ranges

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// <ranges> -*- C++ -*-
 
// Copyright (C) 2019-2026 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
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <concepts>
 
#if __cpp_lib_concepts
 
#include <compare>
#include <initializer_list>
#include <iterator>
#include <optional>
#include <span>
#include <string_view>
#include <tuple>
#if __cplusplus > 202002L
#include <utility>
#include <variant>
#endif
#include <bits/binders.h>
#include <bits/ranges_util.h>
#include <bits/refwrap.h>
 
#define __glibcxx_want_algorithm_default_value_type
#define __glibcxx_want_ranges
#define __glibcxx_want_ranges_as_const
#define __glibcxx_want_ranges_as_rvalue
#define __glibcxx_want_ranges_cache_latest
#define __glibcxx_want_ranges_cartesian_product
#define __glibcxx_want_ranges_concat
#define __glibcxx_want_ranges_chunk
#define __glibcxx_want_ranges_chunk_by
#define __glibcxx_want_ranges_enumerate
#define __glibcxx_want_ranges_filter
#define __glibcxx_want_ranges_indices
#define __glibcxx_want_ranges_join_with
#define __glibcxx_want_ranges_repeat
#define __glibcxx_want_ranges_slide
#define __glibcxx_want_ranges_stride
#define __glibcxx_want_ranges_to_container
#define __glibcxx_want_ranges_as_input
#define __glibcxx_want_ranges_zip
#include <bits/version.h>
 
#ifdef __glibcxx_generator  // C++ >= 23 && __glibcxx_coroutine
# include <bits/elements_of.h>
#endif
 
/**
 * @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
  {
#if __cpp_lib_ranges >= 202207L // C++ >= 23
    // P2494R2 Relaxing range adaptors to allow for move only types
    template<typename _Tp>
      concept __boxable = move_constructible<_Tp> && is_object_v<_Tp>;
#else
    template<typename _Tp>
      concept __boxable = copy_constructible<_Tp> && is_object_v<_Tp>;
#endif
 
    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
        // 3572. copyable-box should be fully constexpr
        constexpr __box&
        operator=(const __box& __that)
        noexcept(is_nothrow_copy_constructible_v<_Tp>)
        requires (!copyable<_Tp>) && copy_constructible<_Tp>
        {
          if (this != std::__addressof(__that))
            {
              if ((bool)__that)
                this->emplace(*__that);
              else
                this->reset();
            }
          return *this;
        }
 
        constexpr __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;
        }
      };
 
    template<typename _Tp>
      concept __boxable_copyable
        = copy_constructible<_Tp>
            && (copyable<_Tp> || (is_nothrow_move_constructible_v<_Tp>
                                    && is_nothrow_copy_constructible_v<_Tp>));
    template<typename _Tp>
      concept __boxable_movable
        = (!copy_constructible<_Tp>)
            && (movable<_Tp> || is_nothrow_move_constructible_v<_Tp>);
 
    // For types which are already copyable (or since C++23, movable)
    // this specialization of the box 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 __boxable_copyable<_Tp> || __boxable_movable<_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>)
        requires copy_constructible<_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 movable<_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
        requires (!copyable<_Tp>) && copy_constructible<_Tp>
        {
          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
        requires (!movable<_Tp>)
        {
          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::move(_M_value); }
 
        constexpr const _Tp&&
        operator*() const && noexcept
        { return std::move(_M_value); }
 
        constexpr _Tp*
        operator->() noexcept
        { return std::__addressof(_M_value); }
 
        constexpr const _Tp*
        operator->() const noexcept
        { return std::__addressof(_M_value); }
      };
 
      namespace __func_handle
      {
        template<typename _Fn>
          struct _Inplace
          {
            _Inplace() = default;
 
            constexpr explicit
            _Inplace(_Fn __func) noexcept
            : _M_fn(__func)
            { }
 
            template<typename... _Iters>
              constexpr decltype(auto)
              _M_call_deref(const _Iters&... __iters) const
              noexcept(noexcept(_M_fn(*__iters...)))
              { return _M_fn(*__iters...); }
 
            template<typename _DistType, typename... _Iters>
              constexpr decltype(auto)
              _M_call_subscript(const _DistType __n, const _Iters&... __iters) const
              noexcept(noexcept(_M_fn(__iters[iter_difference_t<_Iters>(__n)]...)))
              { return _M_fn(__iters[iter_difference_t<_Iters>(__n)]...); }
 
          private:
            [[no_unique_address]] _Fn _M_fn = _Fn();
          };
 
        template<typename _Fn>
          struct _InplaceMemPtr
          {
            _InplaceMemPtr() = default;
 
            constexpr explicit
            _InplaceMemPtr(_Fn __func) noexcept
            : _M_ptr(__func)
            {}
 
            template<typename... _Iters>
            constexpr decltype(auto)
              _M_call_deref(const _Iters&... __iters) const
              noexcept(noexcept(std::__invoke(_M_ptr, *__iters...)))
              { return std::__invoke(_M_ptr, *__iters...); }
 
            template<typename _DistType, typename... _Iters>
              constexpr decltype(auto)
              _M_call_subscript(const _DistType __n, const _Iters&... __iters) const
              noexcept(noexcept(std::__invoke(_M_ptr, __iters[iter_difference_t<_Iters>(__n)]...)))
              { return std::__invoke(_M_ptr, __iters[iter_difference_t<_Iters>(__n)]...); }
 
          private:
            _Fn _M_ptr = nullptr;
          };
 
        template<typename _Fn>
          struct _ViaPointer
          {
            _ViaPointer() = default;
 
            constexpr explicit
            _ViaPointer(_Fn& __func) noexcept
            : _M_ptr(std::addressof(__func))
            { }
 
            template<typename _Un>
              requires (!is_const_v<_Un>) && is_same_v<const _Un, _Fn>
              constexpr
              _ViaPointer(_ViaPointer<_Un> __other) noexcept
              : _M_ptr(__other._M_ptr)
              { }
 
            template<typename... _Iters>
              constexpr decltype(auto)
              _M_call_deref(const _Iters&... __iters) const
              noexcept(noexcept((*_M_ptr)(*__iters...)))
              { return (*_M_ptr)(*__iters...); }
 
            template<typename _DistType, typename... _Iters>
              constexpr decltype(auto)
              _M_call_subscript(const _DistType __n, const _Iters&... __iters) const
              noexcept(noexcept((*_M_ptr)(__iters[iter_difference_t<_Iters>(__n)]...)))
              { return (*_M_ptr)(__iters[iter_difference_t<_Iters>(__n)]...); }
 
          private:
            _Fn* _M_ptr = nullptr;
 
            template<typename>
              friend struct _ViaPointer;
          };
 
        template<typename _Fn>
          struct _StaticCall
          {
            _StaticCall() = default;
 
            constexpr explicit
            _StaticCall(const _Fn&) noexcept
            {}
 
            template<typename... _Iters>
             static constexpr decltype(auto)
              _M_call_deref(const _Iters&... __iters)
              noexcept(noexcept(_Fn::operator()(*__iters...)))
              { return _Fn::operator()(*__iters...); }
 
            template<typename _DistType, typename... _Iters>
              static constexpr decltype(auto)
              _M_call_subscript(_DistType __n, const _Iters&... __iters)
              noexcept(noexcept(_Fn::operator()(__iters[iter_difference_t<_Iters>(__n)]...)))
              { return _Fn::operator()(__iters[iter_difference_t<_Iters>(__n)]...); }
          };
 
          template<typename _Fn, typename... _Iters>
            consteval auto
            __select()
            {
              using _Fd = remove_cv_t<_Fn>;
              if constexpr (is_member_pointer_v<_Fd>)
                return __func_handle::_InplaceMemPtr<_Fd>();
              else if constexpr (is_function_v<remove_pointer_t<_Fd>>)
                return __func_handle::_Inplace<_Fd>();
              else if constexpr (__is_std_op_wrapper<_Fd>)
               return __func_handle::_Inplace<_Fd>();
             else if constexpr (requires (const _Iters&... __iters)
                                { _Fd::operator()(*__iters...); })
               return __func_handle::_StaticCall<_Fd>();
             else
               return __func_handle::_ViaPointer<_Fn>();
           };
      } // __func_handle
 
      template<typename _Fn, typename... _Iters>
        using __func_handle_t = decltype(__func_handle::__select<_Fn, _Iters...>());
  } // namespace __detail
 
  /// A view that contains exactly one element.
#if __cpp_lib_ranges >= 202207L // C++ >= 23
  template<move_constructible _Tp>
#else
  template<copy_constructible _Tp>
#endif
    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>)
      requires copy_constructible<_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; }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 4035. single_view should provide empty
      static constexpr bool
      empty() noexcept
      { return false; }
 
      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:
        _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 __x._M_value == __y._M_bound; }
 
        friend constexpr iter_difference_t<_Winc>
        operator-(const _Iterator& __x, const _Sentinel& __y)
          requires sized_sentinel_for<_Bound, _Winc>
        { return -(__y._M_bound - __x._M_value); }
 
        friend constexpr iter_difference_t<_Winc>
        operator-(const _Sentinel& __x, const _Iterator& __y)
          requires sized_sentinel_for<_Bound, _Winc>
        { return __x._M_bound - __y._M_value; }
 
        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 explicit
      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 explicit
      iota_view(_Iterator __first, _Iterator __last)
        requires same_as<_Winc, _Bound>
        : iota_view(__first._M_value, __last._M_value)
      { }
 
      constexpr explicit
      iota_view(_Iterator __first, unreachable_sentinel_t __last)
        requires same_as<_Bound, unreachable_sentinel_t>
        : iota_view(__first._M_value, __last)
      { }
 
      constexpr explicit
      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}; }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 4001. iota_view should provide empty
      constexpr bool
      empty() const
      { return _M_value == _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<_Bo