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[thirdparty/gcc.git] / libstdc++-v3 / include / std / ranges

// <ranges> -*- C++ -*-

// Copyright (C) 2019-2022 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 <span>
#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> || (is_nothrow_move_constructible_v<_Tp>
				 && is_nothrow_copy_constructible_v<_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)...)
	  { }

	__box(const __box&) = default;
	__box(__box&&) = default;
	__box& operator=(const __box&) requires copyable<_Tp> = default;
	__box& operator=(__box&&) requires copyable<_Tp> = default;

	// When _Tp is nothrow_copy_constructible but not copy_assignable,
	// copy assignment is implemented via destroy-then-copy-construct.
	constexpr __box&
	operator=(const __box& __that) noexcept
	{
	  static_assert(is_nothrow_copy_constructible_v<_Tp>);
	  if (this != std::__addressof(__that))
	    {
	      _M_value.~_Tp();
	      std::construct_at(std::__addressof(_M_value), *__that);
	    }
	  return *this;
	}

	// Likewise for move assignment.
	constexpr __box&
	operator=(__box&& __that) noexcept
	{
	  static_assert(is_nothrow_move_constructible_v<_Tp>);
	  if (this != std::__addressof(__that))
	    {
	      _M_value.~_Tp();
	      std::construct_at(std::__addressof(_M_value), std::move(*__that));
	    }
	  return *this;
	}

	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() requires default_initializable<_Tp> = default;

      constexpr explicit
      single_view(const _Tp& __t)
      noexcept(is_nothrow_copy_constructible_v<_Tp>)
      : _M_value(__t)
      { }

      constexpr explicit
      single_view(_Tp&& __t)
      noexcept(is_nothrow_move_constructible_v<_Tp>)
      : _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)
	noexcept(is_nothrow_constructible_v<_Tp, _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<typename _Winc, typename _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>
      [[nodiscard]]
      constexpr auto
      operator()(_Tp&& __e) const
      noexcept(noexcept(single_view<decay_t<_Tp>>(std::forward<_Tp>(__e))))
      { return single_view<decay_t<_Tp>>(std::forward<_Tp>(__e)); }
  };

  inline constexpr _Single single{};

  struct _Iota
  {
    template<typename _Tp>
      [[nodiscard]]
      constexpr auto
      operator()(_Tp&& __e) const
      { return iota_view(std::forward<_Tp>(__e)); }

    template<typename _Tp, typename _Up>
      [[nodiscard]]
      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:
      constexpr explicit
      basic_istream_view(basic_istream<_CharT, _Traits>& __stream)
	: _M_stream(std::__addressof(__stream))
      { }

      constexpr auto
      begin()
      {
	*_M_stream >> _M_object;
	return _Iterator{this};
      }

      constexpr default_sentinel_t
      end() const noexcept
      { return default_sentinel; }

    private:
      basic_istream<_CharT, _Traits>* _M_stream;
      _Val _M_object = _Val();

      struct _Iterator
      {
      public:
	using iterator_concept = input_iterator_tag;
	using difference_type = ptrdiff_t;
	using value_type = _Val;

	constexpr explicit
	_Iterator(basic_istream_view* __parent) noexcept
	  : _M_parent(__parent)
	{ }

	_Iterator(const _Iterator&) = delete;
	_Iterator(_Iterator&&) = default;
	_Iterator& operator=(const _Iterator&) = delete;
	_Iterator& operator=(_Iterator&&) = default;

	_Iterator&
	operator++()
	{
	  *_M_parent->_M_stream >> _M_parent->_M_object;
	  return *this;
	}

	void
	operator++(int)
	{ ++*this; }

	_Val&
	operator*() const
	{ 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;

	bool
	_M_at_end() const
	{ return !*_M_parent->_M_stream; }
      };

      friend _Iterator;
    };

  template<typename _Val>
    using istream_view = basic_istream_view<_Val, char>;

  template<typename _Val>
    using wistream_view = basic_istream_view<_Val, wchar_t>;

namespace views
{
  template<typename _Tp>
    struct _Istream
    {
      template<typename _CharT, typename _Traits>
	[[nodiscard]]
	constexpr auto
	operator()(basic_istream<_CharT, _Traits>& __e) const
	{ return basic_istream_view<_Tp, _CharT, _Traits>(__e); }
    };

  template<typename _Tp>
    inline constexpr _Istream<_Tp> istream;
}

  // 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;

      static void _S_fun(_Range&); // not defined
      static void _S_fun(_Range&&) = delete;

    public:
      template<__detail::__different_from<ref_view> _Tp>
	requires convertible_to<_Tp, _Range&>
	  && requires { _S_fun(declval<_Tp>()); }
	constexpr
	ref_view(_Tp&& __t)
	noexcept(noexcept(static_cast<_Range&>(std::declval<_Tp>())))
	  : _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<typename _Range>
	static constexpr bool
	_S_noexcept()
	{
	  if constexpr (view<decay_t<_Range>>)
	    return is_nothrow_constructible_v<decay_t<_Range>, _Range>;
	  else if constexpr (__detail::__can_ref_view<_Range>)
	    return true;
	  else
	    return noexcept(subrange{std::declval<_Range>()});
	}

      template<viewable_range _Range>
	requires view<decay_t<_Range>>
	  || __detail::__can_ref_view<_Range>
	  || __detail::__can_subrange<_Range>
	constexpr auto
	operator() [[nodiscard]] (_Range&& __r) const
	noexcept(_S_noexcept<_Range>())
	{
	  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 __non_propagating_cache&
	operator=(_Tp __val)
	{
	  this->_M_reset();
	  this->_M_payload._M_construct(std::move(__val));
	  return *this;
	}

	constexpr explicit
	operator bool() const noexcept
	{ return this->_M_is_engaged(); }

	constexpr _Tp&
	operator*() noexcept
	{ return this->_M_get(); }

	constexpr const _Tp&
	operator*() const noexcept
	{ return this->_M_get(); }

	template<typename _Iter>
	  constexpr _Tp&
	  _M_emplace_deref(const _Iter& __i)
	  {
	    this->_M_reset();
	    auto __f = [] (auto& __x) { return *__x; };
	    this->_M_payload._M_apply(_Optional_func{__f}, __i);
	    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); }
      };

      _Vp _M_base = _Vp();
      [[no_unique_address]] __detail::__box<_Pred> _M_pred;
      [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;

    public:
      filter_view() requires (default_initializable<_Vp>
			      && default_initializable<_Pred>)
	= default;

      constexpr
      filter_view(_Vp __base, _Pred __pred)
	: _M_base(std::move(__base)), _M_pred(std::move(__pred))
      { }

      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() [[nodiscard]] (_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>;
	};

      _Vp _M_base = _Vp();
      [[no_unique_address]] __detail::__box<_Fp> _M_fun;

    public:
      transform_view() requires (default_initializable<_Vp>
				 && default_initializable<_Fp>)
	= default;

      constexpr
      transform_view(_Vp __base, _Fp __fun)
	: _M_base(std::move(__base)), _M_fun(std::move(__fun))
      { }

      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() [[nodiscard]] (_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>;
	};

      _Vp _M_base = _Vp();
      range_difference_t<_Vp> _M_count = 0;

    public:
      take_view() requires default_initializable<_Vp> = default;

      constexpr
      take_view(_Vp base, range_difference_t<_Vp> __count)
	: _M_base(std::move(base)), _M_count(std::move(__count))
      { }

      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>
	inline constexpr bool __is_empty_view = false;

      template<typename _Tp>
	inline constexpr bool __is_empty_view<empty_view<_Tp>> = true;

      template<typename _Range>
	inline constexpr bool __is_basic_string_view = false;

      template<typename _CharT, typename _Traits>
	inline constexpr bool __is_basic_string_view<basic_string_view<_CharT, _Traits>>
	  = true;

      template<typename _Range>
	inline constexpr bool __is_subrange = false;

      template<typename _Iter, typename _Sent, subrange_kind _Kind>
	inline constexpr bool __is_subrange<subrange<_Iter, _Sent, _Kind>> = true;

      template<typename _Range>
	inline constexpr bool __is_iota_view = false;

      template<typename _Winc, typename _Bound>
	inline constexpr bool __is_iota_view<iota_view<_Winc, _Bound>> = true;

      template<typename _Range, typename _Dp>
	concept __can_take_view
	  = requires { take_view(std::declval<_Range>(), std::declval<_Dp>()); };
    } // namespace __detail

    struct _Take : __adaptor::_RangeAdaptor<_Take>
    {
      template<viewable_range _Range, typename _Dp = range_difference_t<_Range>>
	requires __detail::__can_take_view<_Range, _Dp>
	constexpr auto
	operator() [[nodiscard]] (_Range&& __r, type_identity_t<_Dp> __n) const
	{
	  using _Tp = remove_cvref_t<_Range>;
	  if constexpr (__detail::__is_empty_view<_Tp>)
	    return _Tp();
	  else if constexpr (random_access_range<_Tp>
			     && sized_range<_Tp>
			     && (std::__detail::__is_span<_Tp>
				 || __detail::__is_basic_string_view<_Tp>
				 || __detail::__is_subrange<_Tp>
				 || __detail::__is_iota_view<_Tp>))
	    {
	      __n = std::min<_Dp>(ranges::distance(__r), __n);
	      auto __begin = ranges::begin(__r);
	      auto __end = __begin + __n;
	      if constexpr (std::__detail::__is_span<_Tp>)
		return span<typename _Tp::element_type>(__begin, __end);
	      else if constexpr (__detail::__is_basic_string_view<_Tp>)
		return _Tp(__begin, __end);
	      else if constexpr (__detail::__is_subrange<_Tp>)
		return subrange<iterator_t<_Tp>>(__begin, __end);
	      else
		return iota_view(*__begin, *__end);
	    }
	  else
	    return take_view(std::forward<_Range>(__r), __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>;
	};

      _Vp _M_base = _Vp();
      [[no_unique_address]] __detail::__box<_Pred> _M_pred;

    public:
      take_while_view() requires (default_initializable<_Vp>
				  && default_initializable<_Pred>)
	= default;

      constexpr
      take_while_view(_Vp base, _Pred __pred)
	: _M_base(std::move(base)), _M_pred(std::move(__pred))
      { }

      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() [[nodiscard]] (_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:
      _Vp _M_base = _Vp();
      range_difference_t<_Vp> _M_count = 0;

      // 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_base(std::move(__base)), _M_count(__count)
      { __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 _Dp>
	concept __can_drop_view
	  = requires { drop_view(std::declval<_Range>(), std::declval<_Dp>()); };
    } // namespace __detail

    struct _Drop : __adaptor::_RangeAdaptor<_Drop>
    {
      template<viewable_range _Range, typename _Dp = range_difference_t<_Range>>
	requires __detail::__can_drop_view<_Range, _Dp>
	constexpr auto
	operator() [[nodiscard]] (_Range&& __r, type_identity_t<_Dp> __n) const
	{
	  using _Tp = remove_cvref_t<_Range>;
	  if constexpr (__detail::__is_empty_view<_Tp>)
	    return _Tp();
	  else if constexpr (random_access_range<_Tp>
			     && sized_range<_Tp>
			     && (std::__detail::__is_span<_Tp>
				 || __detail::__is_basic_string_view<_Tp>
				 || __detail::__is_iota_view<_Tp>
				 || __detail::__is_subrange<_Tp>))
	    {
	      __n = std::min<_Dp>(ranges::distance(__r), __n);
	      auto __begin = ranges::begin(__r) + __n;
	      auto __end = ranges::end(__r);
	      if constexpr (std::__detail::__is_span<_Tp>)
		return span<typename _Tp::element_type>(__begin, __end);
	      else if constexpr (__detail::__is_subrange<_Tp>)
		{
		  if constexpr (_Tp::_S_store_size)
		    {
		      using ranges::__detail::__to_unsigned_like;
		      auto __m = ranges::distance(__r) - __n;
		      return _Tp(__begin, __end, __to_unsigned_like(__m));
		    }
		  else
		    return _Tp(__begin, __end);
		}
	      else
		return _Tp(__begin, __end);
	    }
	  else
	    return drop_view(std::forward<_Range>(__r), __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:
      _Vp _M_base = _Vp();
      [[no_unique_address]] __detail::__box<_Pred> _M_pred;
      [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;

    public:
      drop_while_view() requires (default_initializable<_Vp>
				  && default_initializable<_Pred>)
	= default;

      constexpr
      drop_while_view(_Vp __base, _Pred __pred)
	: _M_base(std::move(__base)), _M_pred(std::move(__pred))
      { }

      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() [[nodiscard]] (_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>;
	};

      _Vp _M_base = _Vp();
      [[no_unique_address]]
	__detail::__non_propagating_cache<remove_cv_t<_InnerRange>> _M_inner;

    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() [[nodiscard]] (_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 __lazy_split_view_outer_iter_cat
      { };

    template<forward_range _Base>
      struct __lazy_split_view_outer_iter_cat<_Base>
      { using iterator_category = input_iterator_tag; };

    template<typename _Base>
      struct __lazy_split_view_inner_iter_cat
      { };

    template<forward_range _Base>
      struct __lazy_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 lazy_split_view : public view_interface<lazy_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::__lazy_split_view_outer_iter_cat<_Base<_Const>>
	{
	private:
	  using _Parent = __detail::__maybe_const_t<_Const, lazy_split_view>;
	  using _Base = lazy_split_view::_Base<_Const>;

	  constexpr bool
	  __at_end() const
	  { return __current() == ranges::end(_M_parent->_M_base) && !_M_trailing_empty; }

	  // [range.lazy.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;
	  bool _M_trailing_empty = false;

	public:
	  using iterator_concept = __conditional_t<forward_range<_Base>,
						   forward_iterator_tag,
						   input_iterator_tag>;
	  // iterator_category defined in __lazy_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
	    { 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. lazy_split_view::outer-iterator::operator++ misspecified
	    const auto __end = ranges::end(_M_parent->_M_base);
	    if (__current() == __end)
	      {
		_M_trailing_empty = false;
		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();
		    if (__current() == __end)
		      _M_trailing_empty = true;
		  }
	      }
	    else
	      do
		{
		  auto [__b, __p]
		    = ranges::mismatch(__current(), __end, __pbegin, __pend);
		  if (__p == __pend)
		    {
		      __current() = __b;
		      if (__current() == __end)
			_M_trailing_empty = true;
		      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
	      && __x._M_trailing_empty == __y._M_trailing_empty;
	  }

	  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::__lazy_split_view_inner_iter_cat<_Base<_Const>>
	{
	private:
	  using _Base = lazy_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 __lazy_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()); }
	};

      _Vp _M_base = _Vp();
      _Pattern _M_pattern = _Pattern();
      // XXX: _M_current is "present only if !forward_range<V>"
      [[no_unique_address]]
	__detail::__maybe_present_t<!forward_range<_Vp>,
	  __detail::__non_propagating_cache<iterator_t<_Vp>>> _M_current;


    public:
      lazy_split_view() requires (default_initializable<_Vp>
				  && default_initializable<_Pattern>)
	= default;

      constexpr
      lazy_split_view(_Vp __base, _Pattern __pattern)
	: _M_base(std::move(__base)), _M_pattern(std::move(__pattern))
      { }

      template<input_range _Range>
	requires constructible_from<_Vp, views::all_t<_Range>>
	  && constructible_from<_Pattern, single_view<range_value_t<_Range>>>
	constexpr
	lazy_split_view(_Range&& __r, range_value_t<_Range> __e)
	  : _M_base(views::all(std::forward<_Range>(__r))),
	    _M_pattern(views::single(std::move(__e)))
	{ }

      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>
    lazy_split_view(_Range&&, _Pattern&&)
      -> lazy_split_view<views::all_t<_Range>, views::all_t<_Pattern>>;

  template<input_range _Range>
    lazy_split_view(_Range&&, range_value_t<_Range>)
      -> lazy_split_view<views::all_t<_Range>, single_view<range_value_t<_Range>>>;

  namespace views
  {
    namespace __detail
    {
      template<typename _Range, typename _Pattern>
	concept __can_lazy_split_view
	  = requires { lazy_split_view(std::declval<_Range>(), std::declval<_Pattern>()); };
    } // namespace __detail

    struct _LazySplit : __adaptor::_RangeAdaptor<_LazySplit>
    {
      template<viewable_range _Range, typename _Pattern>
	requires __detail::__can_lazy_split_view<_Range, _Pattern>
	constexpr auto
	operator() [[nodiscard]] (_Range&& __r, _Pattern&& __f) const
	{
	  return lazy_split_view(std::forward<_Range>(__r), std::forward<_Pattern>(__f));
	}

      using _RangeAdaptor<_LazySplit>::operator();
      static constexpr int _S_arity = 2;
      // The pattern argument of views::lazy_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 _LazySplit lazy_split;
  } // namespace views

  template<forward_range _Vp, forward_range _Pattern>
    requires view<_Vp> && view<_Pattern>
      && indirectly_comparable<iterator_t<_Vp>, iterator_t<_Pattern>,
			       ranges::equal_to>
  class split_view : public view_interface<split_view<_Vp, _Pattern>>
  {
  private:
    _Vp _M_base = _Vp();
    _Pattern _M_pattern = _Pattern();
    __detail::__non_propagating_cache<subrange<iterator_t<_Vp>>> _M_cached_begin;

    struct _Iterator;
    struct _Sentinel;

  public:
    split_view() requires (default_initializable<_Vp>
			   && default_initializable<_Pattern>)
      = default;

    constexpr
    split_view(_Vp __base, _Pattern __pattern)
      : _M_base(std::move(__base)), _M_pattern(std::move(__pattern))
    { }

    template<forward_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_base(views::all(std::forward<_Range>(__r))),
	_M_pattern(views::single(std::move(__e)))
    { }

    constexpr _Vp
    base() const& requires copy_constructible<_Vp>
    { return _M_base; }

    constexpr _Vp
    base() &&
    { return std::move(_M_base); }

    constexpr _Iterator
    begin()
    {
      if (!_M_cached_begin)
	_M_cached_begin = _M_find_next(ranges::begin(_M_base));
      return {this, ranges::begin(_M_base), *_M_cached_begin};
    }

    constexpr auto
    end()
    {
      if constexpr (common_range<_Vp>)
	return _Iterator{this, ranges::end(_M_base), {}};
      else
	return _Sentinel{this};
    }

    constexpr subrange<iterator_t<_Vp>>
    _M_find_next(iterator_t<_Vp> __it)
    {
      auto [__b, __e] = ranges::search(subrange(__it, ranges::end(_M_base)), _M_pattern);
      if (__b != ranges::end(_M_base) && ranges::empty(_M_pattern))
	{
	  ++__b;
	  ++__e;
	}
      return {__b, __e};
    }

  private:
    struct _Iterator
    {
    private:
      split_view* _M_parent = nullptr;
      iterator_t<_Vp> _M_cur = iterator_t<_Vp>();
      subrange<iterator_t<_Vp>> _M_next = subrange<iterator_t<_Vp>>();
      bool _M_trailing_empty = false;

      friend struct _Sentinel;

    public:
      using iterator_concept = forward_iterator_tag;
      using iterator_category = input_iterator_tag;
      using value_type = subrange<iterator_t<_Vp>>;
      using difference_type = range_difference_t<_Vp>;

      _Iterator() = default;

      constexpr
      _Iterator(split_view* __parent,
		iterator_t<_Vp> __current,
		subrange<iterator_t<_Vp>> __next)
	: _M_parent(__parent),
	  _M_cur(std::move(__current)),
	  _M_next(std::move(__next))
      { }

      constexpr iterator_t<_Vp>
      base() const
      { return _M_cur; }

      constexpr value_type
      operator*() const
      { return {_M_cur, _M_next.begin()}; }

      constexpr _Iterator&
      operator++()
      {
	_M_cur = _M_next.begin();
	if (_M_cur != ranges::end(_M_parent->_M_base))
	  {
	    _M_cur = _M_next.end();
	    if (_M_cur == ranges::end(_M_parent->_M_base))
	      {
		_M_trailing_empty = true;
		_M_next = {_M_cur, _M_cur};
	      }
	    else
	      _M_next = _M_parent->_M_find_next(_M_cur);
	  }
	else
	  _M_trailing_empty = false;
	return *this;
      }

      constexpr _Iterator
      operator++(int)
      {
	auto __tmp = *this;
	++*this;
	return __tmp;
      }

      friend constexpr bool
      operator==(const _Iterator& __x, const _Iterator& __y)
      {
	return __x._M_cur == __y._M_cur
	  && __x._M_trailing_empty == __y._M_trailing_empty;
      }
    };

    struct _Sentinel
    {
    private:
      sentinel_t<_Vp> _M_end = sentinel_t<_Vp>();

      constexpr bool
      _M_equal(const _Iterator& __x) const
      { return __x._M_cur == _M_end && !__x._M_trailing_empty; }

    public:
      _Sentinel() = default;

      constexpr explicit
      _Sentinel(split_view* __parent)
	: _M_end(ranges::end(__parent->_M_base))
      { }

      friend constexpr bool
      operator==(const _Iterator& __x, const _Sentinel& __y)
      { return __y._M_equal(__x); }
    };
  };

  template<typename _Range, typename _Pattern>
    split_view(_Range&&, _Pattern&&)
      -> split_view<views::all_t<_Range>, views::all_t<_Pattern>>;

  template<forward_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() [[nodiscard]] (_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;
      template<typename _Pattern>
	static constexpr bool _S_has_simple_extra_args
	  = _LazySplit::_S_has_simple_extra_args<_Pattern>;
    };

    inline constexpr _Split split;
  } // namespace views

  namespace views
  {
    struct _Counted
    {
      template<input_or_output_iterator _Iter>
      constexpr auto
      operator() [[nodiscard]] (_Iter __i, iter_difference_t<_Iter> __n) const
      {
	if constexpr (contiguous_iterator<_Iter>)
	  return span(std::__to_address(__i), __n);
	else 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() [[nodiscard]] (_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>
				  && sized_sentinel_for<sentinel_t<_Vp>,
							iterator_t<_Vp>>);

      _Vp _M_base = _Vp();
      [[no_unique_address]]
	__detail::__maybe_present_t<_S_needs_cached_begin,
				    __detail::_CachedPosition<_Vp>>
				      _M_cached_begin;

    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() [[nodiscard]] (_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() [[nodiscard]] (_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 */