ranges

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// <ranges> -*- C++ -*-
 
// Copyright (C) 2019-2021 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/ranges
 *  This is a Standard C++ Library header.
 *  @ingroup concepts
 */
 
#ifndef _GLIBCXX_RANGES
#define _GLIBCXX_RANGES 1
 
#if __cplusplus > 201703L
 
#pragma GCC system_header
 
#include <concepts>
 
#if __cpp_lib_concepts
 
#include <compare>
#include <initializer_list>
#include <iterator>
#include <optional>
#include <tuple>
#include <bits/ranges_util.h>
#include <bits/refwrap.h>
 
/**
 * @defgroup ranges Ranges
 *
 * Components for dealing with ranges of elements.
 */
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
namespace ranges
{
  // [range.access] customization point objects
  // [range.req] range and view concepts
  // [range.dangling] dangling iterator handling
  // Defined in <bits/ranges_base.h>
 
  // [view.interface] View interface
  // [range.subrange] Sub-ranges
  // Defined in <bits/ranges_util.h>
 
  // C++20 24.6 [range.factories] Range factories
 
  /// A view that contains no elements.
  template<typename _Tp> requires is_object_v<_Tp>
    class empty_view
    : public view_interface<empty_view<_Tp>>
    {
    public:
      static constexpr _Tp* begin() noexcept { return nullptr; }
      static constexpr _Tp* end() noexcept { return nullptr; }
      static constexpr _Tp* data() noexcept { return nullptr; }
      static constexpr size_t size() noexcept { return 0; }
      static constexpr bool empty() noexcept { return true; }
    };
 
  template<typename _Tp>
    inline constexpr bool enable_borrowed_range<empty_view<_Tp>> = true;
 
  namespace __detail
  {
    template<typename _Tp>
      concept __boxable = copy_constructible<_Tp> && is_object_v<_Tp>;
 
    template<__boxable _Tp>
      struct __box : std::optional<_Tp>
      {
        using std::optional<_Tp>::optional;
 
        constexpr
        __box()
        noexcept(is_nothrow_default_constructible_v<_Tp>)
        requires default_initializable<_Tp>
        : std::optional<_Tp>{std::in_place}
        { }
 
        __box(const __box&) = default;
        __box(__box&&) = default;
 
        using std::optional<_Tp>::operator=;
 
        // _GLIBCXX_RESOLVE_LIB_DEFECTS
        // 3477. Simplify constraints for semiregular-box
        __box&
        operator=(const __box& __that)
        noexcept(is_nothrow_copy_constructible_v<_Tp>)
        requires (!copyable<_Tp>)
        {
          if (this != std::__addressof(__that))
            {
              if ((bool)__that)
                this->emplace(*__that);
              else
                this->reset();
            }
          return *this;
        }
 
        __box&
        operator=(__box&& __that)
        noexcept(is_nothrow_move_constructible_v<_Tp>)
        requires (!movable<_Tp>)
        {
          if (this != std::__addressof(__that))
            {
              if ((bool)__that)
                this->emplace(std::move(*__that));
              else
                this->reset();
            }
          return *this;
        }
      };
 
    // For types which are already copyable, this specialization of the
    // copyable wrapper stores the object directly without going through
    // std::optional.  It provides just the subset of the primary template's
    // API that we currently use.
    template<__boxable _Tp>
      requires copyable<_Tp>
      struct __box<_Tp>
      {
      private:
        [[no_unique_address]] _Tp _M_value = _Tp();
 
      public:
        __box() requires default_initializable<_Tp> = default;
 
        constexpr explicit
        __box(const _Tp& __t)
        noexcept(is_nothrow_copy_constructible_v<_Tp>)
        : _M_value(__t)
        { }
 
        constexpr explicit
        __box(_Tp&& __t)
        noexcept(is_nothrow_move_constructible_v<_Tp>)
        : _M_value(std::move(__t))
        { }
 
        template<typename... _Args>
          requires constructible_from<_Tp, _Args...>
          constexpr explicit
          __box(in_place_t, _Args&&... __args)
          noexcept(is_nothrow_constructible_v<_Tp, _Args...>)
          : _M_value(std::forward<_Args>(__args)...)
          { }
 
        constexpr bool
        has_value() const noexcept
        { return true; };
 
        constexpr _Tp&
        operator*() noexcept
        { return _M_value; }
 
        constexpr const _Tp&
        operator*() const noexcept
        { return _M_value; }
 
        constexpr _Tp*
        operator->() noexcept
        { return std::__addressof(_M_value); }
 
        constexpr const _Tp*
        operator->() const noexcept
        { return std::__addressof(_M_value); }
      };
  } // namespace __detail
 
  /// A view that contains exactly one element.
  template<copy_constructible _Tp> requires is_object_v<_Tp>
    class single_view : public view_interface<single_view<_Tp>>
    {
    public:
      single_view() = default;
 
      constexpr explicit
      single_view(const _Tp& __t)
      : _M_value(__t)
      { }
 
      constexpr explicit
      single_view(_Tp&& __t)
      : _M_value(std::move(__t))
      { }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3428. single_view's in place constructor should be explicit
      template<typename... _Args>
        requires constructible_from<_Tp, _Args...>
        constexpr explicit
        single_view(in_place_t, _Args&&... __args)
        : _M_value{in_place, std::forward<_Args>(__args)...}
        { }
 
      constexpr _Tp*
      begin() noexcept
      { return data(); }
 
      constexpr const _Tp*
      begin() const noexcept
      { return data(); }
 
      constexpr _Tp*
      end() noexcept
      { return data() + 1; }
 
      constexpr const _Tp*
      end() const noexcept
      { return data() + 1; }
 
      static constexpr size_t
      size() noexcept
      { return 1; }
 
      constexpr _Tp*
      data() noexcept
      { return _M_value.operator->(); }
 
      constexpr const _Tp*
      data() const noexcept
      { return _M_value.operator->(); }
 
    private:
      [[no_unique_address]] __detail::__box<_Tp> _M_value;
    };
 
  template<typename _Tp>
    single_view(_Tp) -> single_view<_Tp>;
 
  namespace __detail
  {
    template<typename _Wp>
      constexpr auto __to_signed_like(_Wp __w) noexcept
      {
        if constexpr (!integral<_Wp>)
          return iter_difference_t<_Wp>();
        else if constexpr (sizeof(iter_difference_t<_Wp>) > sizeof(_Wp))
          return iter_difference_t<_Wp>(__w);
        else if constexpr (sizeof(ptrdiff_t) > sizeof(_Wp))
          return ptrdiff_t(__w);
        else if constexpr (sizeof(long long) > sizeof(_Wp))
          return (long long)(__w);
#ifdef __SIZEOF_INT128__
        else if constexpr (__SIZEOF_INT128__ > sizeof(_Wp))
          return __int128(__w);
#endif
        else
          return __max_diff_type(__w);
      }
 
    template<typename _Wp>
      using __iota_diff_t = decltype(__to_signed_like(std::declval<_Wp>()));
 
    template<typename _It>
      concept __decrementable = incrementable<_It>
        && requires(_It __i)
        {
            { --__i } -> same_as<_It&>;
            { __i-- } -> same_as<_It>;
        };
 
    template<typename _It>
      concept __advanceable = __decrementable<_It> && totally_ordered<_It>
        && requires( _It __i, const _It __j, const __iota_diff_t<_It> __n)
        {
          { __i += __n } -> same_as<_It&>;
          { __i -= __n } -> same_as<_It&>;
          _It(__j + __n);
          _It(__n + __j);
          _It(__j - __n);
          { __j - __j } -> convertible_to<__iota_diff_t<_It>>;
        };
 
    template<typename _Winc>
      struct __iota_view_iter_cat
      { };
 
    template<incrementable _Winc>
      struct __iota_view_iter_cat<_Winc>
      { using iterator_category = input_iterator_tag; };
  } // namespace __detail
 
  template<weakly_incrementable _Winc,
           semiregular _Bound = unreachable_sentinel_t>
    requires std::__detail::__weakly_eq_cmp_with<_Winc, _Bound>
      && copyable<_Winc>
    class iota_view : public view_interface<iota_view<_Winc, _Bound>>
    {
    private:
      struct _Sentinel;
 
      struct _Iterator : __detail::__iota_view_iter_cat<_Winc>
      {
      private:
        static auto
        _S_iter_concept()
        {
          using namespace __detail;
          if constexpr (__advanceable<_Winc>)
            return random_access_iterator_tag{};
          else if constexpr (__decrementable<_Winc>)
            return bidirectional_iterator_tag{};
          else if constexpr (incrementable<_Winc>)
            return forward_iterator_tag{};
          else
            return input_iterator_tag{};
        }
 
      public:
        using iterator_concept = decltype(_S_iter_concept());
        // iterator_category defined in __iota_view_iter_cat
        using value_type = _Winc;
        using difference_type = __detail::__iota_diff_t<_Winc>;
 
        _Iterator() requires default_initializable<_Winc> = default;
 
        constexpr explicit
        _Iterator(_Winc __value)
        : _M_value(__value) { }
 
        constexpr _Winc
        operator*() const noexcept(is_nothrow_copy_constructible_v<_Winc>)
        { return _M_value; }
 
        constexpr _Iterator&
        operator++()
        {
          ++_M_value;
          return *this;
        }
 
        constexpr void
        operator++(int)
        { ++*this; }
 
        constexpr _Iterator
        operator++(int) requires incrementable<_Winc>
        {
          auto __tmp = *this;
          ++*this;
          return __tmp;
        }
 
        constexpr _Iterator&
        operator--() requires __detail::__decrementable<_Winc>
        {
          --_M_value;
          return *this;
        }
 
        constexpr _Iterator
        operator--(int) requires __detail::__decrementable<_Winc>
        {
          auto __tmp = *this;
          --*this;
          return __tmp;
        }
 
        constexpr _Iterator&
        operator+=(difference_type __n) requires __detail::__advanceable<_Winc>
        {
          using __detail::__is_integer_like;
          using __detail::__is_signed_integer_like;
          if constexpr (__is_integer_like<_Winc>
              && !__is_signed_integer_like<_Winc>)
            {
              if (__n >= difference_type(0))
                _M_value += static_cast<_Winc>(__n);
              else
                _M_value -= static_cast<_Winc>(-__n);
            }
          else
            _M_value += __n;
          return *this;
        }
 
        constexpr _Iterator&
        operator-=(difference_type __n) requires __detail::__advanceable<_Winc>
        {
          using __detail::__is_integer_like;
          using __detail::__is_signed_integer_like;
          if constexpr (__is_integer_like<_Winc>
              && !__is_signed_integer_like<_Winc>)
            {
              if (__n >= difference_type(0))
                _M_value -= static_cast<_Winc>(__n);
              else
                _M_value += static_cast<_Winc>(-__n);
            }
          else
            _M_value -= __n;
          return *this;
        }
 
        constexpr _Winc
        operator[](difference_type __n) const
        requires __detail::__advanceable<_Winc>
        { return _Winc(_M_value + __n); }
 
        friend constexpr bool
        operator==(const _Iterator& __x, const _Iterator& __y)
        requires equality_comparable<_Winc>
        { return __x._M_value == __y._M_value; }
 
        friend constexpr bool
        operator<(const _Iterator& __x, const _Iterator& __y)
        requires totally_ordered<_Winc>
        { return __x._M_value < __y._M_value; }
 
        friend constexpr bool
        operator>(const _Iterator& __x, const _Iterator& __y)
          requires totally_ordered<_Winc>
        { return __y < __x; }
 
        friend constexpr bool
        operator<=(const _Iterator& __x, const _Iterator& __y)
          requires totally_ordered<_Winc>
        { return !(__y < __x); }
 
        friend constexpr bool
        operator>=(const _Iterator& __x, const _Iterator& __y)
          requires totally_ordered<_Winc>
        { return !(__x < __y); }
 
#ifdef __cpp_lib_three_way_comparison
        friend constexpr auto
        operator<=>(const _Iterator& __x, const _Iterator& __y)
          requires totally_ordered<_Winc> && three_way_comparable<_Winc>
        { return __x._M_value <=> __y._M_value; }
#endif
 
        friend constexpr _Iterator
        operator+(_Iterator __i, difference_type __n)
          requires __detail::__advanceable<_Winc>
        {
          __i += __n;
          return __i;
        }
 
        friend constexpr _Iterator
        operator+(difference_type __n, _Iterator __i)
          requires __detail::__advanceable<_Winc>
        { return __i += __n; }
 
        friend constexpr _Iterator
        operator-(_Iterator __i, difference_type __n)
          requires __detail::__advanceable<_Winc>
        {
          __i -= __n;
          return __i;
        }
 
        friend constexpr difference_type
        operator-(const _Iterator& __x, const _Iterator& __y)
          requires __detail::__advanceable<_Winc>
        {
          using __detail::__is_integer_like;
          using __detail::__is_signed_integer_like;
          using _Dt = difference_type;
          if constexpr (__is_integer_like<_Winc>)
            {
              if constexpr (__is_signed_integer_like<_Winc>)
                return _Dt(_Dt(__x._M_value) - _Dt(__y._M_value));
              else
                return (__y._M_value > __x._M_value)
                  ? _Dt(-_Dt(__y._M_value - __x._M_value))
                  : _Dt(__x._M_value - __y._M_value);
            }
          else
            return __x._M_value - __y._M_value;
        }
 
      private:
        _Winc _M_value = _Winc();
 
        friend iota_view;
        friend _Sentinel;
      };
 
      struct _Sentinel
      {
      private:
        constexpr bool
        _M_equal(const _Iterator& __x) const
        { return __x._M_value == _M_bound; }
 
        constexpr auto
        _M_distance_from(const _Iterator& __x) const
        { return _M_bound - __x._M_value; }
 
        _Bound _M_bound = _Bound();
 
      public:
        _Sentinel() = default;
 
        constexpr explicit
        _Sentinel(_Bound __bound)
        : _M_bound(__bound) { }
 
        friend constexpr bool
        operator==(const _Iterator& __x, const _Sentinel& __y)
        { return __y._M_equal(__x); }
 
        friend constexpr iter_difference_t<_Winc>
        operator-(const _Iterator& __x, const _Sentinel& __y)
          requires sized_sentinel_for<_Bound, _Winc>
        { return -__y._M_distance_from(__x); }
 
        friend constexpr iter_difference_t<_Winc>
        operator-(const _Sentinel& __x, const _Iterator& __y)
          requires sized_sentinel_for<_Bound, _Winc>
        { return __x._M_distance_from(__y); }
 
        friend iota_view;
      };
 
      _Winc _M_value = _Winc();
      [[no_unique_address]] _Bound _M_bound = _Bound();
 
    public:
      iota_view() requires default_initializable<_Winc> = default;
 
      constexpr explicit
      iota_view(_Winc __value)
      : _M_value(__value)
      { }
 
      constexpr
      iota_view(type_identity_t<_Winc> __value,
                type_identity_t<_Bound> __bound)
      : _M_value(__value), _M_bound(__bound)
      {
        if constexpr (totally_ordered_with<_Winc, _Bound>)
          __glibcxx_assert( bool(__value <= __bound) );
      }
 
      constexpr
      iota_view(_Iterator __first, _Iterator __last)
        requires same_as<_Winc, _Bound>
        : iota_view(__first._M_value, __last._M_value)
      { }
 
      constexpr
      iota_view(_Iterator __first, unreachable_sentinel_t __last)
        requires same_as<_Bound, unreachable_sentinel_t>
        : iota_view(__first._M_value, __last)
      { }
 
      constexpr
      iota_view(_Iterator __first, _Sentinel __last)
        requires (!same_as<_Winc, _Bound>) && (!same_as<_Bound, unreachable_sentinel_t>)
        : iota_view(__first._M_value, __last._M_bound)
      { }
 
      constexpr _Iterator
      begin() const { return _Iterator{_M_value}; }
 
      constexpr auto
      end() const
      {
        if constexpr (same_as<_Bound, unreachable_sentinel_t>)
          return unreachable_sentinel;
        else
          return _Sentinel{_M_bound};
      }
 
      constexpr _Iterator
      end() const requires same_as<_Winc, _Bound>
      { return _Iterator{_M_bound}; }
 
      constexpr auto
      size() const
      requires (same_as<_Winc, _Bound> && __detail::__advanceable<_Winc>)
      || (integral<_Winc> && integral<_Bound>)
      || sized_sentinel_for<_Bound, _Winc>
      {
        using __detail::__is_integer_like;
        using __detail::__to_unsigned_like;
        if constexpr (integral<_Winc> && integral<_Bound>)
          {
            using _Up = make_unsigned_t<decltype(_M_bound - _M_value)>;
            return _Up(_M_bound) - _Up(_M_value);
          }
        else if constexpr (__is_integer_like<_Winc>)
          return __to_unsigned_like(_M_bound) - __to_unsigned_like(_M_value);
        else
          return __to_unsigned_like(_M_bound - _M_value);
      }
    };
 
  template<typename _Winc, typename _Bound>
    requires (!__detail::__is_integer_like<_Winc>
        || !__detail::__is_integer_like<_Bound>
        || (__detail::__is_signed_integer_like<_Winc>
            == __detail::__is_signed_integer_like<_Bound>))
    iota_view(_Winc, _Bound) -> iota_view<_Winc, _Bound>;
 
  template<weakly_incrementable _Winc, semiregular _Bound>
    inline constexpr bool
      enable_borrowed_range<iota_view<_Winc, _Bound>> = true;
 
namespace views
{
  template<typename _Tp>
    inline constexpr empty_view<_Tp> empty{};
 
  struct _Single
  {
    template<typename _Tp>
      constexpr auto
      operator()(_Tp&& __e) const
      { return single_view<decay_t<_Tp>>(std::forward<_Tp>(__e)); }
  };
 
  inline constexpr _Single single{};
 
  struct _Iota
  {
    template<typename _Tp>
      constexpr auto
      operator()(_Tp&& __e) const
      { return iota_view(std::forward<_Tp>(__e)); }
 
    template<typename _Tp, typename _Up>
      constexpr auto
      operator()(_Tp&& __e, _Up&& __f) const
      { return iota_view(std::forward<_Tp>(__e), std::forward<_Up>(__f)); }
  };
 
  inline constexpr _Iota iota{};
} // namespace views
 
  namespace __detail
  {
    template<typename _Val, typename _CharT, typename _Traits>
      concept __stream_extractable
        = requires(basic_istream<_CharT, _Traits>& is, _Val& t) { is >> t; };
  } // namespace __detail
 
  template<movable _Val, typename _CharT,
           typename _Traits = char_traits<_CharT>>
    requires default_initializable<_Val>
      && __detail::__stream_extractable<_Val, _CharT, _Traits>
    class basic_istream_view
    : public view_interface<basic_istream_view<_Val, _CharT, _Traits>>
    {
    public:
      basic_istream_view() = default;
 
      constexpr explicit
      basic_istream_view(basic_istream<_CharT, _Traits>& __stream)
        : _M_stream(std::__addressof(__stream))
      { }
 
      constexpr auto
      begin()
      {
        if (_M_stream != nullptr)
          *_M_stream >> _M_object;
        return _Iterator{this};
      }
 
      constexpr default_sentinel_t
      end() const noexcept
      { return default_sentinel; }
 
    private:
      basic_istream<_CharT, _Traits>* _M_stream = nullptr;
      _Val _M_object = _Val();
 
      struct _Iterator
      {
      public:
        using iterator_concept = input_iterator_tag;
        using difference_type = ptrdiff_t;
        using value_type = _Val;
 
        _Iterator() = default;
 
        constexpr explicit
        _Iterator(basic_istream_view* __parent) noexcept
          : _M_parent(__parent)
        { }
 
        _Iterator(const _Iterator&) = delete;
        _Iterator(_Iterator&&) = default;
        _Iterator& operator=(const _Iterator&) = delete;
        _Iterator& operator=(_Iterator&&) = default;
 
        _Iterator&
        operator++()
        {
          __glibcxx_assert(_M_parent->_M_stream != nullptr);
          *_M_parent->_M_stream >> _M_parent->_M_object;
          return *this;
        }
 
        void
        operator++(int)
        { ++*this; }
 
        _Val&
        operator*() const
        {
          __glibcxx_assert(_M_parent->_M_stream != nullptr);
          return _M_parent->_M_object;
        }
 
        friend bool
        operator==(const _Iterator& __x, default_sentinel_t)
        { return __x._M_at_end(); }
 
      private:
        basic_istream_view* _M_parent = nullptr;
 
        bool
        _M_at_end() const
        { return _M_parent == nullptr || !*_M_parent->_M_stream; }
      };
 
      friend _Iterator;
    };
 
  template<typename _Val, typename _CharT, typename _Traits>
    basic_istream_view<_Val, _CharT, _Traits>
    istream_view(basic_istream<_CharT, _Traits>& __s)
    { return basic_istream_view<_Val, _CharT, _Traits>{__s}; }
 
  // C++20 24.7 [range.adaptors] Range adaptors
 
namespace __detail
{
  struct _Empty { };
 
  // Alias for a type that is conditionally present
  // (and is an empty type otherwise).
  // Data members using this alias should use [[no_unique_address]] so that
  // they take no space when not needed.
  template<bool _Present, typename _Tp>
    using __maybe_present_t = conditional_t<_Present, _Tp, _Empty>;
 
  // Alias for a type that is conditionally const.
  template<bool _Const, typename _Tp>
    using __maybe_const_t = conditional_t<_Const, const _Tp, _Tp>;
 
} // namespace __detail
 
namespace views::__adaptor
{
  // True if the range adaptor _Adaptor can be applied with _Args.
  template<typename _Adaptor, typename... _Args>
    concept __adaptor_invocable
      = requires { std::declval<_Adaptor>()(declval<_Args>()...); };
 
  // True if the range adaptor non-closure _Adaptor can be partially applied
  // with _Args.
  template<typename _Adaptor, typename... _Args>
    concept __adaptor_partial_app_viable = (_Adaptor::_S_arity > 1)
      && (sizeof...(_Args) == _Adaptor::_S_arity - 1)
      && (constructible_from<decay_t<_Args>, _Args> && ...);
 
  template<typename _Adaptor, typename... _Args>
    struct _Partial;
 
  template<typename _Lhs, typename _Rhs>
    struct _Pipe;
 
  // The base class of every range adaptor closure.
  //
  // The derived class should define the optional static data member
  // _S_has_simple_call_op to true if the behavior of this adaptor is
  // independent of the constness/value category of the adaptor object.
  struct _RangeAdaptorClosure
  {
    // range | adaptor is equivalent to adaptor(range).
    template<typename _Self, typename _Range>
      requires derived_from<remove_cvref_t<_Self>, _RangeAdaptorClosure>
        && __adaptor_invocable<_Self, _Range>
      friend constexpr auto
      operator|(_Range&& __r, _Self&& __self)
      { return std::forward<_Self>(__self)(std::forward<_Range>(__r)); }
 
    // Compose the adaptors __lhs and __rhs into a pipeline, returning
    // another range adaptor closure object.
    template<typename _Lhs, typename _Rhs>
      requires derived_from<_Lhs, _RangeAdaptorClosure>
        && derived_from<_Rhs, _RangeAdaptorClosure>
      friend constexpr auto
      operator|(_Lhs __lhs, _Rhs __rhs)
      { return _Pipe<_Lhs, _Rhs>{std::move(__lhs), std::move(__rhs)}; }
  };
 
  // The base class of every range adaptor non-closure.
  //
  // The static data member _Derived::_S_arity must contain the total number of
  // arguments that the adaptor takes, and the class _Derived must introduce
  // _RangeAdaptor::operator() into the class scope via a using-declaration.
  //
  // The optional static data member _Derived::_S_has_simple_extra_args should
  // be defined to true if the behavior of this adaptor is independent of the
  // constness/value category of the extra arguments.  This data member could
  // also be defined as a variable template parameterized by the types of the
  // extra arguments.
  template<typename _Derived>
    struct _RangeAdaptor
    {
      // Partially apply the arguments __args to the range adaptor _Derived,
      // returning a range adaptor closure object.
      template<typename... _Args>
        requires __adaptor_partial_app_viable<_Derived, _Args...>
        constexpr auto
        operator()(_Args&&... __args) const
        {
          return _Partial<_Derived, decay_t<_Args>...>{std::forward<_Args>(__args)...};
        }
    };
 
  // True if the range adaptor closure _Adaptor has a simple operator(), i.e.
  // one that's not overloaded according to constness or value category of the
  // _Adaptor object.
  template<typename _Adaptor>
    concept __closure_has_simple_call_op = _Adaptor::_S_has_simple_call_op;
 
  // True if the behavior of the range adaptor non-closure _Adaptor is
  // independent of the value category of its extra arguments _Args.
  template<typename _Adaptor, typename... _Args>
    concept __adaptor_has_simple_extra_args = _Adaptor::_S_has_simple_extra_args
      || _Adaptor::template _S_has_simple_extra_args<_Args...>;
 
  // A range adaptor closure that represents partial application of
  // the range adaptor _Adaptor with arguments _Args.
  template<typename _Adaptor, typename... _Args>
    struct _Partial : _RangeAdaptorClosure
    {
      tuple<_Args...> _M_args;
 
      constexpr
      _Partial(_Args... __args)
        : _M_args(std::move(__args)...)
      { }
 
      // Invoke _Adaptor with arguments __r, _M_args... according to the
      // value category of this _Partial object.
      template<typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
        constexpr auto
        operator()(_Range&& __r) const &
        {
          auto __forwarder = [&__r] (const auto&... __args) {
            return _Adaptor{}(std::forward<_Range>(__r), __args...);
          };
          return std::apply(__forwarder, _M_args);
        }
 
      template<typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, _Args...>
        constexpr auto
        operator()(_Range&& __r) &&
        {
          auto __forwarder = [&__r] (auto&... __args) {
            return _Adaptor{}(std::forward<_Range>(__r), std::move(__args)...);
          };
          return std::apply(__forwarder, _M_args);
        }
 
      template<typename _Range>
        constexpr auto
        operator()(_Range&& __r) const && = delete;
    };
 
  // A lightweight specialization of the above primary template for
  // the common case where _Adaptor accepts a single extra argument.
  template<typename _Adaptor, typename _Arg>
    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
    {
      _Arg _M_arg;
 
      constexpr
      _Partial(_Arg __arg)
        : _M_arg(std::move(__arg))
      { }
 
      template<typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
        constexpr auto
        operator()(_Range&& __r) const &
        { return _Adaptor{}(std::forward<_Range>(__r), _M_arg); }
 
      template<typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, _Arg>
        constexpr auto
        operator()(_Range&& __r) &&
        { return _Adaptor{}(std::forward<_Range>(__r), std::move(_M_arg)); }
 
      template<typename _Range>
        constexpr auto
        operator()(_Range&& __r) const && = delete;
    };
 
  // Partial specialization of the primary template for the case where the extra
  // arguments of the adaptor can always be safely and efficiently forwarded by
  // const reference.  This lets us get away with a single operator() overload,
  // which makes overload resolution failure diagnostics more concise.
  template<typename _Adaptor, typename... _Args>
    requires __adaptor_has_simple_extra_args<_Adaptor, _Args...>
      && (is_trivially_copyable_v<_Args> && ...)
    struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure
    {
      tuple<_Args...> _M_args;
 
      constexpr
      _Partial(_Args... __args)
        : _M_args(std::move(__args)...)
      { }
 
      // Invoke _Adaptor with arguments __r, const _M_args&... regardless
      // of the value category of this _Partial object.
      template<typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
        constexpr auto
        operator()(_Range&& __r) const
        {
          auto __forwarder = [&__r] (const auto&... __args) {
            return _Adaptor{}(std::forward<_Range>(__r), __args...);
          };
          return std::apply(__forwarder, _M_args);
        }
 
      static constexpr bool _S_has_simple_call_op = true;
    };
 
  // A lightweight specialization of the above template for the common case
  // where _Adaptor accepts a single extra argument.
  template<typename _Adaptor, typename _Arg>
    requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
      && is_trivially_copyable_v<_Arg>
    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
    {
      _Arg _M_arg;
 
      constexpr
      _Partial(_Arg __arg)
        : _M_arg(std::move(__arg))
      { }
 
      template<typename _Range>
        requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
        constexpr auto
        operator()(_Range&& __r) const
        { return _Adaptor{}(std::forward<_Range>(__r), _M_arg); }
 
      static constexpr bool _S_has_simple_call_op = true;
    };
 
  template<typename _Lhs, typename _Rhs, typename _Range>
    concept __pipe_invocable
      = requires { std::declval<_Rhs>()(std::declval<_Lhs>()(std::declval<_Range>())); };
 
  // A range adaptor closure that represents composition of the range
  // adaptor closures _Lhs and _Rhs.
  template<typename _Lhs, typename _Rhs>
    struct _Pipe : _RangeAdaptorClosure
    {
      [[no_unique_address]] _Lhs _M_lhs;
      [[no_unique_address]] _Rhs _M_rhs;
 
      constexpr
      _Pipe(_Lhs __lhs, _Rhs __rhs)
        : _M_lhs(std::move(__lhs)), _M_rhs(std::move(__rhs))
      { }
 
      // Invoke _M_rhs(_M_lhs(__r)) according to the value category of this
      // range adaptor closure object.
      template<typename _Range>
        requires __pipe_invocable<const _Lhs&, const _Rhs&, _Range>
        constexpr auto
        operator()(_Range&& __r) const &
        { return _M_rhs(_M_lhs(std::forward<_Range>(__r))); }
 
      template<typename _Range>
        requires __pipe_invocable<_Lhs, _Rhs, _Range>
        constexpr auto
        operator()(_Range&& __r) &&
        { return std::move(_M_rhs)(std::move(_M_lhs)(std::forward<_Range>(__r))); }
 
      template<typename _Range>
        constexpr auto
        operator()(_Range&& __r) const && = delete;
    };
 
  // A partial specialization of the above primary template for the case where
  // both adaptor operands have a simple operator().  This in turn lets us
  // implement composition using a single simple operator(), which makes
  // overload resolution failure diagnostics more concise.
  template<typename _Lhs, typename _Rhs>
    requires __closure_has_simple_call_op<_Lhs>
      && __closure_has_simple_call_op<_Rhs>
    struct _Pipe<_Lhs, _Rhs> : _RangeAdaptorClosure
    {
      [[no_unique_address]] _Lhs _M_lhs;
      [[no_unique_address]] _Rhs _M_rhs;
 
      constexpr
      _Pipe(_Lhs __lhs, _Rhs __rhs)
        : _M_lhs(std::move(__lhs)), _M_rhs(std::move(__rhs))
      { }
 
      template<typename _Range>
        requires __pipe_invocable<const _Lhs&, const _Rhs&, _Range>
        constexpr auto
        operator()(_Range&& __r) const
        { return _M_rhs(_M_lhs(std::forward<_Range>(__r))); }
 
      static constexpr bool _S_has_simple_call_op = true;
    };
} // namespace views::__adaptor
 
  template<range _Range> requires is_object_v<_Range>
    class ref_view : public view_interface<ref_view<_Range>>
    {
    private:
      _Range* _M_r = nullptr;
 
      static void _S_fun(_Range&); // not defined
      static void _S_fun(_Range&&) = delete;
 
    public:
      constexpr
      ref_view() noexcept = default;
 
      template<__detail::__not_same_as<ref_view> _Tp>
        requires convertible_to<_Tp, _Range&>
          && requires { _S_fun(declval<_Tp>()); }
        constexpr
        ref_view(_Tp&& __t)
          : _M_r(std::__addressof(static_cast<_Range&>(std::forward<_Tp>(__t))))
        { }
 
      constexpr _Range&
      base() const
      { return *_M_r; }
 
      constexpr iterator_t<_Range>
      begin() const
      { return ranges::begin(*_M_r); }
 
      constexpr sentinel_t<_Range>
      end() const
      { return ranges::end(*_M_r); }
 
      constexpr bool
      empty() const requires requires { ranges::empty(*_M_r); }
      { return ranges::empty(*_M_r); }
 
      constexpr auto
      size() const requires sized_range<_Range>
      { return ranges::size(*_M_r); }
 
      constexpr auto
      data() const requires contiguous_range<_Range>
      { return ranges::data(*_M_r); }
    };
 
  template<typename _Range>
    ref_view(_Range&) -> ref_view<_Range>;
 
  template<typename _Tp>
    inline constexpr bool enable_borrowed_range<ref_view<_Tp>> = true;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range>
        concept __can_ref_view = requires { ref_view{std::declval<_Range>()}; };
 
      template<typename _Range>
        concept __can_subrange = requires { subrange{std::declval<_Range>()}; };
    } // namespace __detail
 
    struct _All : __adaptor::_RangeAdaptorClosure
    {
      template<viewable_range _Range>
        requires view<decay_t<_Range>>
          || __detail::__can_ref_view<_Range>
          || __detail::__can_subrange<_Range>
        constexpr auto
        operator()(_Range&& __r) const
        {
          if constexpr (view<decay_t<_Range>>)
            return std::forward<_Range>(__r);
          else if constexpr (__detail::__can_ref_view<_Range>)
            return ref_view{std::forward<_Range>(__r)};
          else
            return subrange{std::forward<_Range>(__r)};
        }
 
      static constexpr bool _S_has_simple_call_op = true;
    };
 
    inline constexpr _All all;
 
    template<viewable_range _Range>
      using all_t = decltype(all(std::declval<_Range>()));
  } // namespace views
 
  namespace __detail
  {
    template<typename _Tp>
      struct __non_propagating_cache
      {
        // When _Tp is not an object type (e.g. is a reference type), we make
        // __non_propagating_cache<_Tp> empty rather than ill-formed so that
        // users can easily conditionally declare data members with this type
        // (such as join_view::_M_inner).
      };
 
    template<typename _Tp>
      requires is_object_v<_Tp>
      struct __non_propagating_cache<_Tp> : protected _Optional_base<_Tp>
      {
        __non_propagating_cache() = default;
 
        constexpr
        __non_propagating_cache(const __non_propagating_cache&) noexcept
        { }
 
        constexpr
        __non_propagating_cache(__non_propagating_cache&& __other) noexcept
        { __other._M_reset(); }
 
        constexpr __non_propagating_cache&
        operator=(const __non_propagating_cache& __other) noexcept
        {
          if (std::__addressof(__other) != this)
            this->_M_reset();
          return *this;
        }
 
        constexpr __non_propagating_cache&
        operator=(__non_propagating_cache&& __other) noexcept
        {
          this->_M_reset();
          __other._M_reset();
          return *this;
        }
 
        constexpr _Tp&
        operator*() noexcept
        { return this->_M_get(); }
 
        constexpr const _Tp&
        operator*() const noexcept
        { return this->_M_get(); }
 
        template<typename _Iter>
          _Tp&
          _M_emplace_deref(const _Iter& __i)
          {
            this->_M_reset();
            // Using _Optional_base::_M_construct to initialize from '*__i'
            // would incur an extra move due to the indirection, so we instead
            // use placement new directly.
            ::new ((void *) std::__addressof(this->_M_payload._M_payload)) _Tp(*__i);
            this->_M_payload._M_engaged = true;
            return this->_M_get();
          }
      };
 
    template<range _Range>
      struct _CachedPosition
      {
        constexpr bool
        _M_has_value() const
        { return false; }
 
        constexpr iterator_t<_Range>
        _M_get(const _Range&) const
        {
          __glibcxx_assert(false);
          __builtin_unreachable();
        }
 
        constexpr void
        _M_set(const _Range&, const iterator_t<_Range>&) const
        { }
      };
 
    template<forward_range _Range>
      struct _CachedPosition<_Range>
        : protected __non_propagating_cache<iterator_t<_Range>>
      {
        constexpr bool
        _M_has_value() const
        { return this->_M_is_engaged(); }
 
        constexpr iterator_t<_Range>
        _M_get(const _Range&) const
        {
          __glibcxx_assert(_M_has_value());
          return **this;
        }
 
        constexpr void
        _M_set(const _Range&, const iterator_t<_Range>& __it)
        {
          __glibcxx_assert(!_M_has_value());
          std::construct_at(std::__addressof(this->_M_payload._M_payload),
                            in_place, __it);
          this->_M_payload._M_engaged = true;
        }
      };
 
    template<random_access_range _Range>
      requires (sizeof(range_difference_t<_Range>)
                <= sizeof(iterator_t<_Range>))
      struct _CachedPosition<_Range>
      {
      private:
        range_difference_t<_Range> _M_offset = -1;
 
      public:
        _CachedPosition() = default;
 
        constexpr
        _CachedPosition(const _CachedPosition&) = default;
 
        constexpr
        _CachedPosition(_CachedPosition&& __other) noexcept
        { *this = std::move(__other); }
 
        constexpr _CachedPosition&
        operator=(const _CachedPosition&) = default;
 
        constexpr _CachedPosition&
        operator=(_CachedPosition&& __other) noexcept
        {
          // Propagate the cached offset, but invalidate the source.
          _M_offset = __other._M_offset;
          __other._M_offset = -1;
          return *this;
        }
 
        constexpr bool
        _M_has_value() const
        { return _M_offset >= 0; }
 
        constexpr iterator_t<_Range>
        _M_get(_Range& __r) const
        {
          __glibcxx_assert(_M_has_value());
          return ranges::begin(__r) + _M_offset;
        }
 
        constexpr void
        _M_set(_Range& __r, const iterator_t<_Range>& __it)
        {
          __glibcxx_assert(!_M_has_value());
          _M_offset = __it - ranges::begin(__r);
        }
      };
  } // namespace __detail
 
  namespace __detail
  {
    template<typename _Base>
      struct __filter_view_iter_cat
      { };
 
    template<forward_range _Base>
      struct __filter_view_iter_cat<_Base>
      {
      private:
        static auto
        _S_iter_cat()
        {
          using _Cat = typename iterator_traits<iterator_t<_Base>>::iterator_category;
          if constexpr (derived_from<_Cat, bidirectional_iterator_tag>)
            return bidirectional_iterator_tag{};
          else if constexpr (derived_from<_Cat, forward_iterator_tag>)
            return forward_iterator_tag{};
          else
            return _Cat{};
        }
      public:
        using iterator_category = decltype(_S_iter_cat());
      };
  } // namespace __detail
 
  template<input_range _Vp,
           indirect_unary_predicate<iterator_t<_Vp>> _Pred>
    requires view<_Vp> && is_object_v<_Pred>
    class filter_view : public view_interface<filter_view<_Vp, _Pred>>
    {
    private:
      struct _Sentinel;
 
      struct _Iterator : __detail::__filter_view_iter_cat<_Vp>
      {
      private:
        static constexpr auto
        _S_iter_concept()
        {
          if constexpr (bidirectional_range<_Vp>)
            return bidirectional_iterator_tag{};
          else if constexpr (forward_range<_Vp>)
            return forward_iterator_tag{};
          else
            return input_iterator_tag{};
        }
 
        friend filter_view;
 
        using _Vp_iter = iterator_t<_Vp>;
 
        _Vp_iter _M_current = _Vp_iter();
        filter_view* _M_parent = nullptr;
 
      public:
        using iterator_concept = decltype(_S_iter_concept());
        // iterator_category defined in __filter_view_iter_cat
        using value_type = range_value_t<_Vp>;
        using difference_type = range_difference_t<_Vp>;
 
        _Iterator() requires default_initializable<_Vp_iter> = default;
 
        constexpr
        _Iterator(filter_view* __parent, _Vp_iter __current)
          : _M_current(std::move(__current)),
            _M_parent(__parent)
        { }
 
        constexpr const _Vp_iter&
        base() const & noexcept
        { return _M_current; }
 
        constexpr _Vp_iter
        base() &&
        { return std::move(_M_current); }
 
        constexpr range_reference_t<_Vp>
        operator*() const
        { return *_M_current; }
 
        constexpr _Vp_iter
        operator->() const
          requires __detail::__has_arrow<_Vp_iter>
            && copyable<_Vp_iter>
        { return _M_current; }
 
        constexpr _Iterator&
        operator++()
        {
          _M_current = ranges::find_if(std::move(++_M_current),
                                       ranges::end(_M_parent->_M_base),
                                       std::ref(*_M_parent->_M_pred));
          return *this;
        }
 
        constexpr void
        operator++(int)
        { ++*this; }
 
        constexpr _Iterator
        operator++(int) requires forward_range<_Vp>
        {
          auto __tmp = *this;
          ++*this;
          return __tmp;
        }
 
        constexpr _Iterator&
        operator--() requires bidirectional_range<_Vp>
        {
          do
            --_M_current;
          while (!std::__invoke(*_M_parent->_M_pred, *_M_current));
          return *this;
        }
 
        constexpr _Iterator
        operator--(int) requires bidirectional_range<_Vp>
        {
          auto __tmp = *this;
          --*this;
          return __tmp;
        }
 
        friend constexpr bool
        operator==(const _Iterator& __x, const _Iterator& __y)
          requires equality_comparable<_Vp_iter>
        { return __x._M_current == __y._M_current; }
 
        friend constexpr range_rvalue_reference_t<_Vp>
        iter_move(const _Iterator& __i)
          noexcept(noexcept(ranges::iter_move(__i._M_current)))
        { return ranges::iter_move(__i._M_current); }
 
        friend constexpr void
        iter_swap(const _Iterator& __x, const _Iterator& __y)
          noexcept(noexcept(ranges::iter_swap(__x._M_current, __y._M_current)))
          requires indirectly_swappable<_Vp_iter>
        { ranges::iter_swap(__x._M_current, __y._M_current); }
      };
 
      struct _Sentinel
      {
      private:
        sentinel_t<_Vp> _M_end = sentinel_t<_Vp>();
 
        constexpr bool
        __equal(const _Iterator& __i) const
        { return __i._M_current == _M_end; }
 
      public:
        _Sentinel() = default;
 
        constexpr explicit
        _Sentinel(filter_view* __parent)
          : _M_end(ranges::end(__parent->_M_base))
        { }
 
        constexpr sentinel_t<_Vp>
        base() const
        { return _M_end; }
 
        friend constexpr bool
        operator==(const _Iterator& __x, const _Sentinel& __y)
        { return __y.__equal(__x); }
      };
 
      [[no_unique_address]] __detail::__box<_Pred> _M_pred;
      [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
      _Vp _M_base = _Vp();
 
    public:
      filter_view() requires default_initializable<_Vp> = default;
 
      constexpr
      filter_view(_Vp __base, _Pred __pred)
        : _M_pred(std::move(__pred)), _M_base(std::move(__base))
      { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr const _Pred&
      pred() const
      { return *_M_pred; }
 
      constexpr _Iterator
      begin()
      {
        if (_M_cached_begin._M_has_value())
          return {this, _M_cached_begin._M_get(_M_base)};
 
        __glibcxx_assert(_M_pred.has_value());
        auto __it = ranges::find_if(ranges::begin(_M_base),
                                    ranges::end(_M_base),
                                    std::ref(*_M_pred));
        _M_cached_begin._M_set(_M_base, __it);
        return {this, std::move(__it)};
      }
 
      constexpr auto
      end()
      {
        if constexpr (common_range<_Vp>)
          return _Iterator{this, ranges::end(_M_base)};
        else
          return _Sentinel{this};
      }
    };
 
  template<typename _Range, typename _Pred>
    filter_view(_Range&&, _Pred) -> filter_view<views::all_t<_Range>, _Pred>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Pred>
        concept __can_filter_view
          = requires { filter_view(std::declval<_Range>(), std::declval<_Pred>()); };
    } // namespace __detail
 
    struct _Filter : __adaptor::_RangeAdaptor<_Filter>
    {
      template<viewable_range _Range, typename _Pred>
        requires __detail::__can_filter_view<_Range, _Pred>
        constexpr auto
        operator()(_Range&& __r, _Pred&& __p) const
        {
          return filter_view(std::forward<_Range>(__r), std::forward<_Pred>(__p));
        }
 
      using _RangeAdaptor<_Filter>::operator();
      static constexpr int _S_arity = 2;
      static constexpr bool _S_has_simple_extra_args = true;
    };
 
    inline constexpr _Filter filter;
  } // namespace views
 
  template<input_range _Vp, copy_constructible _Fp>
    requires view<_Vp> && is_object_v<_Fp>
      && regular_invocable<_Fp&, range_reference_t<_Vp>>
      && std::__detail::__can_reference<invoke_result_t<_Fp&,
                                                        range_reference_t<_Vp>>>
    class transform_view : public view_interface<transform_view<_Vp, _Fp>>
    {
    private:
      template<bool _Const>
        using _Base = __detail::__maybe_const_t<_Const, _Vp>;
 
      template<bool _Const>
        struct __iter_cat
        { };
 
      template<bool _Const>
        requires forward_range<_Base<_Const>>
        struct __iter_cat<_Const>
        {
        private:
          static auto
          _S_iter_cat()
          {
            using _Base = transform_view::_Base<_Const>;
            using _Res = invoke_result_t<_Fp&, range_reference_t<_Base>>;
            if constexpr (is_lvalue_reference_v<_Res>)
              {
                using _Cat
                  = typename iterator_traits<iterator_t<_Base>>::iterator_category;
                if constexpr (derived_from<_Cat, contiguous_iterator_tag>)
                  return random_access_iterator_tag{};
                else
                  return _Cat{};
              }
            else
              return input_iterator_tag{};
          }
        public:
          using iterator_category = decltype(_S_iter_cat());
        };
 
      template<bool _Const>
        struct _Sentinel;
 
      template<bool _Const>
        struct _Iterator : __iter_cat<_Const>
        {
        private:
          using _Parent = __detail::__maybe_const_t<_Const, transform_view>;
          using _Base = transform_view::_Base<_Const>;
 
          static auto
          _S_iter_concept()
          {
            if constexpr (random_access_range<_Base>)
              return random_access_iterator_tag{};
            else if constexpr (bidirectional_range<_Base>)
              return bidirectional_iterator_tag{};
            else if constexpr (forward_range<_Base>)
              return forward_iterator_tag{};
            else
              return input_iterator_tag{};
          }
 
          using _Base_iter = iterator_t<_Base>;
 
          _Base_iter _M_current = _Base_iter();
          _Parent* _M_parent = nullptr;
 
        public:
          using iterator_concept = decltype(_S_iter_concept());
          // iterator_category defined in __transform_view_iter_cat
          using value_type
            = remove_cvref_t<invoke_result_t<_Fp&, range_reference_t<_Base>>>;
          using difference_type = range_difference_t<_Base>;
 
          _Iterator() requires default_initializable<_Base_iter> = default;
 
          constexpr
          _Iterator(_Parent* __parent, _Base_iter __current)
            : _M_current(std::move(__current)),
              _M_parent(__parent)
          { }
 
          constexpr
          _Iterator(_Iterator<!_Const> __i)
            requires _Const
              && convertible_to<iterator_t<_Vp>, _Base_iter>
            : _M_current(std::move(__i._M_current)), _M_parent(__i._M_parent)
          { }
 
          constexpr const _Base_iter&
          base() const & noexcept
          { return _M_current; }
 
          constexpr _Base_iter
          base() &&
          { return std::move(_M_current); }
 
          constexpr decltype(auto)
          operator*() const
            noexcept(noexcept(std::__invoke(*_M_parent->_M_fun, *_M_current)))
          { return std::__invoke(*_M_parent->_M_fun, *_M_current); }
 
          constexpr _Iterator&
          operator++()
          {
            ++_M_current;
            return *this;
          }
 
          constexpr void
          operator++(int)
          { ++_M_current; }
 
          constexpr _Iterator
          operator++(int) requires forward_range<_Base>
          {
            auto __tmp = *this;
            ++*this;
            return __tmp;
          }
 
          constexpr _Iterator&
          operator--() requires bidirectional_range<_Base>
          {
            --_M_current;
            return *this;
          }
 
          constexpr _Iterator
          operator--(int) requires bidirectional_range<_Base>
          {
            auto __tmp = *this;
            --*this;
            return __tmp;
          }
 
          constexpr _Iterator&
          operator+=(difference_type __n) requires random_access_range<_Base>
          {
            _M_current += __n;
            return *this;
          }
 
          constexpr _Iterator&
          operator-=(difference_type __n) requires random_access_range<_Base>
          {
            _M_current -= __n;
            return *this;
          }
 
          constexpr decltype(auto)
          operator[](difference_type __n) const
            requires random_access_range<_Base>
          { return std::__invoke(*_M_parent->_M_fun, _M_current[__n]); }
 
          friend constexpr bool
          operator==(const _Iterator& __x, const _Iterator& __y)
            requires equality_comparable<_Base_iter>
          { return __x._M_current == __y._M_current; }
 
          friend constexpr bool
          operator<(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return __x._M_current < __y._M_current; }
 
          friend constexpr bool
          operator>(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return __y < __x; }
 
          friend constexpr bool
          operator<=(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return !(__y < __x); }
 
          friend constexpr bool
          operator>=(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return !(__x < __y); }
 
#ifdef __cpp_lib_three_way_comparison
          friend constexpr auto
          operator<=>(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
              && three_way_comparable<_Base_iter>
          { return __x._M_current <=> __y._M_current; }
#endif
 
          friend constexpr _Iterator
          operator+(_Iterator __i, difference_type __n)
            requires random_access_range<_Base>
          { return {__i._M_parent, __i._M_current + __n}; }
 
          friend constexpr _Iterator
          operator+(difference_type __n, _Iterator __i)
            requires random_access_range<_Base>
          { return {__i._M_parent, __i._M_current + __n}; }
 
          friend constexpr _Iterator
          operator-(_Iterator __i, difference_type __n)
            requires random_access_range<_Base>
          { return {__i._M_parent, __i._M_current - __n}; }
 
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 3483. transform_view::iterator's difference is overconstrained
          friend constexpr difference_type
          operator-(const _Iterator& __x, const _Iterator& __y)
            requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
          { return __x._M_current - __y._M_current; }
 
          friend constexpr decltype(auto)
          iter_move(const _Iterator& __i) noexcept(noexcept(*__i))
          {
            if constexpr (is_lvalue_reference_v<decltype(*__i)>)
              return std::move(*__i);
            else
              return *__i;
          }
 
          friend _Iterator<!_Const>;
          template<bool> friend struct _Sentinel;
        };
 
      template<bool _Const>
        struct _Sentinel
        {
        private:
          using _Parent = __detail::__maybe_const_t<_Const, transform_view>;
          using _Base = transform_view::_Base<_Const>;
 
          template<bool _Const2>
            constexpr auto
            __distance_from(const _Iterator<_Const2>& __i) const
            { return _M_end - __i._M_current; }
 
          template<bool _Const2>
            constexpr bool
            __equal(const _Iterator<_Const2>& __i) const
            { return __i._M_current == _M_end; }
 
          sentinel_t<_Base> _M_end = sentinel_t<_Base>();
 
        public:
          _Sentinel() = default;
 
          constexpr explicit
          _Sentinel(sentinel_t<_Base> __end)
            : _M_end(__end)
          { }
 
          constexpr
          _Sentinel(_Sentinel<!_Const> __i)
            requires _Const
              && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
            : _M_end(std::move(__i._M_end))
          { }
 
          constexpr sentinel_t<_Base>
          base() const
          { return _M_end; }
 
          template<bool _Const2>
            requires sentinel_for<sentinel_t<_Base>,
                       iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
            friend constexpr bool
            operator==(const _Iterator<_Const2>& __x, const _Sentinel& __y)
            { return __y.__equal(__x); }
 
          template<bool _Const2,
                   typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
            requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
            friend constexpr range_difference_t<_Base2>
            operator-(const _Iterator<_Const2>& __x, const _Sentinel& __y)
            { return -__y.__distance_from(__x); }
 
          template<bool _Const2,
                   typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
            requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
            friend constexpr range_difference_t<_Base2>
            operator-(const _Sentinel& __y, const _Iterator<_Const2>& __x)
            { return __y.__distance_from(__x); }
 
          friend _Sentinel<!_Const>;
        };
 
      [[no_unique_address]] __detail::__box<_Fp> _M_fun;
      _Vp _M_base = _Vp();
 
    public:
      transform_view() requires default_initializable<_Vp> = default;
 
      constexpr
      transform_view(_Vp __base, _Fp __fun)
        : _M_fun(std::move(__fun)), _M_base(std::move(__base))
      { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base ; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr _Iterator<false>
      begin()
      { return _Iterator<false>{this, ranges::begin(_M_base)}; }
 
      constexpr _Iterator<true>
      begin() const
        requires range<const _Vp>
          && regular_invocable<const _Fp&, range_reference_t<const _Vp>>
      { return _Iterator<true>{this, ranges::begin(_M_base)}; }
 
      constexpr _Sentinel<false>
      end()
      { return _Sentinel<false>{ranges::end(_M_base)}; }
 
      constexpr _Iterator<false>
      end() requires common_range<_Vp>
      { return _Iterator<false>{this, ranges::end(_M_base)}; }
 
      constexpr _Sentinel<true>
      end() const
        requires range<const _Vp>
          && regular_invocable<const _Fp&, range_reference_t<const _Vp>>
      { return _Sentinel<true>{ranges::end(_M_base)}; }
 
      constexpr _Iterator<true>
      end() const
        requires common_range<const _Vp>
          && regular_invocable<const _Fp&, range_reference_t<const _Vp>>
      { return _Iterator<true>{this, ranges::end(_M_base)}; }
 
      constexpr auto
      size() requires sized_range<_Vp>
      { return ranges::size(_M_base); }
 
      constexpr auto
      size() const requires sized_range<const _Vp>
      { return ranges::size(_M_base); }
    };
 
  template<typename _Range, typename _Fp>
    transform_view(_Range&&, _Fp) -> transform_view<views::all_t<_Range>, _Fp>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Fp>
        concept __can_transform_view
          = requires { transform_view(std::declval<_Range>(), std::declval<_Fp>()); };
    } // namespace __detail
 
    struct _Transform : __adaptor::_RangeAdaptor<_Transform>
    {
      template<viewable_range _Range, typename _Fp>
        requires __detail::__can_transform_view<_Range, _Fp>
        constexpr auto
        operator()(_Range&& __r, _Fp&& __f) const
        {
          return transform_view(std::forward<_Range>(__r), std::forward<_Fp>(__f));
        }
 
      using _RangeAdaptor<_Transform>::operator();
      static constexpr int _S_arity = 2;
      static constexpr bool _S_has_simple_extra_args = true;
    };
 
    inline constexpr _Transform transform;
  } // namespace views
 
  template<view _Vp>
    class take_view : public view_interface<take_view<_Vp>>
    {
    private:
      template<bool _Const>
        using _CI = counted_iterator<
          iterator_t<__detail::__maybe_const_t<_Const, _Vp>>>;
 
      template<bool _Const>
        struct _Sentinel
        {
        private:
          using _Base = __detail::__maybe_const_t<_Const, _Vp>;
          sentinel_t<_Base> _M_end = sentinel_t<_Base>();
 
        public:
          _Sentinel() = default;
 
          constexpr explicit
          _Sentinel(sentinel_t<_Base> __end)
            : _M_end(__end)
          { }
 
          constexpr
          _Sentinel(_Sentinel<!_Const> __s)
            requires _Const && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
            : _M_end(std::move(__s._M_end))
          { }
 
          constexpr sentinel_t<_Base>
          base() const
          { return _M_end; }
 
          friend constexpr bool
          operator==(const _CI<_Const>& __y, const _Sentinel& __x)
          { return __y.count() == 0 || __y.base() == __x._M_end; }
 
          template<bool _OtherConst = !_Const,
                   typename _Base2 = __detail::__maybe_const_t<_OtherConst, _Vp>>
            requires sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
          friend constexpr bool
          operator==(const _CI<_OtherConst>& __y, const _Sentinel& __x)
          { return __y.count() == 0 || __y.base() == __x._M_end; }
 
          friend _Sentinel<!_Const>;
        };
 
      range_difference_t<_Vp> _M_count = 0;
      _Vp _M_base = _Vp();
 
    public:
      take_view() requires default_initializable<_Vp> = default;
 
      constexpr
      take_view(_Vp base, range_difference_t<_Vp> __count)
        : _M_count(std::move(__count)), _M_base(std::move(base))
      { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr auto
      begin() requires (!__detail::__simple_view<_Vp>)
      {
        if constexpr (sized_range<_Vp>)
          {
            if constexpr (random_access_range<_Vp>)
              return ranges::begin(_M_base);
            else
              {
                auto __sz = size();
                return counted_iterator(ranges::begin(_M_base), __sz);
              }
          }
        else
          return counted_iterator(ranges::begin(_M_base), _M_count);
      }
 
      constexpr auto
      begin() const requires range<const _Vp>
      {
        if constexpr (sized_range<const _Vp>)
          {
            if constexpr (random_access_range<const _Vp>)
              return ranges::begin(_M_base);
            else
              {
                auto __sz = size();
                return counted_iterator(ranges::begin(_M_base), __sz);
              }
          }
        else
          return counted_iterator(ranges::begin(_M_base), _M_count);
      }
 
      constexpr auto
      end() requires (!__detail::__simple_view<_Vp>)
      {
        if constexpr (sized_range<_Vp>)
          {
            if constexpr (random_access_range<_Vp>)
              return ranges::begin(_M_base) + size();
            else
              return default_sentinel;
          }
        else
          return _Sentinel<false>{ranges::end(_M_base)};
      }
 
      constexpr auto
      end() const requires range<const _Vp>
      {
        if constexpr (sized_range<const _Vp>)
          {
            if constexpr (random_access_range<const _Vp>)
              return ranges::begin(_M_base) + size();
            else
              return default_sentinel;
          }
        else
          return _Sentinel<true>{ranges::end(_M_base)};
      }
 
      constexpr auto
      size() requires sized_range<_Vp>
      {
        auto __n = ranges::size(_M_base);
        return std::min(__n, static_cast<decltype(__n)>(_M_count));
      }
 
      constexpr auto
      size() const requires sized_range<const _Vp>
      {
        auto __n = ranges::size(_M_base);
        return std::min(__n, static_cast<decltype(__n)>(_M_count));
      }
    };
 
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 3447. Deduction guides for take_view and drop_view have different
  // constraints
  template<typename _Range>
    take_view(_Range&&, range_difference_t<_Range>)
      -> take_view<views::all_t<_Range>>;
 
  template<typename _Tp>
    inline constexpr bool enable_borrowed_range<take_view<_Tp>>
      = enable_borrowed_range<_Tp>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Tp>
        concept __can_take_view
          = requires { take_view(std::declval<_Range>(), std::declval<_Tp>()); };
    } // namespace __detail
 
    struct _Take : __adaptor::_RangeAdaptor<_Take>
    {
      template<viewable_range _Range, typename _Tp>
        requires __detail::__can_take_view<_Range, _Tp>
        constexpr auto
        operator()(_Range&& __r, _Tp&& __n) const
        {
          return take_view(std::forward<_Range>(__r), std::forward<_Tp>(__n));
        }
 
      using _RangeAdaptor<_Take>::operator();
      static constexpr int _S_arity = 2;
      // The count argument of views::take is not always simple -- it can be
      // e.g. a move-only class that's implicitly convertible to the difference
      // type.  But an integer-like count argument is surely simple.
      template<typename _Tp>
        static constexpr bool _S_has_simple_extra_args
          = ranges::__detail::__is_integer_like<_Tp>;
    };
 
    inline constexpr _Take take;
  } // namespace views
 
  template<view _Vp, typename _Pred>
    requires input_range<_Vp> && is_object_v<_Pred>
      && indirect_unary_predicate<const _Pred, iterator_t<_Vp>>
    class take_while_view : public view_interface<take_while_view<_Vp, _Pred>>
    {
      template<bool _Const>
        struct _Sentinel
        {
        private:
          using _Base = __detail::__maybe_const_t<_Const, _Vp>;
 
          sentinel_t<_Base> _M_end = sentinel_t<_Base>();
          const _Pred* _M_pred = nullptr;
 
        public:
          _Sentinel() = default;
 
          constexpr explicit
          _Sentinel(sentinel_t<_Base> __end, const _Pred* __pred)
            : _M_end(__end), _M_pred(__pred)
          { }
 
          constexpr
          _Sentinel(_Sentinel<!_Const> __s)
            requires _Const && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
            : _M_end(__s._M_end), _M_pred(__s._M_pred)
          { }
 
          constexpr sentinel_t<_Base>
          base() const { return _M_end; }
 
          friend constexpr bool
          operator==(const iterator_t<_Base>& __x, const _Sentinel& __y)
          { return __y._M_end == __x || !std::__invoke(*__y._M_pred, *__x); }
 
          template<bool _OtherConst = !_Const,
                   typename _Base2 = __detail::__maybe_const_t<_OtherConst, _Vp>>
            requires sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
          friend constexpr bool
          operator==(const iterator_t<_Base2>& __x, const _Sentinel& __y)
          { return __y._M_end == __x || !std::__invoke(*__y._M_pred, *__x); }
 
          friend _Sentinel<!_Const>;
        };
 
      [[no_unique_address]] __detail::__box<_Pred> _M_pred;
      _Vp _M_base = _Vp();
 
    public:
      take_while_view() requires default_initializable<_Vp> = default;
 
      constexpr
      take_while_view(_Vp base, _Pred __pred)
        : _M_pred(std::move(__pred)), _M_base(std::move(base))
      { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr const _Pred&
      pred() const
      { return *_M_pred; }
 
      constexpr auto
      begin() requires (!__detail::__simple_view<_Vp>)
      { return ranges::begin(_M_base); }
 
      constexpr auto
      begin() const requires range<const _Vp>
        && indirect_unary_predicate<const _Pred, iterator_t<const _Vp>>
      { return ranges::begin(_M_base); }
 
      constexpr auto
      end() requires (!__detail::__simple_view<_Vp>)
      { return _Sentinel<false>(ranges::end(_M_base),
                                std::__addressof(*_M_pred)); }
 
      constexpr auto
      end() const requires range<const _Vp>
        && indirect_unary_predicate<const _Pred, iterator_t<const _Vp>>
      { return _Sentinel<true>(ranges::end(_M_base),
                               std::__addressof(*_M_pred)); }
    };
 
  template<typename _Range, typename _Pred>
    take_while_view(_Range&&, _Pred)
      -> take_while_view<views::all_t<_Range>, _Pred>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Pred>
        concept __can_take_while_view
          = requires { take_while_view(std::declval<_Range>(), std::declval<_Pred>()); };
    } // namespace __detail
 
    struct _TakeWhile : __adaptor::_RangeAdaptor<_TakeWhile>
    {
      template<viewable_range _Range, typename _Pred>
        requires __detail::__can_take_while_view<_Range, _Pred>
        constexpr auto
        operator()(_Range&& __r, _Pred&& __p) const
        {
          return take_while_view(std::forward<_Range>(__r), std::forward<_Pred>(__p));
        }
 
      using _RangeAdaptor<_TakeWhile>::operator();
      static constexpr int _S_arity = 2;
      static constexpr bool _S_has_simple_extra_args = true;
    };
 
    inline constexpr _TakeWhile take_while;
  } // namespace views
 
  template<view _Vp>
    class drop_view : public view_interface<drop_view<_Vp>>
    {
    private:
      range_difference_t<_Vp> _M_count = 0;
      _Vp _M_base = _Vp();
 
      // ranges::next(begin(base), count, end(base)) is O(1) if _Vp satisfies
      // both random_access_range and sized_range. Otherwise, cache its result.
      static constexpr bool _S_needs_cached_begin
        = !(random_access_range<const _Vp> && sized_range<const _Vp>);
      [[no_unique_address]]
        __detail::__maybe_present_t<_S_needs_cached_begin,
                                    __detail::_CachedPosition<_Vp>>
                                      _M_cached_begin;
 
    public:
      drop_view() requires default_initializable<_Vp> = default;
 
      constexpr
      drop_view(_Vp __base, range_difference_t<_Vp> __count)
        : _M_count(__count), _M_base(std::move(__base))
      { __glibcxx_assert(__count >= 0); }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      // This overload is disabled for simple views with constant-time begin().
      constexpr auto
      begin()
        requires (!(__detail::__simple_view<_Vp>
                    && random_access_range<const _Vp>
                    && sized_range<const _Vp>))
      {
        if constexpr (_S_needs_cached_begin)
          if (_M_cached_begin._M_has_value())
            return _M_cached_begin._M_get(_M_base);
 
        auto __it = ranges::next(ranges::begin(_M_base),
                                 _M_count, ranges::end(_M_base));
        if constexpr (_S_needs_cached_begin)
          _M_cached_begin._M_set(_M_base, __it);
        return __it;
      }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3482. drop_view's const begin should additionally require sized_range
      constexpr auto
      begin() const
        requires random_access_range<const _Vp> && sized_range<const _Vp>
      {
        return ranges::next(ranges::begin(_M_base), _M_count,
                            ranges::end(_M_base));
      }
 
      constexpr auto
      end() requires (!__detail::__simple_view<_Vp>)
      { return ranges::end(_M_base); }
 
      constexpr auto
      end() const requires range<const _Vp>
      { return ranges::end(_M_base); }
 
      constexpr auto
      size() requires sized_range<_Vp>
      {
        const auto __s = ranges::size(_M_base);
        const auto __c = static_cast<decltype(__s)>(_M_count);
        return __s < __c ? 0 : __s - __c;
      }
 
      constexpr auto
      size() const requires sized_range<const _Vp>
      {
        const auto __s = ranges::size(_M_base);
        const auto __c = static_cast<decltype(__s)>(_M_count);
        return __s < __c ? 0 : __s - __c;
      }
    };
 
  template<typename _Range>
    drop_view(_Range&&, range_difference_t<_Range>)
      -> drop_view<views::all_t<_Range>>;
 
  template<typename _Tp>
    inline constexpr bool enable_borrowed_range<drop_view<_Tp>>
      = enable_borrowed_range<_Tp>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Tp>
        concept __can_drop_view
          = requires { drop_view(std::declval<_Range>(), std::declval<_Tp>()); };
    } // namespace __detail
 
    struct _Drop : __adaptor::_RangeAdaptor<_Drop>
    {
      template<viewable_range _Range, typename _Tp>
        requires __detail::__can_drop_view<_Range, _Tp>
        constexpr auto
        operator()(_Range&& __r, _Tp&& __n) const
        {
          return drop_view(std::forward<_Range>(__r), std::forward<_Tp>(__n));
        }
 
      using _RangeAdaptor<_Drop>::operator();
      static constexpr int _S_arity = 2;
      template<typename _Tp>
        static constexpr bool _S_has_simple_extra_args
          = _Take::_S_has_simple_extra_args<_Tp>;
    };
 
    inline constexpr _Drop drop;
  } // namespace views
 
  template<view _Vp, typename _Pred>
    requires input_range<_Vp> && is_object_v<_Pred>
      && indirect_unary_predicate<const _Pred, iterator_t<_Vp>>
    class drop_while_view : public view_interface<drop_while_view<_Vp, _Pred>>
    {
    private:
      [[no_unique_address]] __detail::__box<_Pred> _M_pred;
      [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
      _Vp _M_base = _Vp();
 
    public:
      drop_while_view() requires default_initializable<_Vp> = default;
 
      constexpr
      drop_while_view(_Vp __base, _Pred __pred)
        : _M_pred(std::move(__pred)), _M_base(std::move(__base))
      { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr const _Pred&
      pred() const
      { return *_M_pred; }
 
      constexpr auto
      begin()
      {
        if (_M_cached_begin._M_has_value())
          return _M_cached_begin._M_get(_M_base);
 
        __glibcxx_assert(_M_pred.has_value());
        auto __it = ranges::find_if_not(ranges::begin(_M_base),
                                        ranges::end(_M_base),
                                        std::cref(*_M_pred));
        _M_cached_begin._M_set(_M_base, __it);
        return __it;
      }
 
      constexpr auto
      end()
      { return ranges::end(_M_base); }
    };
 
  template<typename _Range, typename _Pred>
    drop_while_view(_Range&&, _Pred)
      -> drop_while_view<views::all_t<_Range>, _Pred>;
 
  template<typename _Tp, typename _Pred>
    inline constexpr bool enable_borrowed_range<drop_while_view<_Tp, _Pred>>
      = enable_borrowed_range<_Tp>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Pred>
        concept __can_drop_while_view
          = requires { drop_while_view(std::declval<_Range>(), std::declval<_Pred>()); };
    } // namespace __detail
 
    struct _DropWhile : __adaptor::_RangeAdaptor<_DropWhile>
    {
      template<viewable_range _Range, typename _Pred>
        requires __detail::__can_drop_while_view<_Range, _Pred>
        constexpr auto
        operator()(_Range&& __r, _Pred&& __p) const
        {
          return drop_while_view(std::forward<_Range>(__r),
                                 std::forward<_Pred>(__p));
        }
 
      using _RangeAdaptor<_DropWhile>::operator();
      static constexpr int _S_arity = 2;
      static constexpr bool _S_has_simple_extra_args = true;
    };
 
    inline constexpr _DropWhile drop_while;
  } // namespace views
 
  template<input_range _Vp>
    requires view<_Vp> && input_range<range_reference_t<_Vp>>
    class join_view : public view_interface<join_view<_Vp>>
    {
    private:
      using _InnerRange = range_reference_t<_Vp>;
 
      template<bool _Const>
        using _Base = __detail::__maybe_const_t<_Const, _Vp>;
 
      template<bool _Const>
        using _Outer_iter = iterator_t<_Base<_Const>>;
 
      template<bool _Const>
        using _Inner_iter = iterator_t<range_reference_t<_Base<_Const>>>;
 
      template<bool _Const>
        static constexpr bool _S_ref_is_glvalue
          = is_reference_v<range_reference_t<_Base<_Const>>>;
 
      template<bool _Const>
        struct __iter_cat
        { };
 
      template<bool _Const>
        requires _S_ref_is_glvalue<_Const>
          && forward_range<_Base<_Const>>
          && forward_range<range_reference_t<_Base<_Const>>>
        struct __iter_cat<_Const>
        {
        private:
          static constexpr auto
          _S_iter_cat()
          {
            using _Outer_iter = join_view::_Outer_iter<_Const>;
            using _Inner_iter = join_view::_Inner_iter<_Const>;
            using _OuterCat = typename iterator_traits<_Outer_iter>::iterator_category;
            using _InnerCat = typename iterator_traits<_Inner_iter>::iterator_category;
            if constexpr (derived_from<_OuterCat, bidirectional_iterator_tag>
                          && derived_from<_InnerCat, bidirectional_iterator_tag>
                          && common_range<range_reference_t<_Base<_Const>>>)
              return bidirectional_iterator_tag{};
            else if constexpr (derived_from<_OuterCat, forward_iterator_tag>
                               && derived_from<_InnerCat, forward_iterator_tag>)
              return forward_iterator_tag{};
            else
              return input_iterator_tag{};
          }
        public:
          using iterator_category = decltype(_S_iter_cat());
        };
 
      template<bool _Const>
        struct _Sentinel;
 
      template<bool _Const>
        struct _Iterator : __iter_cat<_Const>
        {
        private:
          using _Parent = __detail::__maybe_const_t<_Const, join_view>;
          using _Base = join_view::_Base<_Const>;
 
          static constexpr bool _S_ref_is_glvalue
            = join_view::_S_ref_is_glvalue<_Const>;
 
          constexpr void
          _M_satisfy()
          {
            auto __update_inner = [this] (const iterator_t<_Base>& __x) -> auto&& {
              if constexpr (_S_ref_is_glvalue)
                return *__x;
              else
                return _M_parent->_M_inner._M_emplace_deref(__x);
            };
 
            for (; _M_outer != ranges::end(_M_parent->_M_base); ++_M_outer)
              {
                auto&& __inner = __update_inner(_M_outer);
                _M_inner = ranges::begin(__inner);
                if (_M_inner != ranges::end(__inner))
                  return;
              }
 
            if constexpr (_S_ref_is_glvalue)
              _M_inner = _Inner_iter();
          }
 
          static constexpr auto
          _S_iter_concept()
          {
            if constexpr (_S_ref_is_glvalue
                          && bidirectional_range<_Base>
                          && bidirectional_range<range_reference_t<_Base>>
                          && common_range<range_reference_t<_Base>>)
              return bidirectional_iterator_tag{};
            else if constexpr (_S_ref_is_glvalue
                               && forward_range<_Base>
                               && forward_range<range_reference_t<_Base>>)
              return forward_iterator_tag{};
            else
              return input_iterator_tag{};
          }
 
          using _Outer_iter = join_view::_Outer_iter<_Const>;
          using _Inner_iter = join_view::_Inner_iter<_Const>;
 
          _Outer_iter _M_outer = _Outer_iter();
          _Inner_iter _M_inner = _Inner_iter();
          _Parent* _M_parent = nullptr;
 
        public:
          using iterator_concept = decltype(_S_iter_concept());
          // iterator_category defined in __join_view_iter_cat
          using value_type = range_value_t<range_reference_t<_Base>>;
          using difference_type
            = common_type_t<range_difference_t<_Base>,
                            range_difference_t<range_reference_t<_Base>>>;
 
          _Iterator() requires (default_initializable<_Outer_iter>
                                && default_initializable<_Inner_iter>)
            = default;
 
          constexpr
          _Iterator(_Parent* __parent, _Outer_iter __outer)
            : _M_outer(std::move(__outer)),
              _M_parent(__parent)
          { _M_satisfy(); }
 
          constexpr
          _Iterator(_Iterator<!_Const> __i)
            requires _Const
              && convertible_to<iterator_t<_Vp>, _Outer_iter>
              && convertible_to<iterator_t<_InnerRange>, _Inner_iter>
            : _M_outer(std::move(__i._M_outer)), _M_inner(std::move(__i._M_inner)),
              _M_parent(__i._M_parent)
          { }
 
          constexpr decltype(auto)
          operator*() const
          { return *_M_inner; }
 
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 3500. join_view::iterator::operator->() is bogus
          constexpr _Inner_iter
          operator->() const
            requires __detail::__has_arrow<_Inner_iter>
              && copyable<_Inner_iter>
          { return _M_inner; }
 
          constexpr _Iterator&
          operator++()
          {
            auto&& __inner_range = [this] () -> auto&& {
              if constexpr (_S_ref_is_glvalue)
                return *_M_outer;
              else
                return *_M_parent->_M_inner;
            }();
            if (++_M_inner == ranges::end(__inner_range))
              {
                ++_M_outer;
                _M_satisfy();
              }
            return *this;
          }
 
          constexpr void
          operator++(int)
          { ++*this; }
 
          constexpr _Iterator
          operator++(int)
            requires _S_ref_is_glvalue && forward_range<_Base>
              && forward_range<range_reference_t<_Base>>
          {
            auto __tmp = *this;
            ++*this;
            return __tmp;
          }
 
          constexpr _Iterator&
          operator--()
            requires _S_ref_is_glvalue && bidirectional_range<_Base>
              && bidirectional_range<range_reference_t<_Base>>
              && common_range<range_reference_t<_Base>>
          {
            if (_M_outer == ranges::end(_M_parent->_M_base))
              _M_inner = ranges::end(*--_M_outer);
            while (_M_inner == ranges::begin(*_M_outer))
              _M_inner = ranges::end(*--_M_outer);
            --_M_inner;
            return *this;
          }
 
          constexpr _Iterator
          operator--(int)
            requires _S_ref_is_glvalue && bidirectional_range<_Base>
              && bidirectional_range<range_reference_t<_Base>>
              && common_range<range_reference_t<_Base>>
          {
            auto __tmp = *this;
            --*this;
            return __tmp;
          }
 
          friend constexpr bool
          operator==(const _Iterator& __x, const _Iterator& __y)
            requires _S_ref_is_glvalue
              && equality_comparable<_Outer_iter>
              && equality_comparable<_Inner_iter>
          {
            return (__x._M_outer == __y._M_outer
                    && __x._M_inner == __y._M_inner);
          }
 
          friend constexpr decltype(auto)
          iter_move(const _Iterator& __i)
          noexcept(noexcept(ranges::iter_move(__i._M_inner)))
          { return ranges::iter_move(__i._M_inner); }
 
          friend constexpr void
          iter_swap(const _Iterator& __x, const _Iterator& __y)
            noexcept(noexcept(ranges::iter_swap(__x._M_inner, __y._M_inner)))
            requires indirectly_swappable<_Inner_iter>
          { return ranges::iter_swap(__x._M_inner, __y._M_inner); }
 
          friend _Iterator<!_Const>;
          template<bool> friend struct _Sentinel;
        };
 
      template<bool _Const>
        struct _Sentinel
        {
        private:
          using _Parent = __detail::__maybe_const_t<_Const, join_view>;
          using _Base = join_view::_Base<_Const>;
 
          template<bool _Const2>
            constexpr bool
            __equal(const _Iterator<_Const2>& __i) const
            { return __i._M_outer == _M_end; }
 
          sentinel_t<_Base> _M_end = sentinel_t<_Base>();
 
        public:
          _Sentinel() = default;
 
          constexpr explicit
          _Sentinel(_Parent* __parent)
            : _M_end(ranges::end(__parent->_M_base))
          { }
 
          constexpr
          _Sentinel(_Sentinel<!_Const> __s)
            requires _Const && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
            : _M_end(std::move(__s._M_end))
          { }
 
          template<bool _Const2>
            requires sentinel_for<sentinel_t<_Base>,
                       iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
            friend constexpr bool
            operator==(const _Iterator<_Const2>& __x, const _Sentinel& __y)
            { return __y.__equal(__x); }
 
          friend _Sentinel<!_Const>;
        };
 
      [[no_unique_address]]
        __detail::__non_propagating_cache<remove_cv_t<_InnerRange>> _M_inner;
      _Vp _M_base = _Vp();
 
    public:
      join_view() requires default_initializable<_Vp> = default;
 
      constexpr explicit
      join_view(_Vp __base)
        : _M_base(std::move(__base))
      { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr auto
      begin()
      {
        constexpr bool __use_const
          = (__detail::__simple_view<_Vp>
             && is_reference_v<range_reference_t<_Vp>>);
        return _Iterator<__use_const>{this, ranges::begin(_M_base)};
      }
 
      constexpr auto
      begin() const
        requires input_range<const _Vp>
          && is_reference_v<range_reference_t<const _Vp>>
      {
        return _Iterator<true>{this, ranges::begin(_M_base)};
      }
 
      constexpr auto
      end()
      {
        if constexpr (forward_range<_Vp> && is_reference_v<_InnerRange>
                      && forward_range<_InnerRange>
                      && common_range<_Vp> && common_range<_InnerRange>)
          return _Iterator<__detail::__simple_view<_Vp>>{this,
                                                         ranges::end(_M_base)};
        else
          return _Sentinel<__detail::__simple_view<_Vp>>{this};
      }
 
      constexpr auto
      end() const
        requires input_range<const _Vp>
          && is_reference_v<range_reference_t<const _Vp>>
      {
        if constexpr (forward_range<const _Vp>
                      && is_reference_v<range_reference_t<const _Vp>>
                      && forward_range<range_reference_t<const _Vp>>
                      && common_range<const _Vp>
                      && common_range<range_reference_t<const _Vp>>)
          return _Iterator<true>{this, ranges::end(_M_base)};
        else
          return _Sentinel<true>{this};
      }
    };
 
  template<typename _Range>
    explicit join_view(_Range&&) -> join_view<views::all_t<_Range>>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range>
        concept __can_join_view
          = requires { join_view<all_t<_Range>>{std::declval<_Range>()}; };
    } // namespace __detail
 
    struct _Join : __adaptor::_RangeAdaptorClosure
    {
      template<viewable_range _Range>
        requires __detail::__can_join_view<_Range>
        constexpr auto
        operator()(_Range&& __r) const
        {
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 3474. Nesting join_views is broken because of CTAD
          return join_view<all_t<_Range>>{std::forward<_Range>(__r)};
        }
 
      static constexpr bool _S_has_simple_call_op = true;
    };
 
    inline constexpr _Join join;
  } // namespace views
 
  namespace __detail
  {
    template<auto>
      struct __require_constant;
 
    template<typename _Range>
      concept __tiny_range = sized_range<_Range>
        && requires
           { typename __require_constant<remove_reference_t<_Range>::size()>; }
        && (remove_reference_t<_Range>::size() <= 1);
 
    template<typename _Base>
      struct __split_view_outer_iter_cat
      { };
 
    template<forward_range _Base>
      struct __split_view_outer_iter_cat<_Base>
      { using iterator_category = input_iterator_tag; };
 
    template<typename _Base>
      struct __split_view_inner_iter_cat
      { };
 
    template<forward_range _Base>
      struct __split_view_inner_iter_cat<_Base>
      {
      private:
        static constexpr auto
        _S_iter_cat()
        {
          using _Cat = typename iterator_traits<iterator_t<_Base>>::iterator_category;
          if constexpr (derived_from<_Cat, forward_iterator_tag>)
            return forward_iterator_tag{};
          else
            return _Cat{};
        }
      public:
        using iterator_category = decltype(_S_iter_cat());
      };
  }
 
  template<input_range _Vp, forward_range _Pattern>
    requires view<_Vp> && view<_Pattern>
      && indirectly_comparable<iterator_t<_Vp>, iterator_t<_Pattern>,
                               ranges::equal_to>
      && (forward_range<_Vp> || __detail::__tiny_range<_Pattern>)
    class split_view : public view_interface<split_view<_Vp, _Pattern>>
    {
    private:
      template<bool _Const>
        using _Base = __detail::__maybe_const_t<_Const, _Vp>;
 
      template<bool _Const>
        struct _InnerIter;
 
      template<bool _Const>
        struct _OuterIter
          : __detail::__split_view_outer_iter_cat<_Base<_Const>>
        {
        private:
          using _Parent = __detail::__maybe_const_t<_Const, split_view>;
          using _Base = split_view::_Base<_Const>;
 
          constexpr bool
          __at_end() const
          { return __current() == ranges::end(_M_parent->_M_base); }
 
          // [range.split.outer] p1
          //  Many of the following specifications refer to the notional member
          //  current of outer-iterator.  current is equivalent to current_ if
          //  V models forward_range, and parent_->current_ otherwise.
          constexpr auto&
          __current() noexcept
          {
            if constexpr (forward_range<_Vp>)
              return _M_current;
            else
              return _M_parent->_M_current;
          }
 
          constexpr auto&
          __current() const noexcept
          {
            if constexpr (forward_range<_Vp>)
              return _M_current;
            else
              return _M_parent->_M_current;
          }
 
          _Parent* _M_parent = nullptr;
 
          // XXX: _M_current is present only if "V models forward_range"
          [[no_unique_address]]
            __detail::__maybe_present_t<forward_range<_Vp>,
                                        iterator_t<_Base>> _M_current;
 
        public:
          using iterator_concept = conditional_t<forward_range<_Base>,
                                                 forward_iterator_tag,
                                                 input_iterator_tag>;
          // iterator_category defined in __split_view_outer_iter_cat
          using difference_type = range_difference_t<_Base>;
 
          struct value_type : view_interface<value_type>
          {
          private:
            _OuterIter _M_i = _OuterIter();
 
          public:
            value_type() = default;
 
            constexpr explicit
            value_type(_OuterIter __i)
              : _M_i(std::move(__i))
            { }
 
            constexpr _InnerIter<_Const>
            begin() const
            { return _InnerIter<_Const>{_M_i}; }
 
            constexpr default_sentinel_t
            end() const noexcept
            { return default_sentinel; }
          };
 
          _OuterIter() = default;
 
          constexpr explicit
          _OuterIter(_Parent* __parent) requires (!forward_range<_Base>)
            : _M_parent(__parent)
          { }
 
          constexpr
          _OuterIter(_Parent* __parent, iterator_t<_Base> __current)
            requires forward_range<_Base>
            : _M_parent(__parent),
              _M_current(std::move(__current))
          { }
 
          constexpr
          _OuterIter(_OuterIter<!_Const> __i)
            requires _Const
              && convertible_to<iterator_t<_Vp>, iterator_t<_Base>>
            : _M_parent(__i._M_parent), _M_current(std::move(__i._M_current))
          { }
 
          constexpr value_type
          operator*() const
          { return value_type{*this}; }
 
          constexpr _OuterIter&
          operator++()
          {
            // _GLIBCXX_RESOLVE_LIB_DEFECTS
            // 3505. split_view::outer-iterator::operator++ misspecified
            const auto __end = ranges::end(_M_parent->_M_base);
            if (__current() == __end)
              return *this;
            const auto [__pbegin, __pend] = subrange{_M_parent->_M_pattern};
            if (__pbegin == __pend)
              ++__current();
            else if constexpr (__detail::__tiny_range<_Pattern>)
              {
                __current() = ranges::find(std::move(__current()), __end,
                                           *__pbegin);
                if (__current() != __end)
                  ++__current();
              }
            else
              do
                {
                  auto [__b, __p]
                    = ranges::mismatch(__current(), __end, __pbegin, __pend);
                  if (__p == __pend)
                    {
                      __current() = __b;
                      break;
                    }
                } while (++__current() != __end);
            return *this;
          }
 
          constexpr decltype(auto)
          operator++(int)
          {
            if constexpr (forward_range<_Base>)
              {
                auto __tmp = *this;
                ++*this;
                return __tmp;
              }
            else
              ++*this;
          }
 
          friend constexpr bool
          operator==(const _OuterIter& __x, const _OuterIter& __y)
            requires forward_range<_Base>
          { return __x._M_current == __y._M_current; }
 
          friend constexpr bool
          operator==(const _OuterIter& __x, default_sentinel_t)
          { return __x.__at_end(); };
 
          friend _OuterIter<!_Const>;
          friend _InnerIter<_Const>;
        };
 
      template<bool _Const>
        struct _InnerIter
          : __detail::__split_view_inner_iter_cat<_Base<_Const>>
        {
        private:
          using _Base = split_view::_Base<_Const>;
 
          constexpr bool
          __at_end() const
          {
            auto [__pcur, __pend] = subrange{_M_i._M_parent->_M_pattern};
            auto __end = ranges::end(_M_i._M_parent->_M_base);
            if constexpr (__detail::__tiny_range<_Pattern>)
              {
                const auto& __cur = _M_i_current();
                if (__cur == __end)
                  return true;
                if (__pcur == __pend)
                  return _M_incremented;
                return *__cur == *__pcur;
              }
            else
              {
                auto __cur = _M_i_current();
                if (__cur == __end)
                  return true;
                if (__pcur == __pend)
                  return _M_incremented;
                do
                  {
                    if (*__cur != *__pcur)
                      return false;
                    if (++__pcur == __pend)
                      return true;
                  } while (++__cur != __end);
                return false;
              }
          }
 
          constexpr auto&
          _M_i_current() noexcept
          { return _M_i.__current(); }
 
          constexpr auto&
          _M_i_current() const noexcept
          { return _M_i.__current(); }
 
          _OuterIter<_Const> _M_i = _OuterIter<_Const>();
          bool _M_incremented = false;
 
        public:
          using iterator_concept
            = typename _OuterIter<_Const>::iterator_concept;
          // iterator_category defined in __split_view_inner_iter_cat
          using value_type = range_value_t<_Base>;
          using difference_type = range_difference_t<_Base>;
 
          _InnerIter() = default;
 
          constexpr explicit
          _InnerIter(_OuterIter<_Const> __i)
            : _M_i(std::move(__i))
          { }
 
          constexpr const iterator_t<_Base>&
          base() const& noexcept
          { return _M_i_current(); }
 
          constexpr iterator_t<_Base>
          base() && requires forward_range<_Vp>
          { return std::move(_M_i_current()); }
 
          constexpr decltype(auto)
          operator*() const
          { return *_M_i_current(); }
 
          constexpr _InnerIter&
          operator++()
          {
            _M_incremented = true;
            if constexpr (!forward_range<_Base>)
              if constexpr (_Pattern::size() == 0)
                return *this;
            ++_M_i_current();
            return *this;
          }
 
          constexpr decltype(auto)
          operator++(int)
          {
            if constexpr (forward_range<_Base>)
              {
                auto __tmp = *this;
                ++*this;
                return __tmp;
              }
            else
              ++*this;
          }
 
          friend constexpr bool
          operator==(const _InnerIter& __x, const _InnerIter& __y)
            requires forward_range<_Base>
          { return __x._M_i == __y._M_i; }
 
          friend constexpr bool
          operator==(const _InnerIter& __x, default_sentinel_t)
          { return __x.__at_end(); }
 
          friend constexpr decltype(auto)
          iter_move(const _InnerIter& __i)
            noexcept(noexcept(ranges::iter_move(__i._M_i_current())))
          { return ranges::iter_move(__i._M_i_current()); }
 
          friend constexpr void
          iter_swap(const _InnerIter& __x, const _InnerIter& __y)
            noexcept(noexcept(ranges::iter_swap(__x._M_i_current(),
                                                __y._M_i_current())))
            requires indirectly_swappable<iterator_t<_Base>>
          { ranges::iter_swap(__x._M_i_current(), __y._M_i_current()); }
        };
 
      _Pattern _M_pattern = _Pattern();
      // XXX: _M_current is "present only if !forward_range<V>"
      [[no_unique_address]]
        __detail::__maybe_present_t<!forward_range<_Vp>,
                                    iterator_t<_Vp>> _M_current;
      _Vp _M_base = _Vp();
 
 
    public:
      split_view() requires (default_initializable<_Vp>
                             && default_initializable<_Pattern>
                             && (forward_range<_Vp>
                                 || default_initializable<iterator_t<_Vp>>))
        = default;
 
      constexpr
      split_view(_Vp __base, _Pattern __pattern)
        : _M_pattern(std::move(__pattern)), _M_base(std::move(__base))
      { }
 
      template<input_range _Range>
        requires constructible_from<_Vp, views::all_t<_Range>>
          && constructible_from<_Pattern, single_view<range_value_t<_Range>>>
        constexpr
        split_view(_Range&& __r, range_value_t<_Range> __e)
          : _M_pattern(views::single(std::move(__e))),
            _M_base(views::all(std::forward<_Range>(__r)))
        { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr auto
      begin()
      {
        if constexpr (forward_range<_Vp>)
          {
            constexpr bool __simple
              = __detail::__simple_view<_Vp> && __detail::__simple_view<_Pattern>;
            return _OuterIter<__simple>{this, ranges::begin(_M_base)};
          }
        else
          {
            _M_current = ranges::begin(_M_base);
            return _OuterIter<false>{this};
          }
      }
 
      constexpr auto
      begin() const requires forward_range<_Vp> && forward_range<const _Vp>
      {
        return _OuterIter<true>{this, ranges::begin(_M_base)};
      }
 
      constexpr auto
      end() requires forward_range<_Vp> && common_range<_Vp>
      {
        constexpr bool __simple
          = __detail::__simple_view<_Vp> && __detail::__simple_view<_Pattern>;
        return _OuterIter<__simple>{this, ranges::end(_M_base)};
      }
 
      constexpr auto
      end() const
      {
        if constexpr (forward_range<_Vp>
                      && forward_range<const _Vp>
                      && common_range<const _Vp>)
          return _OuterIter<true>{this, ranges::end(_M_base)};
        else
          return default_sentinel;
      }
    };
 
  template<typename _Range, typename _Pattern>
    split_view(_Range&&, _Pattern&&)
      -> split_view<views::all_t<_Range>, views::all_t<_Pattern>>;
 
  template<input_range _Range>
    split_view(_Range&&, range_value_t<_Range>)
      -> split_view<views::all_t<_Range>, single_view<range_value_t<_Range>>>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Pattern>
        concept __can_split_view
          = requires { split_view(std::declval<_Range>(), std::declval<_Pattern>()); };
    } // namespace __detail
 
    struct _Split : __adaptor::_RangeAdaptor<_Split>
    {
      template<viewable_range _Range, typename _Pattern>
        requires __detail::__can_split_view<_Range, _Pattern>
        constexpr auto
        operator()(_Range&& __r, _Pattern&& __f) const
        {
          return split_view(std::forward<_Range>(__r), std::forward<_Pattern>(__f));
        }
 
      using _RangeAdaptor<_Split>::operator();
      static constexpr int _S_arity = 2;
      // The pattern argument of views::split is not always simple -- it can be
      // a non-view range, the value category of which affects whether the call
      // is well-formed.  But a scalar or a view pattern argument is surely
      // simple.
      template<typename _Pattern>
        static constexpr bool _S_has_simple_extra_args
          = is_scalar_v<_Pattern> || (view<_Pattern>
                                      && copy_constructible<_Pattern>);
    };
 
    inline constexpr _Split split;
  } // namespace views
 
  namespace views
  {
    struct _Counted
    {
      template<input_or_output_iterator _Iter>
      constexpr auto
      operator()(_Iter __i, iter_difference_t<_Iter> __n) const
      {
        if constexpr (random_access_iterator<_Iter>)
          return subrange(__i, __i + __n);
        else
          return subrange(counted_iterator(std::move(__i), __n),
                          default_sentinel);
      }
    };
 
    inline constexpr _Counted counted{};
  } // namespace views
 
  template<view _Vp>
    requires (!common_range<_Vp>) && copyable<iterator_t<_Vp>>
    class common_view : public view_interface<common_view<_Vp>>
    {
    private:
      _Vp _M_base = _Vp();
 
    public:
      common_view() requires default_initializable<_Vp> = default;
 
      constexpr explicit
      common_view(_Vp __r)
        : _M_base(std::move(__r))
      { }
 
      /* XXX: LWG 3280 didn't remove this constructor, but I think it should?
      template<viewable_range _Range>
        requires (!common_range<_Range>)
          && constructible_from<_Vp, views::all_t<_Range>>
        constexpr explicit
        common_view(_Range&& __r)
          : _M_base(views::all(std::forward<_Range>(__r)))
        { }
      */
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr auto
      begin()
      {
        if constexpr (random_access_range<_Vp> && sized_range<_Vp>)
          return ranges::begin(_M_base);
        else
          return common_iterator<iterator_t<_Vp>, sentinel_t<_Vp>>
                  (ranges::begin(_M_base));
      }
 
      constexpr auto
      begin() const requires range<const _Vp>
      {
        if constexpr (random_access_range<const _Vp> && sized_range<const _Vp>)
          return ranges::begin(_M_base);
        else
          return common_iterator<iterator_t<const _Vp>, sentinel_t<const _Vp>>
                  (ranges::begin(_M_base));
      }
 
      constexpr auto
      end()
      {
        if constexpr (random_access_range<_Vp> && sized_range<_Vp>)
          return ranges::begin(_M_base) + ranges::size(_M_base);
        else
          return common_iterator<iterator_t<_Vp>, sentinel_t<_Vp>>
                  (ranges::end(_M_base));
      }
 
      constexpr auto
      end() const requires range<const _Vp>
      {
        if constexpr (random_access_range<const _Vp> && sized_range<const _Vp>)
          return ranges::begin(_M_base) + ranges::size(_M_base);
        else
          return common_iterator<iterator_t<const _Vp>, sentinel_t<const _Vp>>
                  (ranges::end(_M_base));
      }
 
      constexpr auto
      size() requires sized_range<_Vp>
      { return ranges::size(_M_base); }
 
      constexpr auto
      size() const requires sized_range<const _Vp>
      { return ranges::size(_M_base); }
    };
 
  template<typename _Range>
    common_view(_Range&&) -> common_view<views::all_t<_Range>>;
 
  template<typename _Tp>
    inline constexpr bool enable_borrowed_range<common_view<_Tp>>
      = enable_borrowed_range<_Tp>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename _Range>
        concept __already_common = common_range<_Range>
          && requires { views::all(std::declval<_Range>()); };
 
      template<typename _Range>
        concept __can_common_view
          = requires { common_view{std::declval<_Range>()}; };
    } // namespace __detail
 
    struct _Common : __adaptor::_RangeAdaptorClosure
    {
      template<viewable_range _Range>
        requires __detail::__already_common<_Range>
          || __detail::__can_common_view<_Range>
        constexpr auto
        operator()(_Range&& __r) const
        {
          if constexpr (__detail::__already_common<_Range>)
            return views::all(std::forward<_Range>(__r));
          else
            return common_view{std::forward<_Range>(__r)};
        }
 
      static constexpr bool _S_has_simple_call_op = true;
    };
 
    inline constexpr _Common common;
  } // namespace views
 
  template<view _Vp>
    requires bidirectional_range<_Vp>
    class reverse_view : public view_interface<reverse_view<_Vp>>
    {
    private:
      static constexpr bool _S_needs_cached_begin
        = !common_range<_Vp> && !random_access_range<_Vp>;
 
      [[no_unique_address]]
        __detail::__maybe_present_t<_S_needs_cached_begin,
                                    __detail::_CachedPosition<_Vp>>
                                      _M_cached_begin;
      _Vp _M_base = _Vp();
 
    public:
      reverse_view() requires default_initializable<_Vp> = default;
 
      constexpr explicit
      reverse_view(_Vp __r)
        : _M_base(std::move(__r))
        { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr reverse_iterator<iterator_t<_Vp>>
      begin()
      {
        if constexpr (_S_needs_cached_begin)
          if (_M_cached_begin._M_has_value())
            return std::make_reverse_iterator(_M_cached_begin._M_get(_M_base));
 
        auto __it = ranges::next(ranges::begin(_M_base), ranges::end(_M_base));
        if constexpr (_S_needs_cached_begin)
          _M_cached_begin._M_set(_M_base, __it);
        return std::make_reverse_iterator(std::move(__it));
      }
 
      constexpr auto
      begin() requires common_range<_Vp>
      { return std::make_reverse_iterator(ranges::end(_M_base)); }
 
      constexpr auto
      begin() const requires common_range<const _Vp>
      { return std::make_reverse_iterator(ranges::end(_M_base)); }
 
      constexpr reverse_iterator<iterator_t<_Vp>>
      end()
      { return std::make_reverse_iterator(ranges::begin(_M_base)); }
 
      constexpr auto
      end() const requires common_range<const _Vp>
      { return std::make_reverse_iterator(ranges::begin(_M_base)); }
 
      constexpr auto
      size() requires sized_range<_Vp>
      { return ranges::size(_M_base); }
 
      constexpr auto
      size() const requires sized_range<const _Vp>
      { return ranges::size(_M_base); }
    };
 
  template<typename _Range>
    reverse_view(_Range&&) -> reverse_view<views::all_t<_Range>>;
 
  template<typename _Tp>
    inline constexpr bool enable_borrowed_range<reverse_view<_Tp>>
      = enable_borrowed_range<_Tp>;
 
  namespace views
  {
    namespace __detail
    {
      template<typename>
        inline constexpr bool __is_reversible_subrange = false;
 
      template<typename _Iter, subrange_kind _Kind>
        inline constexpr bool
          __is_reversible_subrange<subrange<reverse_iterator<_Iter>,
                                            reverse_iterator<_Iter>,
                                            _Kind>> = true;
 
      template<typename>
        inline constexpr bool __is_reverse_view = false;
 
      template<typename _Vp>
        inline constexpr bool __is_reverse_view<reverse_view<_Vp>> = true;
 
      template<typename _Range>
        concept __can_reverse_view
          = requires { reverse_view{std::declval<_Range>()}; };
    } // namespace __detail
 
    struct _Reverse : __adaptor::_RangeAdaptorClosure
    {
      template<viewable_range _Range>
        requires __detail::__is_reverse_view<remove_cvref_t<_Range>>
          || __detail::__is_reversible_subrange<remove_cvref_t<_Range>>
          || __detail::__can_reverse_view<_Range>
        constexpr auto
        operator()(_Range&& __r) const
        {
          using _Tp = remove_cvref_t<_Range>;
          if constexpr (__detail::__is_reverse_view<_Tp>)
            return std::forward<_Range>(__r).base();
          else if constexpr (__detail::__is_reversible_subrange<_Tp>)
            {
              using _Iter = decltype(ranges::begin(__r).base());
              if constexpr (sized_range<_Tp>)
                return subrange<_Iter, _Iter, subrange_kind::sized>
                        {__r.end().base(), __r.begin().base(), __r.size()};
              else
                return subrange<_Iter, _Iter, subrange_kind::unsized>
                        {__r.end().base(), __r.begin().base()};
            }
          else
            return reverse_view{std::forward<_Range>(__r)};
        }
 
      static constexpr bool _S_has_simple_call_op = true;
    };
 
    inline constexpr _Reverse reverse;
  } // namespace views
 
  namespace __detail
  {
    template<typename _Tp, size_t _Nm>
    concept __has_tuple_element = requires(_Tp __t)
      {
        typename tuple_size<_Tp>::type;
        requires _Nm < tuple_size_v<_Tp>;
        typename tuple_element_t<_Nm, _Tp>;
        { std::get<_Nm>(__t) }
          -> convertible_to<const tuple_element_t<_Nm, _Tp>&>;
      };
 
    template<typename _Tp, size_t _Nm>
      concept __returnable_element
        = is_reference_v<_Tp> || move_constructible<tuple_element_t<_Nm, _Tp>>;
  }
 
  template<input_range _Vp, size_t _Nm>
    requires view<_Vp>
      && __detail::__has_tuple_element<range_value_t<_Vp>, _Nm>
      && __detail::__has_tuple_element<remove_reference_t<range_reference_t<_Vp>>,
                                       _Nm>
      && __detail::__returnable_element<range_reference_t<_Vp>, _Nm>
    class elements_view : public view_interface<elements_view<_Vp, _Nm>>
    {
    public:
      elements_view() requires default_initializable<_Vp> = default;
 
      constexpr explicit
      elements_view(_Vp base)
        : _M_base(std::move(base))
      { }
 
      constexpr _Vp
      base() const& requires copy_constructible<_Vp>
      { return _M_base; }
 
      constexpr _Vp
      base() &&
      { return std::move(_M_base); }
 
      constexpr auto
      begin() requires (!__detail::__simple_view<_Vp>)
      { return _Iterator<false>(ranges::begin(_M_base)); }
 
      constexpr auto
      begin() const requires range<const _Vp>
      { return _Iterator<true>(ranges::begin(_M_base)); }
 
      constexpr auto
      end() requires (!__detail::__simple_view<_Vp> && !common_range<_Vp>)
      { return _Sentinel<false>{ranges::end(_M_base)}; }
 
      constexpr auto
      end() requires (!__detail::__simple_view<_Vp> && common_range<_Vp>)
      { return _Iterator<false>{ranges::end(_M_base)}; }
 
      constexpr auto
      end() const requires range<const _Vp>
      { return _Sentinel<true>{ranges::end(_M_base)}; }
 
      constexpr auto
      end() const requires common_range<const _Vp>
      { return _Iterator<true>{ranges::end(_M_base)}; }
 
      constexpr auto
      size() requires sized_range<_Vp>
      { return ranges::size(_M_base); }
 
      constexpr auto
      size() const requires sized_range<const _Vp>
      { return ranges::size(_M_base); }
 
    private:
      template<bool _Const>
        using _Base = __detail::__maybe_const_t<_Const, _Vp>;
 
      template<bool _Const>
        struct __iter_cat
        { };
 
      template<bool _Const>
        requires forward_range<_Base<_Const>>
        struct __iter_cat<_Const>
        {
        private:
          static auto _S_iter_cat()
          {
            using _Base = elements_view::_Base<_Const>;
            using _Cat = typename iterator_traits<iterator_t<_Base>>::iterator_category;
            using _Res = decltype((std::get<_Nm>(*std::declval<iterator_t<_Base>>())));
            if constexpr (!is_lvalue_reference_v<_Res>)
              return input_iterator_tag{};
            else if constexpr (derived_from<_Cat, random_access_iterator_tag>)
              return random_access_iterator_tag{};
            else
              return _Cat{};
          }
        public:
          using iterator_category = decltype(_S_iter_cat());
        };
 
      template<bool _Const>
        struct _Sentinel;
 
      template<bool _Const>
        struct _Iterator : __iter_cat<_Const>
        {
        private:
          using _Base = elements_view::_Base<_Const>;
 
          iterator_t<_Base> _M_current = iterator_t<_Base>();
 
          static constexpr decltype(auto)
          _S_get_element(const iterator_t<_Base>& __i)
          {
            if constexpr (is_reference_v<range_reference_t<_Base>>)
              return std::get<_Nm>(*__i);
            else
              {
                using _Et = remove_cv_t<tuple_element_t<_Nm, range_reference_t<_Base>>>;
                return static_cast<_Et>(std::get<_Nm>(*__i));
              }
          }
 
          static auto
          _S_iter_concept()
          {
            if constexpr (random_access_range<_Base>)
              return random_access_iterator_tag{};
            else if constexpr (bidirectional_range<_Base>)
              return bidirectional_iterator_tag{};
            else if constexpr (forward_range<_Base>)
              return forward_iterator_tag{};
            else
              return input_iterator_tag{};
          }
 
          friend _Iterator<!_Const>;
 
        public:
          using iterator_concept = decltype(_S_iter_concept());
          // iterator_category defined in elements_view::__iter_cat
          using value_type
            = remove_cvref_t<tuple_element_t<_Nm, 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)
            : _M_current(std::move(current))
          { }
 
          constexpr
          _Iterator(_Iterator<!_Const> i)
            requires _Const && convertible_to<iterator_t<_Vp>, iterator_t<_Base>>
            : _M_current(std::move(i._M_current))
          { }
 
          constexpr const iterator_t<_Base>&
          base() const& noexcept
          { return _M_current; }
 
          constexpr iterator_t<_Base>
          base() &&
          { return std::move(_M_current); }
 
          constexpr decltype(auto)
          operator*() const
          { return _S_get_element(_M_current); }
 
          constexpr _Iterator&
          operator++()
          {
            ++_M_current;
            return *this;
          }
 
          constexpr void
          operator++(int)
          { ++_M_current; }
 
          constexpr _Iterator
          operator++(int) requires forward_range<_Base>
          {
            auto __tmp = *this;
            ++_M_current;
            return __tmp;
          }
 
          constexpr _Iterator&
          operator--() requires bidirectional_range<_Base>
          {
            --_M_current;
            return *this;
          }
 
          constexpr _Iterator
          operator--(int) requires bidirectional_range<_Base>
          {
            auto __tmp = *this;
            --_M_current;
            return __tmp;
          }
 
          constexpr _Iterator&
          operator+=(difference_type __n)
            requires random_access_range<_Base>
          {
            _M_current += __n;
            return *this;
          }
 
          constexpr _Iterator&
          operator-=(difference_type __n)
            requires random_access_range<_Base>
          {
            _M_current -= __n;
            return *this;
          }
 
          constexpr decltype(auto)
          operator[](difference_type __n) const
            requires random_access_range<_Base>
          { return _S_get_element(_M_current + __n); }
 
          friend constexpr bool
          operator==(const _Iterator& __x, const _Iterator& __y)
            requires equality_comparable<iterator_t<_Base>>
          { return __x._M_current == __y._M_current; }
 
          friend constexpr bool
          operator<(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return __x._M_current < __y._M_current; }
 
          friend constexpr bool
          operator>(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return __y._M_current < __x._M_current; }
 
          friend constexpr bool
          operator<=(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return !(__y._M_current > __x._M_current); }
 
          friend constexpr bool
          operator>=(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return !(__x._M_current > __y._M_current); }
 
#ifdef __cpp_lib_three_way_comparison
          friend constexpr auto
          operator<=>(const _Iterator& __x, const _Iterator& __y)
            requires random_access_range<_Base>
              && three_way_comparable<iterator_t<_Base>>
          { return __x._M_current <=> __y._M_current; }
#endif
 
          friend constexpr _Iterator
          operator+(const _Iterator& __x, difference_type __y)
            requires random_access_range<_Base>
          { return _Iterator{__x} += __y; }
 
          friend constexpr _Iterator
          operator+(difference_type __x, const _Iterator& __y)
            requires random_access_range<_Base>
          { return __y + __x; }
 
          friend constexpr _Iterator
          operator-(const _Iterator& __x, difference_type __y)
            requires random_access_range<_Base>
          { return _Iterator{__x} -= __y; }
 
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 3483. transform_view::iterator's difference is overconstrained
          friend constexpr difference_type
          operator-(const _Iterator& __x, const _Iterator& __y)
            requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
          { return __x._M_current - __y._M_current; }
 
          template <bool> friend struct _Sentinel;
        };
 
      template<bool _Const>
        struct _Sentinel
        {
        private:
          template<bool _Const2>
            constexpr bool
            _M_equal(const _Iterator<_Const2>& __x) const
            { return __x._M_current == _M_end; }
 
          template<bool _Const2>
            constexpr auto
            _M_distance_from(const _Iterator<_Const2>& __i) const
            { return _M_end - __i._M_current; }
 
          using _Base = elements_view::_Base<_Const>;
          sentinel_t<_Base> _M_end = sentinel_t<_Base>();
 
        public:
          _Sentinel() = default;
 
          constexpr explicit
          _Sentinel(sentinel_t<_Base> __end)
            : _M_end(std::move(__end))
          { }
 
          constexpr
          _Sentinel(_Sentinel<!_Const> __other)
            requires _Const
              && convertible_to<sentinel_t<_Vp>, sentinel_t<_Base>>
            : _M_end(std::move(__other._M_end))
          { }
 
          constexpr sentinel_t<_Base>
          base() const
          { return _M_end; }
 
          template<bool _Const2>
            requires sentinel_for<sentinel_t<_Base>,
                       iterator_t<__detail::__maybe_const_t<_Const2, _Vp>>>
            friend constexpr bool
            operator==(const _Iterator<_Const2>& __x, const _Sentinel& __y)
            { return __y._M_equal(__x); }
 
          template<bool _Const2,
                   typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
            requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
            friend constexpr range_difference_t<_Base2>
            operator-(const _Iterator<_Const2>& __x, const _Sentinel& __y)
            { return -__y._M_distance_from(__x); }
 
          template<bool _Const2,
                   typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
            requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
            friend constexpr range_difference_t<_Base2>
            operator-(const _Sentinel& __x, const _Iterator<_Const2>& __y)
            { return __x._M_distance_from(__y); }
 
          friend _Sentinel<!_Const>;
        };
 
      _Vp _M_base = _Vp();
    };
 
  template<typename _Tp, size_t _Nm>
    inline constexpr bool enable_borrowed_range<elements_view<_Tp, _Nm>>
      = enable_borrowed_range<_Tp>;
 
  template<typename _Range>
    using keys_view = elements_view<views::all_t<_Range>, 0>;
 
  template<typename _Range>
    using values_view = elements_view<views::all_t<_Range>, 1>;
 
  namespace views
  {
    namespace __detail
    {
      template<size_t _Nm, typename _Range>
        concept __can_elements_view
          = requires { elements_view<all_t<_Range>, _Nm>{std::declval<_Range>()}; };
    } // namespace __detail
 
    template<size_t _Nm>
      struct _Elements : __adaptor::_RangeAdaptorClosure
      {
        template<viewable_range _Range>
          requires __detail::__can_elements_view<_Nm, _Range>
          constexpr auto
          operator()(_Range&& __r) const
          {
            return elements_view<all_t<_Range>, _Nm>{std::forward<_Range>(__r)};
          }
 
        static constexpr bool _S_has_simple_call_op = true;
      };
 
    template<size_t _Nm>
      inline constexpr _Elements<_Nm> elements;
    inline constexpr auto keys = elements<0>;
    inline constexpr auto values = elements<1>;
  } // namespace views
 
} // namespace ranges
 
  namespace views = ranges::views;
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // library concepts
#endif // C++2a
#endif /* _GLIBCXX_RANGES */