          && view<Pattern> && /*compatible-joinable-ranges*/<range_reference_t<V>, Pattern>
  template<bool Const>
  class join_with_view<V, Pattern>::iterator {
    using Parent = /*maybe-const*/<Const, join_with_view>;              // exposition only
    using Base = /*maybe-const*/<Const, V>;                             // exposition only
    using InnerBase = range_reference_t<Base>;                          // exposition only
    using PatternBase = /*maybe-const*/<Const, Pattern>;                // exposition only
 
    using OuterIter = iterator_t<Base>;                                 // exposition only
    using InnerIter = iterator_t<InnerBase>;                            // exposition only
    using PatternIter = iterator_t<PatternBase>;                        // exposition only
 
    static constexpr bool /*ref-is-glvalue*/ = is_reference_v<InnerBase>; // exposition only
 
    Parent* parent_ = nullptr;                                          // exposition only
    OuterIter outer_it_ = OuterIter();                                  // exposition only
    variant<PatternIter, InnerIter> inner_it_;                          // exposition only
 
    constexpr iterator(Parent& parent, iterator_t<Base> outer);         // exposition only
    constexpr auto&& /*update-inner*/(const OuterIter&);                // exposition only
    constexpr auto&& /*get-inner*/(const OuterIter&);                   // exposition only
    constexpr void /*satisfy*/();                                       // exposition only
 
  public:
    using iterator_concept = /* see description */;
    using iterator_category = /* see description */;                // not always present
    using value_type = /* see description */;
    using difference_type = /* see description */;
 
    iterator() requires default_initializable<OuterIter> = default;
    constexpr iterator(iterator<!Const> i)
        requires Const && convertible_to<iterator_t<V>, OuterIter> &&
                 convertible_to<iterator_t<InnerRng>, InnerIter> &&
                 convertible_to<iterator_t<Pattern>, PatternIter>;
 
    constexpr decltype(auto) operator*() const;
 
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int)
        requires /*ref-is-glvalue*/ && forward_iterator<OuterIter> &&
                 forward_iterator<InnerIter>;
 
    constexpr iterator& operator--()
        requires /*ref-is-glvalue*/ && bidirectional_range<Base> &&
                 /*bidirectional-common*/<InnerBase> &&
                 /*bidirectional-common*/<PatternBase>;
    constexpr iterator operator--(int)
        requires /*ref-is-glvalue*/ && bidirectional_range<Base> &&
                 /*bidirectional-common*/<InnerBase> &&
                 /*bidirectional-common*/<PatternBase>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
        requires /*ref-is-glvalue*/ && equality_comparable<OuterIter> &&
                 equality_comparable<InnerIter>;
 
    friend constexpr decltype(auto) iter_move(const iterator& x) {
      using rvalue_reference = common_reference_t<
        iter_rvalue_reference_t<InnerIter>,
        iter_rvalue_reference_t<PatternIter>>;
      return visit<rvalue_reference>(ranges::iter_move, x.inner_it_);
    }
 
    friend constexpr void iter_swap(const iterator& x, const iterator& y)
        requires indirectly_swappable<InnerIter, PatternIter> {
      visit(ranges::iter_swap, x.inner_it_, y.inner_it_);
    }
  };
}

[edit] Class template std::ranges::join_with_view::sentinel

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && input_range<range_reference_t<V>>
          && view<Pattern> && /*compatible-joinable-ranges*/<range_reference_t<V>, Pattern>
  template<bool Const>
  class join_with_view<V, Pattern>::sentinel {
    using Parent = /*maybe-const*/<Const, join_with_view>;  // exposition only
    using Base = /*maybe-const*/<Const, V>;                 // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();             // exposition only
 
    constexpr explicit sentinel(Parent& parent);            // exposition only
 
  public:
    sentinel() = default;
    constexpr sentinel(sentinel<!Const> s)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    template <bool OtherConst>
      requires sentinel_for<sentinel_t<Base>, iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
  };
}

[edit] Class template std::ranges::lazy_split_view

namespace std::ranges {
  template<auto> struct /*require-constant*/;                       // exposition only
 
  template<class R>
  concept /*tiny-range*/ =                                          // exposition only
    sized_range<R> &&
    requires { typename /*require-constant*/<remove_reference_t<R>::size()>; } &&
    (remove_reference_t<R>::size() <= 1);
 
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
             && (forward_range<V> || /*tiny-range*/<Pattern>)
  class lazy_split_view : public view_interface<lazy_split_view<V, Pattern>> {
  private:
    V base_ = V();                                      // exposition only
    Pattern pattern_ = Pattern();                       // exposition only
 
    /*non-propagating-cache*/<iterator_t<V>> current_;  // exposition only, present only
                                                        // if !forward_range<V>
 
    // class template lazy_split_view::/*outer-iterator*/
    template<bool> struct /*outer-iterator*/;           // exposition only
 
    // class template lazy_split_view::/*inner-iterator*/
    template<bool> struct /*inner-iterator*/;           // exposition only
 
  public:
    lazy_split_view()
      requires default_initializable<V> && default_initializable<Pattern> = default;
    constexpr lazy_split_view(V base, Pattern pattern);
 
    template<input_range R>
      requires constructible_from<V, views::all_t<R>> &&
               constructible_from<Pattern, single_view<range_value_t<R>>>
    constexpr lazy_split_view(R&& r, range_value_t<R> e);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin() {
      if constexpr (forward_range<V>) {
        return /*outer-iterator*/</*simple-view*/<V> && /*simple-view*/<Pattern>>
          {*this, ranges::begin(base_)};
      } else {
        current_ = ranges::begin(base_);
        return /*outer-iterator*/<false>{*this};
      }
    }
 
    constexpr auto begin() const requires forward_range<V> && forward_range<const V> {
      return /*outer-iterator*/<true>{*this, ranges::begin(base_)};
    }
 
    constexpr auto end() requires forward_range<V> && common_range<V> {
      return /*outer-iterator*/</*simple-view*/<V> && /*simple-view*/<Pattern>>
        {*this, ranges::end(base_)};
    }
 
    constexpr auto end() const {
      if constexpr (forward_range<V> && forward_range<const V> && common_range<const V>)
        return /*outer-iterator*/<true>{*this, ranges::end(base_)};
      else
        return default_sentinel;
    }
  };
 
  template<class R, class P>
    lazy_split_view(R&&, P&&) -> lazy_split_view<views::all_t<R>, views::all_t<P>>;
 
  template<input_range R>
    lazy_split_view(R&&, range_value_t<R>)
      -> lazy_split_view<views::all_t<R>, single_view<range_value_t<R>>>;
}

[edit] Class template std::ranges::lazy_split_view::outer_iterator

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
             && (forward_range<V> || /*tiny-range*/<Pattern>)
  template<bool Const>
  struct lazy_split_view<V, Pattern>::/*outer-iterator*/ {
  private:
    using Parent = /*maybe-const*/<Const, lazy_split_view>; // exposition only
    using Base = /*maybe-const*/<Const, V>;                 // exposition only
    Parent* parent_ = nullptr;                              // exposition only
 
    iterator_t<Base> current_ = iterator_t<Base>();      // exposition only, present only
                                                         // if V models forward_range
 
    bool trailing_empty_ = false;                        // exposition only
 
  public:
    using iterator_concept  =
      conditional_t<forward_range<Base>, forward_iterator_tag, input_iterator_tag>;
 
    using iterator_category = input_iterator_tag;        // present only if Base
                                                         // models forward_range
 
    // class lazy_split_view::/*outer-iterator*/::value_type
    struct value_type;
    using difference_type   = range_difference_t<Base>;
 
    /*outer-iterator*/() = default;
    constexpr explicit /*outer-iterator*/(Parent& parent)
      requires (!forward_range<Base>);
    constexpr /*outer-iterator*/(Parent& parent, iterator_t<Base> current)
      requires forward_range<Base>;
    constexpr /*outer-iterator*/(/*outer-iterator*/<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
 
    constexpr value_type operator*() const;
 
    constexpr /*outer-iterator*/& operator++();
    constexpr decltype(auto) operator++(int) {
      if constexpr (forward_range<Base>) {
        auto tmp = *this;
        ++*this;
        return tmp;
      } else
        ++*this;
    }
 
    friend constexpr bool operator==(const /*outer-iterator*/& x,
                                     const /*outer-iterator*/& y)
      requires forward_range<Base>;
 
    friend constexpr bool operator==(const /*outer-iterator*/& x, default_sentinel_t);
  };
}

[edit] Class template std::ranges::lazy_split_view::outer_iterator::value_type

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
             && (forward_range<V> || /*tiny-range*/<Pattern>)
  template<bool Const>
  struct lazy_split_view<V, Pattern>::/*outer-iterator*/<Const>::value_type
    : view_interface<value_type> {
  private:
    /*outer-iterator*/ i_ = /*outer-iterator*/();               // exposition only
  public:
    value_type() = default;
    constexpr explicit value_type(/*outer-iterator*/ i);
 
    constexpr /*inner-iterator*/<Const> begin() const;
    constexpr default_sentinel_t end() const noexcept;
  };
}

[edit] Class template std::ranges::lazy_split_view::inner_iterator

namespace std::ranges {
  template<input_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
             && (forward_range<V> || /*tiny-range*/<Pattern>)
  template<bool Const>
  struct lazy_split_view<V, Pattern>::/*inner-iterator*/ {
  private:
    using Base = /*maybe-const*/<Const, V>;                     // exposition only
    /*outer-iterator*/<Const> i_ = /*outer-iterator*/<Const>(); // exposition only
    bool incremented_ = false;                                  // exposition only
 
  public:
    using iterator_concept  = typename /*outer-iterator*/<Const>::iterator_concept;
 
    using iterator_category = /* see description */;        // present only if Base
                                                            // models forward_range
    using value_type        = range_value_t<Base>;
    using difference_type   = range_difference_t<Base>;
 
    /*inner-iterator*/() = default;
    constexpr explicit /*inner-iterator*/(/*outer-iterator*/<Const> i);
 
    constexpr const iterator_t<Base>& base() const & noexcept;
    constexpr iterator_t<Base> base() && requires forward_range<V>;
 
    constexpr decltype(auto) operator*() const { return *i_.current; }
 
    constexpr /*inner-iterator*/& operator++();
    constexpr decltype(auto) operator++(int) {
      if constexpr (forward_range<Base>) {
        auto tmp = *this;
        ++*this;
        return tmp;
      } else
        ++*this;
    }
 
    friend constexpr bool operator==(const /*inner-iterator*/& x,
                                     const /*inner-iterator*/& y)
      requires forward_range<Base>;
 
    friend constexpr bool operator==(const /*inner-iterator*/& x, default_sentinel_t);
 
    friend constexpr decltype(auto) iter_move(const /*inner-iterator*/& i)
    noexcept(noexcept(ranges::iter_move(i.i_.current))) {
      return ranges::iter_move(i.i_.current);
    }
 
    friend constexpr void iter_swap(const /*inner-iterator*/& x,
                                    const /*inner-iterator*/& y)
      noexcept(noexcept(ranges::iter_swap(x.i_.current, y.i_.current)))
      requires indirectly_swappable<iterator_t<Base>>;
  };
}

[edit] Class template std::ranges::split_view

namespace std::ranges {
  template<forward_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
  class split_view : public view_interface<split_view<V, Pattern>> {
  private:
    V base_ = V();                              // exposition only
    Pattern pattern_ = Pattern();               // exposition only
 
    // class split_view::iterator
    struct iterator;                            // exposition only
 
    // class split_view::sentinel
    struct sentinel;                            // exposition only
 
  public:
    split_view()
      requires default_initializable<V> && default_initializable<Pattern> = default;
    constexpr split_view(V base, Pattern pattern);
 
    template<forward_range R>
      requires constructible_from<V, views::all_t<R>> &&
               constructible_from<Pattern, single_view<range_value_t<R>>>
    constexpr split_view(R&& r, range_value_t<R> e);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr iterator begin();
 
    constexpr auto end() {
      if constexpr (common_range<V>) {
        return iterator{*this, ranges::end(base_), {} };
      } else {
        return sentinel{*this};
      }
    }
 
    constexpr subrange<iterator_t<V>> /*find-next*/(iterator_t<V>); // exposition only
  };
 
  template<class R, class P>
    split_view(R&&, P&&) -> split_view<views::all_t<R>, views::all_t<P>>;
 
  template<forward_range R>
    split_view(R&&, range_value_t<R>)
      -> split_view<views::all_t<R>, single_view<range_value_t<R>>>;
}

[edit] Class template std::ranges::split_view::iterator

namespace std::ranges {
  template<forward_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
  class split_view<V, Pattern>::iterator {
  private:
    split_view* parent_ = nullptr;                              // exposition only
    iterator_t<V> cur_ = iterator_t<V>();                       // exposition only
    subrange<iterator_t<V>> next_ = subrange<iterator_t<V>>();  // exposition only
    bool trailing_empty_ = false;                               // exposition only
 
  public:
    using iterator_concept = forward_iterator_tag;
    using iterator_category = input_iterator_tag;
    using value_type = subrange<iterator_t<V>>;
    using difference_type = range_difference_t<V>;
 
    iterator() = default;
    constexpr iterator(split_view& parent, iterator_t<V> current,
                       subrange<iterator_t<V>> next);
    constexpr iterator_t<V> base() const;
    constexpr value_type operator*() const;
 
    constexpr iterator& operator++();
    constexpr iterator operator++(int);
 
    friend constexpr bool operator==(const iterator& x, const iterator& y);
  };
}

[edit] Class template std::ranges::split_view::sentinel

namespace std::ranges {
  template<forward_range V, forward_range Pattern>
    requires view<V> && view<Pattern> &&
             indirectly_comparable<iterator_t<V>, iterator_t<Pattern>, ranges::equal_to>
  struct split_view<V, Pattern>::sentinel {
  private:
    sentinel_t<V> end_ = sentinel_t<V>();               // exposition only
 
  public:
    sentinel() = default;
    constexpr explicit sentinel(split_view& parent);
 
    friend constexpr bool operator==(const iterator& x, const sentinel& y);
  };
}

[edit] Class template std::ranges::common_view

namespace std::ranges {
  template<view V>
    requires (!common_range<V> && copyable<iterator_t<V>>)
  class common_view : public view_interface<common_view<V>> {
  private:
    V base_ = V();  // exposition only
  public:
    common_view() requires default_initializable<V> = default;
 
    constexpr explicit common_view(V r);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin() {
      if constexpr (random_access_range<V> && sized_range<V>)
        return ranges::begin(base_);
      else
        return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_));
    }
 
    constexpr auto begin() const requires range<const V> {
      if constexpr (random_access_range<const V> && sized_range<const V>)
        return ranges::begin(base_);
      else
        return common_iterator<iterator_t<const V>,
          sentinel_t<const V>>(ranges::begin(base_));
    }
 
    constexpr auto end() {
      if constexpr (random_access_range<V> && sized_range<V>)
        return ranges::begin(base_) + ranges::size(base_);
      else
        return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_));
    }
 
    constexpr auto end() const requires range<const V> {
      if constexpr (random_access_range<const V> && sized_range<const V>)
        return ranges::begin(base_) + ranges::size(base_);
      else
        return common_iterator<iterator_t<const V>,
          sentinel_t<const V>>(ranges::end(base_));
    }
 
    constexpr auto size() requires sized_range<V> {
      return ranges::size(base_);
    }
    constexpr auto size() const requires sized_range<const V> {
      return ranges::size(base_);
    }
  };
 
  template<class R>
    common_view(R&&) -> common_view<views::all_t<R>>;
}

[edit] Class template std::ranges::reverse_view

namespace std::ranges {
  template<view V>
    requires bidirectional_range<V>
  class reverse_view : public view_interface<reverse_view<V>> {
  private:
    V base_ = V();  // exposition only
  public:
    reverse_view() requires default_initializable<V> = default;
 
    constexpr explicit reverse_view(V r);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr reverse_iterator<iterator_t<V>> begin();
    constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
    constexpr auto begin() const requires common_range<const V>;
 
    constexpr reverse_iterator<iterator_t<V>> end();
    constexpr auto end() const requires common_range<const V>;
 
    constexpr auto size() requires sized_range<V> {
      return ranges::size(base_);
    }
 
    constexpr auto size() const requires sized_range<const V> {
      return ranges::size(base_);
    }
  };
 
  template<class R>
    reverse_view(R&&) -> reverse_view<views::all_t<R>>;
}

[edit] Class template std::ranges::elements_view

namespace std::ranges {
  template<class T, size_t N>
  concept /*has-tuple-element*/ =                   // exposition only
    requires(T t) {
      typename tuple_size<T>::type;
      requires N < tuple_size_v<T>;
      typename tuple_element_t<N, T>;
      { std::get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
    };
 
  template<class T, size_t N>
  concept /*returnable-element*/ =                  // exposition only
    is_reference_v<T> || move_constructible<tuple_element_t<N, T>>;
 
  template<input_range V, size_t N>
    requires view<V> && /*has-tuple-element*/<range_value_t<V>, N> &&
             /*has-tuple-element*/<remove_reference_t<range_reference_t<V>>, N> &&
             /*returnable-element*/<range_reference_t<V>, N>
  class elements_view : public view_interface<elements_view<V, N>> {
  public:
    elements_view() requires default_initializable<V> = default;
    constexpr explicit elements_view(V base);
 
    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin() requires (!/*simple-view*/<V>)
    { return iterator<false>(ranges::begin(base_)); }
 
    constexpr auto begin() const requires range<const V>
    { return iterator<true>(ranges::begin(base_)); }
 
    constexpr auto end() requires (!/*simple-view*/<V> && !common_range<V>)
    { return sentinel<false>{ranges::end(base_)}; }
 
    constexpr auto end() requires (!/*simple-view*/<V> && common_range<V>)
    { return iterator<false>{ranges::end(base_)}; }
 
    constexpr auto end() const requires range<const V>
    { return sentinel<true>{ranges::end(base_)}; }
 
    constexpr auto end() const requires common_range<const V>
    { return iterator<true>{ranges::end(base_)}; }
 
    constexpr auto size() requires sized_range<V>
    { return ranges::size(base_); }
 
    constexpr auto size() const requires sized_range<const V>
    { return ranges::size(base_); }
 
  private:
    // class template elements_view::iterator
    template<bool> struct iterator;                     // exposition only
 
    // class template elements_view::sentinel
    template<bool> struct sentinel;                     // exposition only
 
    V base_ = V();                                      // exposition only
  };
}

[edit] Class template std::ranges::elements_view::iterator

namespace std::ranges {
  template<input_range V, size_t N>
    requires view<V> && /*has-tuple-element*/<range_value_t<V>, N> &&
             /*has-tuple-element*/<remove_reference_t<range_reference_t<V>>, N> &&
             /*returnable-element*/<range_reference_t<V>, N>
  template<bool Const>
  class elements_view<V, N>::iterator {
    using Base = /*maybe-const*/<Const, V>;             // exposition only
 
    iterator_t<Base> current_ = iterator_t<Base>();     // exposition only
 
    static constexpr decltype(auto) /*get-element*/(const iterator_t<Base>& i);
      // exposition only
 
  public:
    using iterator_concept = /* see description */;
    using iterator_category = /* see description */;                // not always present
    using value_type = remove_cvref_t<tuple_element_t<N, range_value_t<Base>>>;
    using difference_type = range_difference_t<Base>;
 
    iterator() requires default_initializable<iterator_t<Base>> = default;
    constexpr explicit iterator(iterator_t<Base> current);
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
 
    constexpr const iterator_t<Base>& base() const & noexcept;
    constexpr iterator_t<Base> base() &&;
 
    constexpr decltype(auto) operator*() const
    { return /*get-element*/(current_); }
 
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires forward_range<Base>;
 
    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;
 
    constexpr iterator& operator+=(difference_type x)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x)
      requires random_access_range<Base>;
 
    constexpr decltype(auto) operator[](difference_type n) const
      requires random_access_range<Base>
    { return /*get-element*/(current_ + n); }
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<iterator_t<Base>>;
 
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;
 
    friend constexpr iterator operator+(const iterator& x, difference_type y)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& x, difference_type y)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
  };
}

[edit] Class template std::ranges::elements_view::sentinel

namespace std::ranges {
  template<input_range V, size_t N>
    requires view<V> && /*has-tuple-element*/<range_value_t<V>, N> &&
             /*has-tuple-element*/<remove_reference_t<range_reference_t<V>>, N> &&
             /*returnable-element*/<range_reference_t<V>, N>
  template<bool Const>
  class elements_view<V, N>::sentinel {
  private:
    using Base = /*maybe-const*/<Const, V>;             // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();         // exposition only
  public:
    sentinel() = default;
    constexpr explicit sentinel(sentinel_t<Base> end);
    constexpr sentinel(sentinel<!Const> other)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    constexpr sentinel_t<Base> base() const;
 
    template<bool OtherConst>
      requires sentinel_for<sentinel_t<Base>, iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<sentinel_t<Base>,
        iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, V>>
      operator-(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<sentinel_t<Base>,
        iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, V>>
      operator-(const sentinel& x, const iterator<OtherConst>& y);
  };
}

[edit] Class template std::ranges::zip_view

namespace std::ranges {
  template<class... Rs>
  concept /*zip-is-common*/ =                                // exposition only
    (sizeof...(Rs) == 1 && (common_range<Rs> && ...)) ||
    (!(bidirectional_range<Rs> && ...) && (common_range<Rs> && ...)) ||
    ((random_access_range<Rs> && ...) && (sized_range<Rs> && ...));
 
  template<class... Ts>
    using /*tuple-or-pair*/ = /* see description */;         // exposition only
 
  template<class F, class Tuple>
  constexpr auto /*tuple-transform*/(F&& f, Tuple&& tuple) { // exposition only
    return apply([&]<class... Ts>(Ts&&... elements) {
      return /*tuple-or-pair*/<invoke_result_t<F&, Ts>...>(
        invoke(f, std::forward<Ts>(elements))...
      );
    }, std::forward<Tuple>(tuple));
  }
 
  template<class F, class Tuple>
  constexpr void /*tuple-for-each*/(F&& f, Tuple&& tuple) { // exposition only
    apply([&]<class... Ts>(Ts&&... elements) {
      (invoke(f, std::forward<Ts>(elements)), ...);
    }, std::forward<Tuple>(tuple));
  }
 
  template<input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0)
  class zip_view : public view_interface<zip_view<Views...>> {
    tuple<Views...> views_;             // exposition only
 
    // class template zip_view::iterator
    template<bool> class iterator;      // exposition only
 
    // class template zip_view::sentinel
    template<bool> class sentinel;      // exposition only
 
  public:
    zip_view() = default;
    constexpr explicit zip_view(Views... views);
 
    constexpr auto begin() requires (!(/*simple-view*/<Views> && ...)) {
      return iterator<false>(/*tuple-transform*/(ranges::begin, views_));
    }
    constexpr auto begin() const requires (range<const Views> && ...) {
      return iterator<true>(/*tuple-transform*/(ranges::begin, views_));
    }
 
    constexpr auto end() requires (!(/*simple-view*/<Views> && ...)) {
      if constexpr (!/*zip-is-common*/<Views...>) {
        return sentinel<false>(/*tuple-transform*/(ranges::end, views_));
      } else if constexpr ((random_access_range<Views> && ...)) {
        return begin() + iter_difference_t<iterator<false>>(size());
      } else {
        return iterator<false>(/*tuple-transform*/(ranges::end, views_));
      }
    }
 
    constexpr auto end() const requires (range<const Views> && ...) {
      if constexpr (!/*zip-is-common*/<const Views...>) {
        return sentinel<true>(/*tuple-transform*/(ranges::end, views_));
      } else if constexpr ((random_access_range<const Views> && ...)) {
        return begin() + iter_difference_t<iterator<true>>(size());
      } else {
        return iterator<true>(/*tuple-transform*/(ranges::end, views_));
      }
    }
 
    constexpr auto size() requires (sized_range<Views> && ...);
    constexpr auto size() const requires (sized_range<const Views> && ...);
  };
 
  template<class... Rs>
    zip_view(Rs&&...) -> zip_view<views::all_t<Rs>...>;
}

[edit] Class template std::ranges::zip_view::iterator

namespace std::ranges {
  template<bool Const, class... Views>
    concept /*all-random-access*/ =                 // exposition only
      (random_access_range</*maybe-const*/<Const, Views>> && ...);
  template<bool Const, class... Views>
    concept /*all-bidirectional*/ =                 // exposition only
      (bidirectional_range</*maybe-const*/<Const, Views>> && ...);
  template<bool Const, class... Views>
    concept /*all-forward*/ =                       // exposition only
      (forward_range</*maybe-const*/<Const, Views>> && ...);
 
  template<input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0)
  template<bool Const>
  class zip_view<Views...>::iterator {
    /*tuple-or-pair*/<iterator_t</*maybe-const*/<Const, Views>>...>
      current_;       // exposition only
    constexpr explicit iterator(/*tuple-or-pair*/<iterator_t<
      /*maybe-const*/<Const, Views>>...>);          // exposition only
  public:
    using iterator_category = input_iterator_tag;   // not always present
    using iterator_concept  = /* see description */;
    using value_type = /*tuple-or-pair*/<range_value_t<
      /*maybe-const*/<Const, Views>>...>;
    using difference_type = common_type_t<range_difference_t<
      /*maybe-const*/<Const, Views>>...>;
 
    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && (convertible_to<iterator_t<Views>,
        iterator_t</*maybe-const*/<Const, Views>>> && ...);
 
    constexpr auto operator*() const;
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires /*all-forward*/<Const, Views...>;
 
    constexpr iterator& operator--() requires /*all-bidirectional*/<Const, Views...>;
    constexpr iterator operator--(int) requires /*all-bidirectional*/<Const, Views...>;
 
    constexpr iterator& operator+=(difference_type x)
      requires /*all-random-access*/<Const, Views...>;
    constexpr iterator& operator-=(difference_type x)
      requires /*all-random-access*/<Const, Views...>;
 
    constexpr auto operator[](difference_type n) const
      requires /*all-random-access*/<Const, Views...>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires (equality_comparable<iterator_t</*maybe-const*/<Const, Views>>> && ...);
 
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires /*all-random-access*/<Const, Views...>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires /*all-random-access*/<Const, Views...>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires /*all-random-access*/<Const, Views...>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires /*all-random-access*/<Const, Views...>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires /*all-random-access*/<Const, Views...> &&
               (three_way_comparable<iterator_t</*maybe-const*/<Const, Views>>> && ...);
 
    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires /*all-random-access*/<Const, Views...>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires /*all-random-access*/<Const, Views...>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires /*all-random-access*/<Const, Views...>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires (sized_sentinel_for<iterator_t</*maybe-const*/<Const, Views>>,
                                   iterator_t</*maybe-const*/<Const, Views>>> && ...);
 
    friend constexpr auto iter_move(const iterator& i) noexcept(/* see description */);
 
    friend constexpr void iter_swap(const iterator& l, const iterator& r)
      noexcept(/* see description */)
      requires (indirectly_swappable<iterator_t</*maybe-const*/<Const, Views>>> && ...);
  };
}

[edit] Class template std::ranges::zip_view::sentinel

namespace std::ranges {
  template<input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0)
  template<bool Const>
  class zip_view<Views...>::sentinel {
    /*tuple-or-pair*/<sentinel_t</*maybe-const*/<Const, Views>>...>
      end_;   // exposition only
    constexpr explicit sentinel(/*tuple-or-pair*/<sentinel_t<
      /*maybe-const*/<Const, Views>>...> end);  // exposition only
  public:
    sentinel() = default;
    constexpr sentinel(sentinel<!Const> i)
      requires Const &&
               (convertible_to<sentinel_t<Views>, sentinel_t<
                 /*maybe-const*/<Const, Views>>> && ...);
 
    template<bool OtherConst>
      requires (sentinel_for<sentinel_t</*maybe-const*/<Const, Views>>,
                             iterator_t</*maybe-const*/<OtherConst, Views>>> && ...)
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires (sized_sentinel_for<sentinel_t</*maybe-const*/<Const, Views>>,
                                   iterator_t</*maybe-const*/<OtherConst, Views>>> && ...)
    friend constexpr common_type_t<range_difference_t<
      /*maybe-const*/<OtherConst, Views>>...>
      operator-(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires (sized_sentinel_for<sentinel_t</*maybe-const*/<Const, Views>>,
                                   iterator_t</*maybe-const*/<OtherConst, Views>>> && ...)
    friend constexpr common_type_t<range_difference_t<
      /*maybe-const*/<OtherConst, Views>>...>
      operator-(const sentinel& y, const iterator<OtherConst>& x);
  };
}

[edit] Class template std::ranges::zip_transform_view

namespace std::ranges {
  template<copy_constructible F, input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> &&
              regular_invocable<F&, range_reference_t<Views>...> &&
              /*can-reference*/<invoke_result_t<F&, range_reference_t<Views>...>>
  class zip_transform_view : public view_interface<zip_transform_view<F, Views...>> {
    /*copyable-box*/<F> fun_;               // exposition only
    zip_view<Views...> zip_;                // exposition only
 
    using InnerView = zip_view<Views...>;   // exposition only
    template<bool Const>
      using ziperator = iterator_t</*maybe-const*/<Const, InnerView>>; // exposition only
    template<bool Const>
      using zentinel = sentinel_t</*maybe-const*/<Const, InnerView>>;  // exposition only
 
    // class template zip_transform_view::iterator
    template<bool> class iterator;          // exposition only
 
    // class template zip_transform_view::sentinel
    template<bool> class sentinel;          // exposition only
 
  public:
    zip_transform_view() = default;
 
    constexpr explicit zip_transform_view(F fun, Views... views);
 
    constexpr auto begin() { return iterator<false>(*this, zip_.begin()); }
 
    constexpr auto begin() const
      requires range<const InnerView> &&
               regular_invocable<const F&, range_reference_t<const Views>...> {
      return iterator<true>(*this, zip_.begin());
    }
 
    constexpr auto end() {
      if constexpr (common_range<InnerView>) {
        return iterator<false>(*this, zip_.end());
      } else {
        return sentinel<false>(zip_.end());
      }
    }
 
    constexpr auto end() const
      requires range<const InnerView> &&
               regular_invocable<const F&, range_reference_t<const Views>...> {
      if constexpr (common_range<const InnerView>) {
        return iterator<true>(*this, zip_.end());
      } else {
        return sentinel<true>(zip_.end());
      }
    }
 
    constexpr auto size() requires sized_range<InnerView> {
      return zip_.size();
    }
 
    constexpr auto size() const requires sized_range<const InnerView> {
      return zip_.size();
    }
  };
 
  template<class F, class... Rs>
    zip_transform_view(F, Rs&&...) -> zip_transform_view<F, views::all_t<Rs>...>;
}

[edit] Class template std::ranges::zip_transform_view::iterator

namespace std::ranges {
  template<copy_constructible F, input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> &&
              regular_invocable<F&, range_reference_t<Views>...> &&
              /*can-reference*/<invoke_result_t<F&, range_reference_t<Views>...>>
  template<bool Const>
  class zip_transform_view<F, Views...>::iterator {
    using Parent = /*maybe-const*/<Const, zip_transform_view>;  // exposition only
    using Base = /*maybe-const*/<Const, InnerView>;             // exposition only
    Parent* parent_ = nullptr;                                  // exposition only
    ziperator<Const> inner_;                                    // exposition only
 
    constexpr iterator(Parent& parent, ziperator<Const> inner); // exposition only
 
  public:
    using iterator_category = /* see description */;            // not always present
    using iterator_concept  = typename ziperator<Const>::iterator_concept;
    using value_type =
      remove_cvref_t<invoke_result_t</*maybe-const*/<Const, F>&,
                                     range_reference_t<
                                       /*maybe-const*/<Const, Views>>...>>;
    using difference_type = range_difference_t<Base>;
 
    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<ziperator<false>, ziperator<Const>>;
 
    constexpr decltype(auto) operator*() const noexcept(/* see description */);
    constexpr iterator& operator++();
    constexpr void operator++(int);
    constexpr iterator operator++(int) requires forward_range<Base>;
 
    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;
 
    constexpr iterator& operator+=(difference_type x) requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x) requires random_access_range<Base>;
 
    constexpr decltype(auto) operator[](difference_type n) const
      requires random_access_range<Base>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y)
      requires equality_comparable<ziperator<Const>>;
 
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> && three_way_comparable<ziperator<Const>>;
 
    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<ziperator<Const>, ziperator<Const>>;
  };
}

[edit] Class template std::ranges::zip_transform_view::sentinel

namespace std::ranges {
  template<copy_constructible F, input_range... Views>
    requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> &&
              regular_invocable<F&, range_reference_t<Views>...> &&
              /*can-reference*/<invoke_result_t<F&, range_reference_t<Views>...>>
  template<bool Const>
  class zip_transform_view<F, Views...>::sentinel {
    zentinel<Const> inner_;                                     // exposition only
    constexpr explicit sentinel(zentinel<Const> inner);         // exposition only
 
  public:
    sentinel() = default;
    constexpr sentinel(sentinel<!Const> i)
      requires Const && convertible_to<zentinel<false>, zentinel<Const>>;
 
    template<bool OtherConst>
      requires sentinel_for<zentinel<Const>, ziperator<OtherConst>>
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<zentinel<Const>, ziperator<OtherConst>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, InnerView>>
      operator-(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<zentinel<Const>, ziperator<OtherConst>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, InnerView>>
      operator-(const sentinel& x, const iterator<OtherConst>& y);
  };
}

[edit] Class template std::ranges::adjacent_view

namespace std::ranges {
  template<forward_range V, size_t N>
    requires view<V> && (N > 0)
  class adjacent_view : public view_interface<adjacent_view<V, N>> {
    V base_ = V();                      // exposition only
 
    // class template adjacent_view::iterator
    template<bool> class iterator;      // exposition only
 
    // class template adjacent_view::sentinel
    template<bool> class sentinel;      // exposition only
 
    struct /*as-sentinel*/{};           // exposition only
 
  public:
    adjacent_view() requires default_initializable<V> = default;
    constexpr explicit adjacent_view(V base);
 
    constexpr auto begin() requires (!/*simple-view*/<V>) {
      return iterator<false>(ranges::begin(base_), ranges::end(base_));
    }
 
    constexpr auto begin() const requires range<const V> {
      return iterator<true>(ranges::begin(base_), ranges::end(base_));
    }
 
    constexpr auto end() requires (!/*simple-view*/<V>) {
      if constexpr (common_range<V>) {
        return iterator<false>(/*as-sentinel*/{}, ranges::begin(base_),
          ranges::end(base_));
      } else {
        return sentinel<false>(ranges::end(base_));
      }
    }
 
    constexpr auto end() const requires range<const V> {
      if constexpr (common_range<const V>) {
        return iterator<true>(/*as-sentinel*/{}, ranges::begin(base_),
          ranges::end(base_));
      } else {
        return sentinel<true>(ranges::end(base_));
      }
    }
 
    constexpr auto size() requires sized_range<V>;
    constexpr auto size() const requires sized_range<const V>;
  };
}

[edit] Class template std::ranges::adjacent_view::iterator

namespace std::ranges {
  template<forward_range V, size_t N>
    requires view<V> && (N > 0)
  template<bool Const>
  class adjacent_view<V, N>::iterator {
    using Base = /*maybe-const*/<Const, V>;                             // exposition only
    array<iterator_t<Base>, N> current_ = array<iterator_t<Base>, N>(); // exposition only
    constexpr iterator(iterator_t<Base> first, sentinel_t<Base> last);  // exposition only
    constexpr iterator(/*as-sentinel*/, iterator_t<Base> first, iterator_t<Base> last);
                                                                        // exposition only
  public:
    using iterator_category = input_iterator_tag;
    using iterator_concept  = /* see description */;
    using value_type = /*tuple-or-pair*/</*REPEAT*/(range_value_t<Base>, N)...>;
    using difference_type = range_difference_t<Base>;
 
    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
 
    constexpr auto operator*() const;
    constexpr iterator& operator++();
    constexpr iterator operator++(int);
 
    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;
 
    constexpr iterator& operator+=(difference_type x)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x)
      requires random_access_range<Base>;
 
    constexpr auto operator[](difference_type n) const
      requires random_access_range<Base>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y);
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> &&
               three_way_comparable<iterator_t<Base>>;
 
    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
 
    friend constexpr auto iter_move(const iterator& i) noexcept(/* see description */);
    friend constexpr void iter_swap(const iterator& l, const iterator& r)
      noexcept(/* see description */)
      requires indirectly_swappable<iterator_t<Base>>;
  };
}

[edit] Class template std::ranges::adjacent_view::sentinel

namespace std::ranges {
  template<forward_range V, size_t N>
    requires view<V> && (N > 0)
  template<bool Const>
  class adjacent_view<V, N>::sentinel {
    using Base = /*maybe-const*/<Const, V>;                     // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();                 // exposition only
    constexpr explicit sentinel(sentinel_t<Base> end);          // exposition only
 
  public:
    sentinel() = default;
    constexpr sentinel(sentinel<!Const> i)
      requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    template<bool OtherConst>
      requires sentinel_for<sentinel_t<Base>, iterator_t</*maybe-const*/<OtherConst, V>>>
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<sentinel_t<Base>, iterator_t<
        /*maybe-const*/<OtherConst, V>>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, V>>
      operator-(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for<sentinel_t<Base>, iterator_t<
        /*maybe-const*/<OtherConst, V>>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, V>>
      operator-(const sentinel& y, const iterator<OtherConst>& x);
  };
}

[edit] Class template std::ranges::adjacent_transform_view

namespace std::ranges {
  template<forward_range V, copy_constructible F, size_t N>
   requires view<V> && (N > 0) && is_object_v<F> &&
            regular_invocable<F&, /*REPEAT*/(range_reference_t<V>, N)...> &&
            /*can-reference*/<invoke_result_t<F&, /*REPEAT*/(range_reference_t<V>, N)...>>
  class adjacent_transform_view :
    public view_interface<adjacent_transform_view<V, F, N>> {
    /*copyable-box*/<F> fun_;                       // exposition only
    adjacent_view<V, N> inner_;                 // exposition only
 
    using InnerView = adjacent_view<V, N>;      // exposition only
    template<bool Const>
      using /*inner-iterator*/ =
        iterator_t</*maybe-const*/<Const, InnerView>>;         // exposition only
    template<bool Const>
      using /*inner-sentinel*/ =
        sentinel_t</*maybe-const*/<Const, InnerView>>;         // exposition only
 
    // class template adjacent_transform_view::iterator
    template<bool> class iterator;              // exposition only
 
    // class template adjacent_transform_view::sentinel
    template<bool> class sentinel;              // exposition only
 
  public:
    adjacent_transform_view() = default;
    constexpr explicit adjacent_transform_view(V base, F fun);
 
    constexpr auto begin() {
      return iterator<false>(*this, inner_.begin());
    }
 
    constexpr auto begin() const
      requires range<const InnerView> &&
               regular_invocable<const F&, /*REPEAT*/(range_reference_t<const V>, N)...> {
      return iterator<true>(*this, inner_.begin());
    }
 
    constexpr auto end() {
      if constexpr (common_range<InnerView>) {
        return iterator<false>(*this, inner_.end());
      } else {
        return sentinel<false>(inner_.end());
      }
    }
 
    constexpr auto end() const
      requires range<const InnerView> &&
               regular_invocable<const F&, /*REPEAT*/(range_reference_t<const V>, N)...> {
      if constexpr (common_range<const InnerView>) {
        return iterator<true>(*this, inner_.end());
      } else {
        return sentinel<true>(inner_.end());
      }
    }
 
    constexpr auto size() requires sized_range<InnerView> {
      return inner_.size();
    }
 
    constexpr auto size() const requires sized_range<const InnerView> {
      return inner_.size();
    }
  };
}

[edit] Class template std::ranges::adjacent_transform_view::iterator

namespace std::ranges {
  template<forward_range V, copy_constructible F, size_t N>
    requires view<V> && (N > 0) && is_object_v<F> &&
             regular_invocable<F&, /*REPEAT*/(range_reference_t<V>, N)...> &&
             /*can-reference*/<invoke_result_t<F&,
               /*REPEAT*/(range_reference_t<V>, N)...>>
  template<bool Const>
  class adjacent_transform_view<F, V...>::iterator {
    using Parent = /*maybe-const*/<Const, adjacent_transform_view>;  // exposition only
    using Base = /*maybe-const*/<Const, V>;                          // exposition only
    Parent* parent_ = nullptr;                                       // exposition only
    /*inner-iterator*/<Const> inner_;                                // exposition only
 
    constexpr iterator(Parent& parent,
      /*inner-iterator*/<Const> inner);    // exposition only
 
  public:
    using iterator_category = /* see description */;
    using iterator_concept  = typename /*inner-iterator*/<Const>::iterator_concept;
    using value_type =
      remove_cvref_t<invoke_result_t</*maybe-const*/<Const, F>&,
                                     /*REPEAT*/(range_reference_t<Base>, N)...>>;
    using difference_type = range_difference_t<Base>;
 
    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to</*inner-iterator*/<false>,
        /*inner-iterator*/<Const>>;
 
    constexpr decltype(auto) operator*() const noexcept(/* see description */);
    constexpr iterator& operator++();
    constexpr iterator operator++(int);
    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;
    constexpr iterator& operator+=(difference_type x) requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x) requires random_access_range<Base>;
 
    constexpr decltype(auto) operator[](difference_type n) const
      requires random_access_range<Base>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y);
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> && three_way_comparable<
        /*inner-iterator*/<Const>>;
 
    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for</*inner-iterator*/<Const>, /*inner-iterator*/<Const>>;
  };
}

[edit] Class template std::ranges::adjacent_transform_view::sentinel

namespace std::ranges {
  template<forward_range V, copy_constructible F, size_t N>
    requires view<V> && (N > 0) && is_object_v<F> &&
             regular_invocable<F&, /*REPEAT*/(range_reference_t<V>, N)...> &&
             /*can-reference*/<invoke_result_t<F&,
               /*REPEAT*/(range_reference_t<V>, N)...>>
  template<bool Const>
  class adjacent_transform_view<V, F, N>::sentinel {
    /*inner-sentinel*/<Const> inner_;                               // exposition only
    constexpr explicit sentinel(/*inner-sentinel*/<Const> inner);   // exposition only
 
  public:
    sentinel() = default;
    constexpr sentinel(sentinel<!Const> i)
      requires Const && convertible_to</*inner-sentinel*/<false>,
                                       /*inner-sentinel*/<Const>>;
    template<bool OtherConst>
      requires sentinel_for</*inner-sentinel*/<Const>, /*inner-iterator*/<OtherConst>>
    friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for</*inner-sentinel*/<Const>,
                                  /*inner-iterator*/<OtherConst>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, InnerView>>
      operator-(const iterator<OtherConst>& x, const sentinel& y);
 
    template<bool OtherConst>
      requires sized_sentinel_for</*inner-sentinel*/<Const>,
                                  /*inner-iterator*/<OtherConst>>
    friend constexpr range_difference_t</*maybe-const*/<OtherConst, InnerView>>
      operator-(const sentinel& x, const iterator<OtherConst>& y);
  };
}

[edit] Class template std::ranges::chunk_view for input ranges

namespace std::ranges {
  template<class I>
  constexpr I /*div-ceil*/(I num, I denom) {              // exposition only
    I r = num / denom;
    if (num % denom)
      ++r;
    return r;
  }
 
  template<view V>
    requires input_range<V>
  class chunk_view : public view_interface<chunk_view<V>> {
    V base_ = V();                                        // exposition only
    range_difference_t<V> n_ = 0;                         // exposition only
    range_difference_t<V> remainder_ = 0;                 // exposition only
 
    /*non-propagating-cache*/<iterator_t<V>> current_;    // exposition only
 
    // class chunk_view::/*outer-iterator*/
    class /*outer-iterator*/;                             // exposition only
 
    // class chunk_view::/*inner-iterator*/
    class /*inner-iterator*/;                             // exposition only
 
  public:
    chunk_view() requires default_initializable<V> = default;
    constexpr explicit chunk_view(V base, range_difference_t<V> n);
 
    constexpr V base() const & requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr /*outer-iterator*/ begin();
    constexpr default_sentinel_t end() noexcept;
 
    constexpr auto size() requires sized_range<V>;
    constexpr auto size() const requires sized_range<const V>;
  };
 
  template<class R>
    chunk_view(R&& r, range_difference_t<R>) -> chunk_view<views::all_t<R>>;
}

[edit] Class template std::ranges::chunk_view::outer_iterator for input ranges

namespace std::ranges {
  template<view V>
    requires input_range<V>
  class chunk_view<V>::/*outer-iterator*/ {
    chunk_view* parent_;                                        // exposition only
 
    constexpr explicit /*outer-iterator*/(chunk_view& parent);  // exposition only
 
  public:
    using iterator_concept = input_iterator_tag;
    using difference_type  = range_difference_t<V>;
 
    // class chunk_view::/*outer-iterator*/::value_type
    struct value_type;
 
    /*outer-iterator*/(/*outer-iterator*/&&) = default;
    /*outer-iterator*/r& operator=(/*outer-iterator*/&&) = default;
 
    constexpr value_type operator*() const;
    constexpr /*outer-iterator*/& operator++();
    constexpr void operator++(int);
 
    friend constexpr bool operator==(const /*outer-iterator*/& x, default_sentinel_t);
 
    friend constexpr difference_type operator-(default_sentinel_t y,
                                               const /*outer-iterator*/& x)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
    friend constexpr difference_type operator-(const /*outer-iterator*/& x,
                                               default_sentinel_t y)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}

[edit] Class template std::ranges::chunk_view::outer_iterator::value_type for input ranges

namespace std::ranges {
  template<view V>
    requires input_range<V>
  struct chunk_view<V>::/*outer-iterator*/::value_type
  : view_interface<value_type> {
  private:
    chunk_view* parent_;                                        // exposition only
 
    constexpr explicit value_type(chunk_view& parent);          // exposition only
 
  public:
    constexpr /*inner-iterator*/ begin() const noexcept;
    constexpr default_sentinel_t end() const noexcept;
 
    constexpr auto size() const
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}

[edit] Class template std::ranges::chunk_view::inner_iterator for input ranges

namespace std::ranges {
  template<view V>
    requires input_range<V>
  class chunk_view<V>::/*inner-iterator*/ {
    chunk_view* parent_;                                                // exposition only
 
    constexpr explicit /*inner-iterator*/(chunk_view& parent) noexcept;     // exposition only
 
  public:
    using iterator_concept = input_iterator_tag;
    using difference_type = range_difference_t<V>;
    using value_type = range_value_t<V>;
 
    /*inner-iterator*/(/*inner-iterator*/&&) = default;
    /*inner-iterator*/& operator=(/*inner-iterator*/&&) = default;
 
    constexpr const iterator_t<V>& base() const &;
 
    constexpr range_reference_t<V> operator*() const;
    constexpr /*inner-iterator*/& operator++();
    constexpr void operator++(int);
 
    friend constexpr bool operator==(const /*inner-iterator*/& x, default_sentinel_t);
 
    friend constexpr difference_type operator-(default_sentinel_t y,
                                               const /*inner-iterator*/& x)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
    friend constexpr difference_type operator-(const /*inner-iterator*/& x,
                                               default_sentinel_t y)
      requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}

[edit] Class template std::ranges::chunk_view for forward ranges

namespace std::ranges {
  template<view V>
    requires forward_range<V>
  class chunk_view<V> : public view_interface<chunk_view<V>> {
    V base_ = V();                      // exposition only
    range_difference_t<V> n_ = 0;       // exposition only
 
    // class template chunk_view::iterator
    template<bool> class iterator;      // exposition only
 
  public:
    chunk_view() requires default_initializable<V> = default;
    constexpr explicit chunk_view(V base, range_difference_t<V> n);
 
    constexpr V base() const & requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr auto begin() requires (!/*simple-view*/<V>) {
      return iterator<false>(this, ranges::begin(base_));
    }
 
    constexpr auto begin() const requires forward_range<const V> {
      return iterator<true>(this, ranges::begin(base_));
    }
 
    constexpr auto end() requires (!/*simple-view*/<V>) {
      if constexpr (common_range<V> && sized_range<V>) {
        auto missing = (n_ - ranges::distance(base_) % n_) % n_;
        return iterator<false>(this, ranges::end(base_), missing);
      } else if constexpr (common_range<V> && !bidirectional_range<V>) {
        return iterator<false>(this, ranges::end(base_));
      } else {
        return default_sentinel;
      }
    }
 
    constexpr auto end() const requires forward_range<const V> {
      if constexpr (common_range<const V> && sized_range<const V>) {
        auto missing = (n_ - ranges::distance(base_) % n_) % n_;
        return iterator<true>(this, ranges::end(base_), missing);
      } else if constexpr (common_range<const V> && !bidirectional_range<const V>) {
        return iterator<true>(this, ranges::end(base_));
      } else {
        return default_sentinel;
      }
    }
 
    constexpr auto size() requires sized_range<V>;
    constexpr auto size() const requires sized_range<const V>;
  };
}

[edit] Class template std::ranges::chunk_view::iterator for forward ranges

namespace std::ranges {
  template<view V>
    requires forward_range<V>
  template<bool Const>
  class chunk_view<V>::iterator {
    using Parent = /*maybe-const*/<Const, chunk_view>;                  // exposition only
    using Base = /*maybe-const*/<Const, V>;                             // exposition only
 
    iterator_t<Base> current_ = iterator_t<Base>();                     // exposition only
    sentinel_t<Base> end_ = sentinel_t<Base>();                         // exposition only
    range_difference_t<Base> n_ = 0;                                    // exposition only
    range_difference_t<Base> missing_ = 0;                              // exposition only
 
    constexpr iterator(Parent* parent, iterator_t<Base> current,        // exposition only
                       range_difference_t<Base> missing = 0);
 
  public:
    using iterator_category = input_iterator_tag;
    using iterator_concept = /* see description */;
    using value_type = decltype(views::take(subrange(current_, end_), n_));
    using difference_type = range_difference_t<Base>;
 
    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>
                     && convertible_to<sentinel_t<V>, sentinel_t<Base>>;
 
    constexpr iterator_t<Base> base() const;
 
    constexpr value_type operator*() const;
    constexpr iterator& operator++();
    constexpr iterator operator++(int);
 
    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;
 
    constexpr iterator& operator+=(difference_type x)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x)
      requires random_access_range<Base>;
 
    constexpr value_type operator[](difference_type n) const
      requires random_access_range<Base>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y);
    friend constexpr bool operator==(const iterator& x, default_sentinel_t);
 
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> &&
               three_way_comparable<iterator_t<Base>>;
 
    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
 
    friend constexpr difference_type operator-(default_sentinel_t y, const iterator& x)
      requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
    friend constexpr difference_type operator-(const iterator& x, default_sentinel_t y)
      requires sized_sentinel_for<sentinel_t<Base>, iterator_t<Base>>;
  };
}

[edit] Class template std::ranges::slide_view

namespace std::ranges {
  template<class V>
  concept /*slide-caches-nothing*/ = random_access_range<V> // exposition only
                                  && sized_range<V>;
 
  template<class V>
  concept /*slide-caches-last*/ =                           // exposition only
    !/*slide-caches-nothing*/<V> && bidirectional_range<V> && common_range<V>;
 
  template<class V>
  concept /*slide-caches-first*/ =                          // exposition only
    !/*slide-caches-nothing*/<V> && !/*slide-caches-last*/<V>;
 
  template<forward_range V>
    requires view<V>
  class slide_view : public view_interface<slide_view<V>> {
    V base_ = V();                      // exposition only
    range_difference_t<V> n_ = 0;       // exposition only
 
    // class template slide_view::iterator
    template<bool> class iterator;      // exposition only
 
    // class slide_view::sentinel
    class sentinel;                     // exposition only
 
  public:
    slide_view() requires default_initializable<V> = default;
    constexpr explicit slide_view(V base, range_difference_t<V> n);
 
    constexpr auto begin()
      requires (!(/*simple-view*/<V> && /*slide-caches-nothing*/<const V>));
    constexpr auto begin() const requires /*slide-caches-nothing*/<const V>;
 
    constexpr auto end()
      requires (!(/*simple-view*/<V> && /*slide-caches-nothing*/<const V>));
    constexpr auto end() const requires /*slide-caches-nothing*/<const V>;
 
    constexpr auto size() requires sized_range<V>;
    constexpr auto size() const requires sized_range<const V>;
  };
 
  template<class R>
    slide_view(R&& r, range_difference_t<R>) -> slide_view<views::all_t<R>>;
}

[edit] Class template std::ranges::slide_view::iterator

namespace std::ranges {
  template<forward_range V>
    requires view<V>
  template<bool Const>
  class slide_view<V>::iterator {
    using Base = /*maybe-const*/<Const, V>;             // exposition only
    iterator_t<Base> current_   = iterator_t<Base>();   // exposition only
    iterator_t<Base> last_ele_  = iterator_t<Base>();   // exposition only,
                          // present only if Base models slide-caches-first
    range_difference_t<Base> n_ = 0;                    // exposition only
 
    constexpr iterator(iterator_t<Base> current,
                       range_difference_t<Base> n)      // exposition only
      requires (!/*slide-caches-first*/<Base>);
 
    constexpr iterator(iterator_t<Base> current, iterator_t<Base> last_elem,
                       range_difference_t<Base> n)      // exposition only
      requires /*slide-caches-first*/<Base>;
 
  public:
    using iterator_category = input_iterator_tag;
    using iterator_concept = /* see description */;
    using value_type = decltype(views::counted(current_, n_));
    using difference_type = range_difference_t<Base>;
 
    iterator() = default;
    constexpr iterator(iterator<!Const> i)
      requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;
 
    constexpr auto operator*() const;
    constexpr iterator& operator++();
    constexpr iterator operator++(int);
 
    constexpr iterator& operator--() requires bidirectional_range<Base>;
    constexpr iterator operator--(int) requires bidirectional_range<Base>;
 
    constexpr iterator& operator+=(difference_type x)
      requires random_access_range<Base>;
    constexpr iterator& operator-=(difference_type x)
      requires random_access_range<Base>;
 
    constexpr auto operator[](difference_type n) const
      requires random_access_range<Base>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y);
 
    friend constexpr bool operator<(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator<=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr bool operator>=(const iterator& x, const iterator& y)
      requires random_access_range<Base>;
    friend constexpr auto operator<=>(const iterator& x, const iterator& y)
      requires random_access_range<Base> &&
               three_way_comparable<iterator_t<Base>>;
 
    friend constexpr iterator operator+(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr iterator operator+(difference_type n, const iterator& i)
      requires random_access_range<Base>;
    friend constexpr iterator operator-(const iterator& i, difference_type n)
      requires random_access_range<Base>;
    friend constexpr difference_type operator-(const iterator& x, const iterator& y)
      requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;
  };
}

[edit] Class template std::ranges::slide_view::sentinel

namespace std::ranges {
  template<forward_range V>
    requires view<V>
  class slide_view<V>::sentinel {
    sentinel_t<V> end_ = sentinel_t<V>();             // exposition only
    constexpr explicit sentinel(sentinel_t<V> end);   // exposition only
 
  public:
    sentinel() = default;
 
    friend constexpr bool operator==(const iterator<false>& x, const sentinel& y);
 
    friend constexpr range_difference_t<V>
      operator-(const iterator<false>& x, const sentinel& y)
        requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
 
    friend constexpr range_difference_t<V>
      operator-(const sentinel& y, const iterator<false>& x)
        requires sized_sentinel_for<sentinel_t<V>, iterator_t<V>>;
  };
}

[edit] Class template std::ranges::chunk_by_view

namespace std::ranges {
  template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class chunk_by_view : public view_interface<chunk_by_view<V, Pred>> {
    V base_ = V();                          // exposition only
    /*copyable-box*/<Pred> pred_ = Pred();  // exposition only
 
    // class chunk_by_view::iterator
    class iterator;                         // exposition only
 
  public:
    chunk_by_view()
        requires default_initializable<V> && default_initializable<Pred> = default;
    constexpr explicit chunk_by_view(V base, Pred pred);
 
    constexpr V base() const & requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }
 
    constexpr const Pred& pred() const;
 
    constexpr iterator begin();
    constexpr auto end();
 
    constexpr iterator_t<V> /*find-next*/(iterator_t<V>);   // exposition only
    constexpr iterator_t<V> /*find-prev*/(iterator_t<V>)    // exposition only
      requires bidirectional_range<V>;
  };
 
  template<class R, class Pred>
    chunk_by_view(R&&, Pred) -> chunk_by_view<views::all_t<R>, Pred>;
}

[edit] Class template std::ranges::chunk_by_view::iterator

namespace std::ranges {
  template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred>
    requires view<V> && is_object_v<Pred>
  class chunk_by_view<V, Pred>::iterator {
    chunk_by_view* parent_ = nullptr;           // exposition only
    iterator_t<V> current_ = iterator_t<V>();   // exposition only
    iterator_t<V> next_    = iterator_t<V>();   // exposition only
 
    constexpr iterator(chunk_by_view& parent, iterator_t<V> current,    // exposition only
                       iterator_t<V> next);
 
  public:
    using value_type = subrange<iterator_t<V>>;
    using difference_type  = range_difference_t<V>;
    using iterator_category = input_iterator_tag;
    using iterator_concept = /* see description */;
 
    iterator() = default;
 
    constexpr value_type operator*() const;
    constexpr iterator& operator++();
    constexpr iterator operator++(int);
 
    constexpr iterator& operator--() requires bidirectional_range<V>;
    constexpr iterator operator--(int) requires bidirectional_range<V>;
 
    friend constexpr bool operator==(const iterator& x, const iterator& y);
    friend constexpr bool operator==(const iterator& x, default_sentinel_t);
  };
}