chrono

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// <chrono> -*- C++ -*-
 
// Copyright (C) 2008-2025 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/>.
 
// [time]
 
/** @file include/chrono
 *  This is a Standard C++ Library header.
 *  @ingroup chrono
 */
 
#ifndef _GLIBCXX_CHRONO
#define _GLIBCXX_CHRONO 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#ifdef _GLIBCXX_NO_FREESTANDING_CHRONO
# include <bits/requires_hosted.h> // for <ctime> and clocks
#endif
 
#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else
 
#define __glibcxx_want_chrono
#define __glibcxx_want_chrono_udls
#include <bits/version.h>
 
#include <bits/chrono.h>
 
#if __cpp_lib_bitops >= 201907L
# include <bit> // __countr_zero
#endif
#ifdef __glibcxx_chrono_cxx20
# include <bits/stl_algo.h>     // upper_bound
# include <bits/range_access.h> // begin/end for arrays
#endif
#if __cpp_lib_chrono >= 201803L // C++20 && HOSTED && USE_CXX11_ABI
# include <sstream>
# include <string>
# include <vector>
# include <bits/shared_ptr.h>
# include <bits/unique_ptr.h>
#endif
#if __glibcxx_chrono_cxx20 >= 202306L // C++26
# include <bits/functional_hash.h>
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   * @defgroup chrono Time
   * @ingroup utilities
   *
   * Classes and functions for time.
   *
   * @since C++11
   */
 
  /** @namespace std::chrono
   *  @brief ISO C++ 2011 namespace for date and time utilities
   *  @ingroup chrono
   */
  namespace chrono
  {
#ifdef __glibcxx_chrono_cxx20
    /// @addtogroup chrono
    /// @{
    struct local_t { };
    template<typename _Duration>
      using local_time = time_point<local_t, _Duration>;
    using local_seconds = local_time<seconds>;
    using local_days = local_time<days>;
 
#if _GLIBCXX_HOSTED
    class utc_clock;
    class tai_clock;
    class gps_clock;
 
    template<typename _Duration>
      using utc_time = time_point<utc_clock, _Duration>;
    using utc_seconds = utc_time<seconds>;
 
    template<typename _Duration>
      using tai_time = time_point<tai_clock, _Duration>;
    using tai_seconds = tai_time<seconds>;
 
    template<typename _Duration>
      using gps_time = time_point<gps_clock, _Duration>;
    using gps_seconds = gps_time<seconds>;
 
    template<> struct is_clock<utc_clock> : true_type { };
    template<> struct is_clock<tai_clock> : true_type { };
    template<> struct is_clock<gps_clock> : true_type { };
 
    template<> inline constexpr bool is_clock_v<utc_clock> = true;
    template<> inline constexpr bool is_clock_v<tai_clock> = true;
    template<> inline constexpr bool is_clock_v<gps_clock> = true;
 
    struct leap_second_info
    {
      bool is_leap_second;
      seconds elapsed;
    };
 
    template<typename _Duration>
      leap_second_info
      get_leap_second_info(const utc_time<_Duration>& __ut);
 
    /** A clock that measures Universal Coordinated Time (UTC).
     *
     * The epoch is 1970-01-01 00:00:00.
     *
     * @since C++20
     */
    class utc_clock
    {
    public:
      using rep                       = system_clock::rep;
      using period                    = system_clock::period;
      using duration                  = chrono::duration<rep, period>;
      using time_point                = chrono::time_point<utc_clock>;
      static constexpr bool is_steady = false;
 
      [[nodiscard]]
      static time_point
      now()
      { return from_sys(system_clock::now()); }
 
      template<typename _Duration>
        [[nodiscard]]
        static sys_time<common_type_t<_Duration, seconds>>
        to_sys(const utc_time<_Duration>& __t)
        {
          using _CDur = common_type_t<_Duration, seconds>;
          const auto __li = chrono::get_leap_second_info(__t);
          sys_time<_CDur> __s{__t.time_since_epoch() - __li.elapsed};
          if (__li.is_leap_second)
            __s = chrono::floor<seconds>(__s) + seconds{1} - _CDur{1};
          return __s;
        }
 
      template<typename _Duration>
        [[nodiscard]]
        static utc_time<common_type_t<_Duration, seconds>>
        from_sys(const sys_time<_Duration>& __t);
    };
 
    /** A clock that measures International Atomic Time.
     *
     * The epoch is 1958-01-01 00:00:00.
     *
     * @since C++20
     */
    class tai_clock
    {
    public:
      using rep                       = system_clock::rep;
      using period                    = system_clock::period;
      using duration                  = chrono::duration<rep, period>;
      using time_point                = chrono::time_point<tai_clock>;
      static constexpr bool is_steady = false;
 
      [[nodiscard]]
      static time_point
      now();  // in src/c++20/clock.cc
 
      template<typename _Duration>
        [[nodiscard]]
        static utc_time<common_type_t<_Duration, seconds>>
        to_utc(const tai_time<_Duration>& __t)
        {
          using _CDur = common_type_t<_Duration, seconds>;
          return utc_time<_CDur>{__t.time_since_epoch()} - 378691210s;
        }
 
      template<typename _Duration>
        [[nodiscard]]
        static tai_time<common_type_t<_Duration, seconds>>
        from_utc(const utc_time<_Duration>& __t)
        {
          using _CDur = common_type_t<_Duration, seconds>;
          return tai_time<_CDur>{__t.time_since_epoch()} + 378691210s;
        }
    };
 
    /** A clock that measures GPS time.
     *
     * The epoch is 1980-01-06 00:00:00.
     *
     * @since C++20
     */
    class gps_clock
    {
    public:
      using rep                       = system_clock::rep;
      using period                    = system_clock::period;
      using duration                  = chrono::duration<rep, period>;
      using time_point                = chrono::time_point<gps_clock>;
      static constexpr bool is_steady = false;
 
      [[nodiscard]]
      static time_point
      now();  // in src/c++20/clock.cc
 
      template<typename _Duration>
        [[nodiscard]]
        static utc_time<common_type_t<_Duration, seconds>>
        to_utc(const gps_time<_Duration>& __t)
        {
          using _CDur = common_type_t<_Duration, seconds>;
          return utc_time<_CDur>{__t.time_since_epoch()} + 315964809s;
        }
 
      template<typename _Duration>
        [[nodiscard]]
        static gps_time<common_type_t<_Duration, seconds>>
        from_utc(const utc_time<_Duration>& __t)
        {
          using _CDur = common_type_t<_Duration, seconds>;
          return gps_time<_CDur>{__t.time_since_epoch()} - 315964809s;
        }
    };
#endif // _GLIBCXX_HOSTED
 
    template<typename _DestClock, typename _SourceClock>
      struct clock_time_conversion
      { };
 
    // Identity conversions
 
    template<typename _Clock>
      struct clock_time_conversion<_Clock, _Clock>
      {
        template<typename _Duration>
          time_point<_Clock, _Duration>
          operator()(const time_point<_Clock, _Duration>& __t) const
          { return __t; }
      };
 
#if _GLIBCXX_HOSTED
    template<>
      struct clock_time_conversion<system_clock, system_clock>
      {
        template<typename _Duration>
          sys_time<_Duration>
          operator()(const sys_time<_Duration>& __t) const
          { return __t; }
      };
 
    template<>
      struct clock_time_conversion<utc_clock, utc_clock>
      {
        template<typename _Duration>
          utc_time<_Duration>
          operator()(const utc_time<_Duration>& __t) const
          { return __t; }
      };
 
    // Conversions between system_clock and utc_clock
 
    template<>
      struct clock_time_conversion<utc_clock, system_clock>
      {
        template<typename _Duration>
          utc_time<common_type_t<_Duration, seconds>>
          operator()(const sys_time<_Duration>& __t) const
          { return utc_clock::from_sys(__t); }
      };
 
    template<>
      struct clock_time_conversion<system_clock, utc_clock>
      {
        template<typename _Duration>
          sys_time<common_type_t<_Duration, seconds>>
          operator()(const utc_time<_Duration>& __t) const
          { return utc_clock::to_sys(__t); }
      };
 
    template<typename _Tp, typename _Clock>
      inline constexpr bool __is_time_point_for_v = false;
 
    template<typename _Clock, typename _Duration>
      inline constexpr bool
       __is_time_point_for_v<time_point<_Clock, _Duration>, _Clock> = true;
 
    // Conversions between system_clock and other clocks
 
    template<typename _SourceClock>
      struct clock_time_conversion<system_clock, _SourceClock>
      {
        template<typename _Duration, typename _Src = _SourceClock>
          auto
          operator()(const time_point<_SourceClock, _Duration>& __t) const
          -> decltype(_Src::to_sys(__t))
          {
            using _Ret = decltype(_SourceClock::to_sys(__t));
            static_assert(__is_time_point_for_v<_Ret, system_clock>);
            return _SourceClock::to_sys(__t);
          }
      };
 
    template<typename _DestClock>
      struct clock_time_conversion<_DestClock, system_clock>
      {
        template<typename _Duration, typename _Dest = _DestClock>
          auto
          operator()(const sys_time<_Duration>& __t) const
          -> decltype(_Dest::from_sys(__t))
          {
            using _Ret = decltype(_DestClock::from_sys(__t));
            static_assert(__is_time_point_for_v<_Ret, _DestClock>);
            return _DestClock::from_sys(__t);
          }
      };
 
    // Conversions between utc_clock and other clocks
 
    template<typename _SourceClock>
      struct clock_time_conversion<utc_clock, _SourceClock>
      {
        template<typename _Duration, typename _Src = _SourceClock>
          auto
          operator()(const time_point<_SourceClock, _Duration>& __t) const
          -> decltype(_Src::to_utc(__t))
          {
            using _Ret = decltype(_SourceClock::to_utc(__t));
            static_assert(__is_time_point_for_v<_Ret, utc_clock>);
            return _SourceClock::to_utc(__t);
          }
      };
 
    template<typename _DestClock>
      struct clock_time_conversion<_DestClock, utc_clock>
      {
        template<typename _Duration, typename _Dest = _DestClock>
          auto
          operator()(const utc_time<_Duration>& __t) const
          -> decltype(_Dest::from_utc(__t))
          {
            using _Ret = decltype(_DestClock::from_utc(__t));
            static_assert(__is_time_point_for_v<_Ret, _DestClock>);
            return _DestClock::from_utc(__t);
          }
      };
#endif // _GLIBCXX_HOSTED
 
    /// @cond undocumented
    namespace __detail
    {
      template<typename _DestClock, typename _SourceClock, typename _Duration>
       concept __clock_convs
          = requires (const time_point<_SourceClock, _Duration>& __t) {
            clock_time_conversion<_DestClock, _SourceClock>{}(__t);
          };
 
#if _GLIBCXX_HOSTED
      template<typename _DestClock, typename _SourceClock, typename _Duration>
       concept __clock_convs_sys
          = requires (const time_point<_SourceClock, _Duration>& __t) {
            clock_time_conversion<_DestClock, system_clock>{}(
              clock_time_conversion<system_clock, _SourceClock>{}(__t));
          };
 
      template<typename _DestClock, typename _SourceClock, typename _Duration>
       concept __clock_convs_utc
          = requires (const time_point<_SourceClock, _Duration>& __t) {
            clock_time_conversion<_DestClock, utc_clock>{}(
              clock_time_conversion<utc_clock, _SourceClock>{}(__t));
          };
 
      template<typename _DestClock, typename _SourceClock, typename _Duration>
        concept __clock_convs_sys_utc
          = requires (const time_point<_SourceClock, _Duration>& __t) {
            clock_time_conversion<_DestClock, utc_clock>{}(
              clock_time_conversion<utc_clock, system_clock>{}(
                clock_time_conversion<system_clock, _SourceClock>{}(__t)));
          };
 
      template<typename _DestClock, typename _SourceClock, typename _Duration>
       concept __clock_convs_utc_sys
          = requires (const time_point<_SourceClock, _Duration>& __t) {
            clock_time_conversion<_DestClock, system_clock>{}(
              clock_time_conversion<system_clock, utc_clock>{}(
                clock_time_conversion<utc_clock, _SourceClock>{}(__t)));
          };
#endif // _GLIBCXX_HOSTED
    } // namespace __detail
    /// @endcond
 
    /// Convert a time point to a different clock.
    template<typename _DestClock, typename _SourceClock, typename _Duration>
      [[nodiscard]]
      inline auto
      clock_cast(const time_point<_SourceClock, _Duration>& __t)
      requires __detail::__clock_convs<_DestClock, _SourceClock, _Duration>
#if _GLIBCXX_HOSTED
        || __detail::__clock_convs_sys<_DestClock, _SourceClock, _Duration>
        || __detail::__clock_convs_utc<_DestClock, _SourceClock, _Duration>
        || __detail::__clock_convs_sys_utc<_DestClock, _SourceClock, _Duration>
        || __detail::__clock_convs_utc_sys<_DestClock, _SourceClock, _Duration>
#endif // _GLIBCXX_HOSTED
      {
       constexpr bool __direct
         = __detail::__clock_convs<_DestClock, _SourceClock, _Duration>;
       if constexpr (__direct)
         {
           return clock_time_conversion<_DestClock, _SourceClock>{}(__t);
         }
#if _GLIBCXX_HOSTED
       else
         {
           constexpr bool __convert_via_sys_clock
             = __detail::__clock_convs_sys<_DestClock, _SourceClock, _Duration>;
           constexpr bool __convert_via_utc_clock
             = __detail::__clock_convs_utc<_DestClock, _SourceClock, _Duration>;
           if constexpr (__convert_via_sys_clock)
             {
               static_assert(!__convert_via_utc_clock,
                 "clock_cast requires a unique best conversion, but "
                 "conversion is possible via system_clock and also via"
                 "utc_clock");
               return clock_time_conversion<_DestClock, system_clock>{}(
                        clock_time_conversion<system_clock, _SourceClock>{}(__t));
             }
           else if constexpr (__convert_via_utc_clock)
             {
               return clock_time_conversion<_DestClock, utc_clock>{}(
                        clock_time_conversion<utc_clock, _SourceClock>{}(__t));
             }
           else
             {
               constexpr bool __convert_via_sys_and_utc_clocks
                 = __detail::__clock_convs_sys_utc<_DestClock,
                                                   _SourceClock,
                                                   _Duration>;
 
               if constexpr (__convert_via_sys_and_utc_clocks)
                 {
                   constexpr bool __convert_via_utc_and_sys_clocks
                     = __detail::__clock_convs_utc_sys<_DestClock,
                                                       _SourceClock,
                                                       _Duration>;
                   static_assert(!__convert_via_utc_and_sys_clocks,
                     "clock_cast requires a unique best conversion, but "
                     "conversion is possible via system_clock followed by "
                     "utc_clock, and also via utc_clock followed by "
                     "system_clock");
                   return clock_time_conversion<_DestClock, utc_clock>{}(
                            clock_time_conversion<utc_clock, system_clock>{}(
                              clock_time_conversion<system_clock, _SourceClock>{}(__t)));
                 }
               else
                 {
                   return clock_time_conversion<_DestClock, system_clock>{}(
                            clock_time_conversion<system_clock, utc_clock>{}(
                              clock_time_conversion<utc_clock, _SourceClock>{}(__t)));
                 }
             }
         }
#endif // _GLIBCXX_HOSTED
      }
 
    // CALENDRICAL TYPES
 
    // CLASS DECLARATIONS
    class day;
    class month;
    class year;
    class weekday;
    class weekday_indexed;
    class weekday_last;
    class month_day;
    class month_day_last;
    class month_weekday;
    class month_weekday_last;
    class year_month;
    class year_month_day;
    class year_month_day_last;
    class year_month_weekday;
    class year_month_weekday_last;
 
    struct last_spec
    {
      explicit last_spec() = default;
 
      friend constexpr month_day_last
      operator/(int __m, last_spec) noexcept;
 
      friend constexpr month_day_last
      operator/(last_spec, int __m) noexcept;
    };
 
    inline constexpr last_spec last{};
 
    namespace __detail
    {
      // Helper to __add_modulo and __sub_modulo.
      template <unsigned __d, typename _Tp>
      consteval auto
      __modulo_offset()
      {
        using _Up = make_unsigned_t<_Tp>;
        auto constexpr __a = _Up(-1) - _Up(255 + __d - 2);
        auto constexpr __b = _Up(__d * (__a / __d) - 1);
        // Notice: b <= a - 1 <= _Up(-1) - (255 + d - 1) and b % d = d - 1.
        return _Up(-1) - __b; // >= 255 + d - 1
      }
 
      // Compute the remainder of the Euclidean division of __x + __y divided by
      // __d without overflowing.  Typically, __x <= 255 + d - 1 is sum of
      // weekday/month with a shift in [0, d - 1] and __y is a duration count.
      template <unsigned __d, typename _Tp>
      constexpr unsigned
      __add_modulo(unsigned __x, _Tp __y)
      {
        using _Up = make_unsigned_t<_Tp>;
        // For __y >= 0, _Up(__y) has the same mathematical value as __y and
        // this function simply returns (__x + _Up(__y)) % d.  Typically, this
        // doesn't overflow since the range of _Up contains many more positive
        // values than _Tp's.  For __y < 0, _Up(__y) has a mathematical value in
        // the upper-half range of _Up so that adding a positive value to it
        // might overflow.  Moreover, most likely, _Up(__y) != __y mod d.  To
        // fix both issues we subtract from _Up(__y) an __offset >=
        // 255 + d - 1 to make room for the addition to __x and shift the modulo
        // to the correct value.
        auto const __offset = __y >= 0 ? _Up(0) : __modulo_offset<__d, _Tp>();
        return (__x + _Up(__y) - __offset) % __d;
      }
 
      // Similar to __add_modulo but for __x - __y.
      template <unsigned __d, typename _Tp>
      constexpr unsigned
      __sub_modulo(unsigned __x, _Tp __y)
      {
        using _Up = make_unsigned_t<_Tp>;
        auto const __offset = __y <= 0 ? _Up(0) : __modulo_offset<__d, _Tp>();
        return (__x - _Up(__y) - __offset) % __d;
      }
 
      inline constexpr unsigned __days_per_month[12]
        = { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
    }
 
    // DAY
 
    class day
    {
    private:
      unsigned char _M_d;
 
    public:
      day() = default;
 
      explicit constexpr
      day(unsigned __d) noexcept
      : _M_d(__d)
      { }
 
      constexpr day&
      operator++() noexcept
      {
        ++_M_d;
        return *this;
      }
 
      constexpr day
      operator++(int) noexcept
      {
        auto __ret = *this;
        ++(*this);
        return __ret;
      }
 
      constexpr day&
      operator--() noexcept
      {
        --_M_d;
        return *this;
      }
 
      constexpr day
      operator--(int) noexcept
      {
        auto __ret = *this;
        --(*this);
        return __ret;
      }
 
      constexpr day&
      operator+=(const days& __d) noexcept
      {
        *this = *this + __d;
        return *this;
      }
 
      constexpr day&
      operator-=(const days& __d) noexcept
      {
        *this = *this - __d;
        return *this;
      }
 
      constexpr explicit
      operator unsigned() const noexcept
      { return _M_d; }
 
      constexpr bool
      ok() const noexcept
      { return 1 <= _M_d && _M_d <= 31; }
 
      friend constexpr bool
      operator==(const day& __x, const day& __y) noexcept
      { return unsigned{__x} == unsigned{__y}; }
 
      friend constexpr strong_ordering
      operator<=>(const day& __x, const day& __y) noexcept
      { return unsigned{__x} <=> unsigned{__y}; }
 
      friend constexpr day
      operator+(const day& __x, const days& __y) noexcept
      { return day(unsigned{__x} + __y.count()); }
 
      friend constexpr day
      operator+(const days& __x, const day& __y) noexcept
      { return __y + __x; }
 
      friend constexpr day
      operator-(const day& __x, const days& __y) noexcept
      { return __x + -__y; }
 
      friend constexpr days
      operator-(const day& __x, const day& __y) noexcept
      { return days{int(unsigned{__x}) - int(unsigned{__y})}; }
 
      friend constexpr month_day
      operator/(const month& __m, const day& __d) noexcept;
 
      friend constexpr month_day
      operator/(int __m, const day& __d) noexcept;
 
      friend constexpr month_day
      operator/(const day& __d, const month& __m) noexcept;
 
      friend constexpr month_day
      operator/(const day& __d, int __m) noexcept;
 
      friend constexpr year_month_day
      operator/(const year_month& __ym, const day& __d) noexcept;
    };
 
    // MONTH
 
    class month
    {
    private:
      unsigned char _M_m;
 
    public:
      month() = default;
 
      explicit constexpr
      month(unsigned __m) noexcept
      : _M_m(__m)
      { }
 
      constexpr month&
      operator++() noexcept
      {
        *this += months{1};
        return *this;
      }
 
      constexpr month
      operator++(int) noexcept
      {
        auto __ret = *this;
        ++(*this);
        return __ret;
      }
 
      constexpr month&
      operator--() noexcept
      {
        *this -= months{1};
        return *this;
      }
 
      constexpr month
      operator--(int) noexcept
      {
        auto __ret = *this;
        --(*this);
        return __ret;
      }
 
      constexpr month&
      operator+=(const months& __m) noexcept
      {
        *this = *this + __m;
        return *this;
      }
 
      constexpr month&
      operator-=(const months& __m) noexcept
      {
        *this = *this - __m;
        return *this;
      }
 
      explicit constexpr
      operator unsigned() const noexcept
      { return _M_m; }
 
      constexpr bool
      ok() const noexcept
      { return 1 <= _M_m && _M_m <= 12; }
 
      friend constexpr bool
      operator==(const month& __x, const month& __y) noexcept
      { return unsigned{__x} == unsigned{__y}; }
 
      friend constexpr strong_ordering
      operator<=>(const month& __x, const month& __y) noexcept
      { return unsigned{__x} <=> unsigned{__y}; }
 
      friend constexpr month
      operator+(const month& __x, const months& __y) noexcept
      {
        // modulo(x + (y - 1), 12) = modulo(x + (y - 1) + 12, 12)
        //                         = modulo((x + 11) + y    , 12)
        return month{1 + __detail::__add_modulo<12>(
          unsigned{__x} + 11, __y.count())};
      }
 
      friend constexpr month
      operator+(const months& __x,  const month& __y) noexcept
      { return __y + __x; }
 
      friend constexpr month
      operator-(const month& __x, const months& __y) noexcept
      {
        // modulo(x + (-y - 1), 12) = modulo(x + (-y - 1) + 12, 12)
        //                          = modulo((x + 11) - y     , 12)
        return month{1 + __detail::__sub_modulo<12>(
          unsigned{__x} + 11, __y.count())};
      }
 
      friend constexpr months
      operator-(const month& __x,  const month& __y) noexcept
      {
        const auto __dm = int(unsigned(__x)) - int(unsigned(__y));
        return months{__dm < 0 ? 12 + __dm : __dm};
      }
 
      friend constexpr year_month
      operator/(const year& __y, const month& __m) noexcept;
 
      friend constexpr month_day
      operator/(const month& __m, int __d) noexcept;
 
      friend constexpr month_day_last
      operator/(const month& __m, last_spec) noexcept;
 
      friend constexpr month_day_last
      operator/(last_spec, const month& __m) noexcept;
 
      friend constexpr month_weekday
      operator/(const month& __m, const weekday_indexed& __wdi) noexcept;
 
      friend constexpr month_weekday
      operator/(const weekday_indexed& __wdi, const month& __m) noexcept;
 
      friend constexpr month_weekday_last
      operator/(const month& __m, const weekday_last& __wdl) noexcept;
 
      friend constexpr month_weekday_last
      operator/(const weekday_last& __wdl, const month& __m) noexcept;
    };
 
    inline constexpr month January{1};
    inline constexpr month February{2};
    inline constexpr month March{3};
    inline constexpr month April{4};
    inline constexpr month May{5};
    inline constexpr month June{6};
    inline constexpr month July{7};
    inline constexpr month August{8};
    inline constexpr month September{9};
    inline constexpr month October{10};
    inline constexpr month November{11};
    inline constexpr month December{12};
 
    // YEAR
 
    class year
    {
    private:
      short _M_y;
 
    public:
      year() = default;
 
      explicit constexpr
      year(int __y) noexcept
      : _M_y{static_cast<short>(__y)}
      { }
 
      static constexpr year
      min() noexcept
      { return year{-32767}; }
 
      static constexpr year
      max() noexcept
      { return year{32767}; }
 
      constexpr year&
      operator++() noexcept
      {
        ++_M_y;
        return *this;
      }
 
      constexpr year
      operator++(int) noexcept
      {
        auto __ret = *this;
        ++(*this);
        return __ret;
      }
 
      constexpr year&
      operator--() noexcept
      {
        --_M_y;
        return *this;
      }
 
      constexpr year
      operator--(int) noexcept
      {
        auto __ret = *this;
        --(*this);
        return __ret;
      }
 
      constexpr year&
      operator+=(const years& __y) noexcept
      {
        *this = *this + __y;
        return *this;
      }
 
      constexpr year&
      operator-=(const years& __y) noexcept
      {
        *this = *this - __y;
        return *this;
      }
 
      constexpr year
      operator+() const noexcept
      { return *this; }
 
      constexpr year
      operator-() const noexcept
      { return year{-_M_y}; }
 
      constexpr bool
      is_leap() const noexcept
      {
        // Testing divisibility by 100 first gives better performance [1], i.e.,
        //     return _M_y % 100 == 0 ? _M_y % 400 == 0 : _M_y % 16 == 0;
        // Furthermore, if _M_y % 100 == 0, then _M_y % 400 == 0 is equivalent
        // to _M_y % 16 == 0, so we can simplify it to
        //     return _M_y % 100 == 0 ? _M_y % 16 == 0 : _M_y % 4 == 0.  // #1
        // Similarly, we can replace 100 with 25 (which is good since
        // _M_y % 25 == 0 requires one fewer instruction than _M_y % 100 == 0
        // [2]):
        //     return _M_y % 25 == 0 ? _M_y % 16 == 0 : _M_y % 4 == 0.  // #2
        // Indeed, first assume _M_y % 4 != 0.  Then _M_y % 16 != 0 and hence,
        // _M_y % 4 == 0 and _M_y % 16 == 0 are both false.  Therefore, #2
        // returns false as it should (regardless of _M_y % 25.) Now assume
        // _M_y % 4 == 0.  In this case, _M_y % 25 == 0 if, and only if,
        // _M_y % 100 == 0, that is, #1 and #2 are equivalent.  Finally, #2 is
        // equivalent to
        //     return (_M_y & (_M_y % 25 == 0 ? 15 : 3)) == 0.
 
        // References:
        // [1] https://github.com/cassioneri/calendar
        // [2] https://godbolt.org/z/55G8rn77e
        // [3] https://gcc.gnu.org/pipermail/libstdc++/2021-June/052815.html
 
        return (_M_y & (_M_y % 25 == 0 ? 15 : 3)) == 0;
      }
 
      explicit constexpr
      operator int() const noexcept
      { return _M_y; }
 
      constexpr bool
      ok() const noexcept
      { return min()._M_y <= _M_y && _M_y <= max()._M_y; }
 
      friend constexpr bool
      operator==(const year& __x, const year& __y) noexcept
      { return int{__x} == int{__y}; }
 
      friend constexpr strong_ordering
      operator<=>(const year& __x, const year& __y) noexcept
      { return int{__x} <=> int{__y}; }
 
      friend constexpr year
      operator+(const year& __x, const years& __y) noexcept
      { return year{int{__x} + static_cast<int>(__y.count())}; }
 
      friend constexpr year
      operator+(const years& __x, const year& __y) noexcept
      { return __y + __x; }
 
      friend constexpr year
      operator-(const year& __x, const years& __y) noexcept
      { return __x + -__y; }
 
      friend constexpr years
      operator-(const year& __x, const year& __y) noexcept
      { return years{int{__x} - int{__y}}; }
 
      friend constexpr year_month
      operator/(const year& __y, int __m) noexcept;
 
      friend constexpr year_month_day
      operator/(const year& __y, const month_day& __md) noexcept;
 
      friend constexpr year_month_day
      operator/(const month_day& __md, const year& __y) noexcept;
 
      friend constexpr year_month_day_last
      operator/(const year& __y, const month_day_last& __mdl) noexcept;
 
      friend constexpr year_month_day_last
      operator/(const month_day_last& __mdl, const year& __y) noexcept;
 
      friend constexpr year_month_weekday
      operator/(const year& __y, const month_weekday& __mwd) noexcept;
 
      friend constexpr year_month_weekday
      operator/(const month_weekday& __mwd, const year& __y) noexcept;
 
      friend constexpr year_month_weekday_last
      operator/(const year& __y, const month_weekday_last& __mwdl) noexcept;
 
      friend constexpr year_month_weekday_last
      operator/(const month_weekday_last& __mwdl, const year& __y) noexcept;
    };
 
    // WEEKDAY
 
    class weekday
    {
    private:
      unsigned char _M_wd;
 
      static constexpr weekday
      _S_from_days(const days& __d)
      {
        return weekday{__detail::__add_modulo<7>(4, __d.count())};
      }
 
    public:
      weekday() = default;
 
      explicit constexpr
      weekday(unsigned __wd) noexcept
      : _M_wd(__wd == 7 ? 0 : __wd) // __wd % 7 ?
      { }
 
      constexpr
      weekday(const sys_days& __dp) noexcept
      : weekday{_S_from_days(__dp.time_since_epoch())}
      { }
 
      explicit constexpr
      weekday(const local_days& __dp) noexcept
      : weekday{sys_days{__dp.time_since_epoch()}}
      { }
 
      constexpr weekday&
      operator++() noexcept
      {
        *this += days{1};
        return *this;
      }
 
      constexpr weekday
      operator++(int) noexcept
      {
        auto __ret = *this;
        ++(*this);
        return __ret;
      }
 
      constexpr weekday&
      operator--() noexcept
      {
        *this -= days{1};
        return *this;
      }
 
      constexpr weekday
      operator--(int) noexcept
      {
        auto __ret = *this;
        --(*this);
        return __ret;
      }
 
      constexpr weekday&
      operator+=(const days& __d) noexcept
      {
        *this = *this + __d;
        return *this;
      }
 
      constexpr weekday&
      operator-=(const days& __d) noexcept
      {
        *this = *this - __d;
        return *this;
      }
 
      constexpr unsigned
      c_encoding() const noexcept
      { return _M_wd; }
 
      constexpr unsigned
      iso_encoding() const noexcept
      { return _M_wd == 0u ? 7u : _M_wd; }
 
      constexpr bool
      ok() const noexcept
      { return _M_wd <= 6; }
 
      constexpr weekday_indexed
      operator[](unsigned __index) const noexcept;
 
      constexpr weekday_last
      operator[](last_spec) const noexcept;
 
      friend constexpr bool
      operator==(const weekday& __x, const weekday& __y) noexcept
      { return __x._M_wd == __y._M_wd; }
 
      friend constexpr weekday
      operator+(const weekday& __x, const days& __y) noexcept
      {
        return weekday{__detail::__add_modulo<7>(__x._M_wd, __y.count())};
      }
 
      friend constexpr weekday
      operator+(const days& __x, const weekday& __y) noexcept
      { return __y + __x; }
 
      friend constexpr weekday
      operator-(const weekday& __x, const days& __y) noexcept
      {
        return weekday{__detail::__sub_modulo<7>(__x._M_wd, __y.count())};
      }
 
      friend constexpr days
      operator-(const weekday& __x, const weekday& __y) noexcept
      {
        const auto __n = __x.c_encoding() - __y.c_encoding();
        return static_cast<int>(__n) >= 0 ? days{__n} : days{__n + 7};
      }
    };
 
    inline constexpr weekday Sunday{0};
    inline constexpr weekday Monday{1};
    inline constexpr weekday Tuesday{2};
    inline constexpr weekday Wednesday{3};
    inline constexpr weekday Thursday{4};
    inline constexpr weekday Friday{5};
    inline constexpr weekday Saturday{6};
 
    // WEEKDAY_INDEXED
 
    class weekday_indexed
    {
    private:
      chrono::weekday _M_wd;
      unsigned char _M_index;
 
    public:
      weekday_indexed() = default;
 
      constexpr
      weekday_indexed(const chrono::weekday& __wd, unsigned __index) noexcept
      : _M_wd(__wd), _M_index(__index)
      { }
 
      constexpr chrono::weekday
      weekday() const noexcept
      { return _M_wd; }
 
      constexpr unsigned
      index() const noexcept
      { return _M_index; };
 
      constexpr bool
      ok() const noexcept
      { return _M_wd.ok() && 1 <= _M_index && _M_index <= 5; }
 
      friend constexpr bool
      operator==(const weekday_indexed& __x, const weekday_indexed& __y) noexcept
      { return __x.weekday() == __y.weekday() && __x.index() == __y.index(); }
 
      friend constexpr month_weekday
      operator/(const month& __m, const weekday_indexed& __wdi) noexcept;
 
      friend constexpr month_weekday
      operator/(int __m, const weekday_indexed& __wdi) noexcept;
 
      friend constexpr month_weekday
      operator/(const weekday_indexed& __wdi, const month& __m) noexcept;
 
      friend constexpr month_weekday
      operator/(const weekday_indexed& __wdi, int __m) noexcept;
 
      friend constexpr year_month_weekday
      operator/(const year_month& __ym, const weekday_indexed& __wdi) noexcept;
    };
 
    constexpr weekday_indexed
    weekday::operator[](unsigned __index) const noexcept
    { return {*this, __index}; }
 
    // WEEKDAY_LAST
 
    class weekday_last
    {
    private:
      chrono::weekday _M_wd;
 
    public:
      explicit constexpr
      weekday_last(const chrono::weekday& __wd) noexcept
      : _M_wd{__wd}
      { }
 
      constexpr chrono::weekday
      weekday() const noexcept
      { return _M_wd; }
 
      constexpr bool
      ok() const noexcept
      { return _M_wd.ok(); }
 
      friend constexpr bool
      operator==(const weekday_last& __x, const weekday_last& __y) noexcept
      { return __x.weekday() == __y.weekday(); }
 
      friend constexpr month_weekday_last
      operator/(int __m, const weekday_last& __wdl) noexcept;
 
      friend constexpr month_weekday_last
      operator/(const weekday_last& __wdl, int __m) noexcept;
 
      friend constexpr year_month_weekday_last
      operator/(const year_month& __ym, const weekday_last& __wdl) noexcept;
    };
 
    constexpr weekday_last
    weekday::operator[](last_spec) const noexcept
    { return weekday_last{*this}; }
 
    // MONTH_DAY
 
    class month_day
    {
    private:
      chrono::month _M_m;
      chrono::day _M_d;
 
    public:
      month_day() = default;
 
      constexpr
      month_day(const chrono::month& __m, const chrono::day& __d) noexcept
      : _M_m{__m}, _M_d{__d}
      { }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      constexpr chrono::day
      day() const noexcept
      { return _M_d; }
 
      constexpr bool
      ok() const noexcept
      {
        return _M_m.ok()
          && 1u <= unsigned(_M_d)
          && unsigned(_M_d) <= __detail::__days_per_month[unsigned(_M_m) - 1];
      }
 
      friend constexpr bool
      operator==(const month_day& __x, const month_day& __y) noexcept
      { return __x.month() == __y.month() && __x.day() == __y.day(); }
 
      friend constexpr strong_ordering
      operator<=>(const month_day& __x, const month_day& __y) noexcept
        = default;
 
      friend constexpr month_day
      operator/(const chrono::month& __m, const chrono::day& __d) noexcept
      { return {__m, __d}; }
 
      friend constexpr month_day
      operator/(const chrono::month& __m, int __d) noexcept
      { return {__m, chrono::day(unsigned(__d))}; }
 
      friend constexpr month_day
      operator/(int __m, const chrono::day& __d) noexcept
      { return {chrono::month(unsigned(__m)), __d}; }
 
      friend constexpr month_day
      operator/(const chrono::day& __d, const chrono::month& __m) noexcept
      { return {__m, __d}; }
 
      friend constexpr month_day
      operator/(const chrono::day& __d, int __m) noexcept
      { return {chrono::month(unsigned(__m)), __d}; }
 
      friend constexpr year_month_day
      operator/(int __y, const month_day& __md) noexcept;
 
      friend constexpr year_month_day
      operator/(const month_day& __md, int __y) noexcept;
    };
 
    // MONTH_DAY_LAST
 
    class month_day_last
    {
    private:
      chrono::month _M_m;
 
    public:
      explicit constexpr
      month_day_last(const chrono::month& __m) noexcept
      : _M_m{__m}
      { }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      constexpr bool
      ok() const noexcept
      { return _M_m.ok(); }
 
      friend constexpr bool
      operator==(const month_day_last& __x, const month_day_last& __y) noexcept
      { return __x.month() == __y.month(); }
 
      friend constexpr strong_ordering
      operator<=>(const month_day_last& __x, const month_day_last& __y) noexcept
        = default;
 
      friend constexpr month_day_last
      operator/(const chrono::month& __m, last_spec) noexcept
      { return month_day_last{__m}; }
 
      friend constexpr month_day_last
      operator/(int __m, last_spec) noexcept
      { return chrono::month(unsigned(__m)) / last; }
 
      friend constexpr month_day_last
      operator/(last_spec, const chrono::month& __m) noexcept
      { return __m / last; }
 
      friend constexpr month_day_last
      operator/(last_spec, int __m) noexcept
      { return __m / last; }
 
      friend constexpr year_month_day_last
      operator/(int __y, const month_day_last& __mdl) noexcept;
 
      friend constexpr year_month_day_last
      operator/(const month_day_last& __mdl, int __y) noexcept;
    };
 
    // MONTH_WEEKDAY
 
    class month_weekday
    {
    private:
      chrono::month _M_m;
      chrono::weekday_indexed _M_wdi;
 
    public:
      constexpr
      month_weekday(const chrono::month& __m,
                    const chrono::weekday_indexed& __wdi) noexcept
      : _M_m{__m}, _M_wdi{__wdi}
      { }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      constexpr chrono::weekday_indexed
      weekday_indexed() const noexcept
      { return _M_wdi; }
 
      constexpr bool
      ok() const noexcept
      { return _M_m.ok() && _M_wdi.ok(); }
 
      friend constexpr bool
      operator==(const month_weekday& __x, const month_weekday& __y) noexcept
      {
        return __x.month() == __y.month()
          && __x.weekday_indexed() == __y.weekday_indexed();
      }
 
      friend constexpr month_weekday
      operator/(const chrono::month& __m,
                const chrono::weekday_indexed& __wdi) noexcept
      { return {__m, __wdi}; }
 
      friend constexpr month_weekday
      operator/(int __m, const chrono::weekday_indexed& __wdi) noexcept
      { return chrono::month(unsigned(__m)) / __wdi; }
 
      friend constexpr month_weekday
      operator/(const chrono::weekday_indexed& __wdi,
                const chrono::month& __m) noexcept
      { return __m / __wdi; }
 
      friend constexpr month_weekday
      operator/(const chrono::weekday_indexed& __wdi, int __m) noexcept
      { return __m / __wdi; }
 
      friend constexpr year_month_weekday
      operator/(int __y, const month_weekday& __mwd) noexcept;
 
      friend constexpr year_month_weekday
      operator/(const month_weekday& __mwd, int __y) noexcept;
    };
 
    // MONTH_WEEKDAY_LAST
 
    class month_weekday_last
    {
    private:
      chrono::month _M_m;
      chrono::weekday_last _M_wdl;
 
    public:
      constexpr
      month_weekday_last(const chrono::month& __m,
                         const chrono::weekday_last& __wdl) noexcept
      :_M_m{__m}, _M_wdl{__wdl}
      { }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      constexpr chrono::weekday_last
      weekday_last() const noexcept
      { return _M_wdl; }
 
      constexpr bool
      ok() const noexcept
      { return _M_m.ok() && _M_wdl.ok(); }
 
      friend constexpr bool
      operator==(const month_weekday_last& __x,
                 const month_weekday_last& __y) noexcept
      {
        return __x.month() == __y.month()
          && __x.weekday_last() == __y.weekday_last();
      }
 
      friend constexpr month_weekday_last
      operator/(const chrono::month& __m,
                const chrono::weekday_last& __wdl) noexcept
      { return {__m, __wdl}; }
 
      friend constexpr month_weekday_last
      operator/(int __m, const chrono::weekday_last& __wdl) noexcept
      { return chrono::month(unsigned(__m)) / __wdl; }
 
      friend constexpr month_weekday_last
      operator/(const chrono::weekday_last& __wdl,
                const chrono::month& __m) noexcept
      { return __m / __wdl; }
 
      friend constexpr month_weekday_last
      operator/(const chrono::weekday_last& __wdl, int __m) noexcept
      { return chrono::month(unsigned(__m)) / __wdl; }
 
      friend constexpr year_month_weekday_last
      operator/(int __y, const month_weekday_last& __mwdl) noexcept;
 
      friend constexpr year_month_weekday_last
      operator/(const month_weekday_last& __mwdl, int __y) noexcept;
    };
 
    // YEAR_MONTH
 
    namespace __detail
    {
      // [time.cal.ym], [time.cal.ymd], etc constrain the 'months'-based
      // addition/subtraction operator overloads like so:
      //
      //   Constraints: if the argument supplied by the caller for the months
      //   parameter is convertible to years, its implicit conversion sequence
      //   to years is worse than its implicit conversion sequence to months.
      //
      // We realize this constraint by templatizing the 'months'-based
      // overloads (using a dummy defaulted template parameter), so that
      // overload resolution doesn't select the 'months'-based overload unless
      // the implicit conversion sequence to 'months' is better than that to
      // 'years'.
      using __months_years_conversion_disambiguator = void;
    }
 
    class year_month
    {
    private:
      chrono::year _M_y;
      chrono::month _M_m;
 
    public:
      year_month() = default;
 
      constexpr
      year_month(const chrono::year& __y, const chrono::month& __m) noexcept
      : _M_y{__y}, _M_m{__m}
      { }
 
      constexpr chrono::year
      year() const noexcept
      { return _M_y; }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month&
        operator+=(const months& __dm) noexcept
        {
          *this = *this + __dm;
          return *this;
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month&
        operator-=(const months& __dm) noexcept
        {
          *this = *this - __dm;
          return *this;
        }
 
      constexpr year_month&
      operator+=(const years& __dy)  noexcept
      {
        *this = *this + __dy;
        return *this;
      }
 
      constexpr year_month&
      operator-=(const years& __dy)  noexcept
      {
        *this = *this - __dy;
        return *this;
      }
 
      constexpr bool
      ok() const noexcept
      { return _M_y.ok() && _M_m.ok(); }
 
      friend constexpr bool
      operator==(const year_month& __x, const year_month& __y) noexcept
      { return __x.year() == __y.year() && __x.month() == __y.month(); }
 
      friend constexpr strong_ordering
      operator<=>(const year_month& __x, const year_month& __y) noexcept
        = default;
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month
        operator+(const year_month& __ym, const months& __dm) noexcept
        {
          // TODO: Optimize?
          auto __m = __ym.month() + __dm;
          auto __i = int(unsigned(__ym.month())) - 1 + __dm.count();
          auto __y = (__i < 0
                      ? __ym.year() + years{(__i - 11) / 12}
                      : __ym.year() + years{__i / 12});
          return __y / __m;
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month
        operator+(const months& __dm, const year_month& __ym) noexcept
        { return __ym + __dm; }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month
        operator-(const year_month& __ym, const months& __dm) noexcept
        { return __ym + -__dm; }
 
      friend constexpr months
      operator-(const year_month& __x, const year_month& __y) noexcept
      {
        return (__x.year() - __y.year()
                + months{static_cast<int>(unsigned{__x.month()})
                         - static_cast<int>(unsigned{__y.month()})});
      }
 
      friend constexpr year_month
      operator+(const year_month& __ym, const years& __dy) noexcept
      { return (__ym.year() + __dy) / __ym.month(); }
 
      friend constexpr year_month
      operator+(const years& __dy, const year_month& __ym) noexcept
      { return __ym + __dy; }
 
      friend constexpr year_month
      operator-(const year_month& __ym, const years& __dy) noexcept
      { return __ym + -__dy; }
 
      friend constexpr year_month
      operator/(const chrono::year& __y, const chrono::month& __m) noexcept
      { return {__y, __m}; }
 
      friend constexpr year_month
      operator/(const chrono::year& __y, int __m) noexcept
      { return {__y, chrono::month(unsigned(__m))}; }
 
      friend constexpr year_month_day
      operator/(const year_month& __ym, int __d) noexcept;
 
      friend constexpr year_month_day_last
      operator/(const year_month& __ym, last_spec) noexcept;
    };
 
    // YEAR_MONTH_DAY
 
    class year_month_day
    {
    private:
      chrono::year _M_y;
      chrono::month _M_m;
      chrono::day _M_d;
 
      static constexpr year_month_day _S_from_days(const days& __dp) noexcept;
 
      constexpr days _M_days_since_epoch() const noexcept;
 
    public:
      year_month_day() = default;
 
      constexpr
      year_month_day(const chrono::year& __y, const chrono::month& __m,
                     const chrono::day& __d) noexcept
      : _M_y{__y}, _M_m{__m}, _M_d{__d}
      { }
 
      constexpr
      year_month_day(const year_month_day_last& __ymdl) noexcept;
 
      constexpr
      year_month_day(const sys_days& __dp) noexcept
      : year_month_day(_S_from_days(__dp.time_since_epoch()))
      { }
 
      explicit constexpr
      year_month_day(const local_days& __dp) noexcept
      : year_month_day(sys_days{__dp.time_since_epoch()})
      { }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_day&
        operator+=(const months& __m) noexcept
        {
          *this = *this + __m;
          return *this;
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_day&
        operator-=(const months& __m) noexcept
        {
          *this = *this - __m;
          return *this;
        }
 
      constexpr year_month_day&
      operator+=(const years& __y) noexcept
      {
        *this = *this + __y;
        return *this;
      }
 
      constexpr year_month_day&
      operator-=(const years& __y) noexcept
      {
        *this = *this - __y;
        return *this;
      }
 
      constexpr chrono::year
      year() const noexcept
      { return _M_y; }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      constexpr chrono::day
      day() const noexcept
      { return _M_d; }
 
      constexpr
      operator sys_days() const noexcept
      { return sys_days{_M_days_since_epoch()}; }
 
      explicit constexpr
      operator local_days() const noexcept
      { return local_days{sys_days{*this}.time_since_epoch()}; }
 
      constexpr bool ok() const noexcept;
 
      friend constexpr bool
      operator==(const year_month_day& __x, const year_month_day& __y) noexcept
      {
        return __x.year() == __y.year()
          && __x.month() == __y.month()
          && __x.day() == __y.day();
      }
 
      friend constexpr strong_ordering
      operator<=>(const year_month_day& __x, const year_month_day& __y) noexcept
        = default;
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_day
        operator+(const year_month_day& __ymd, const months& __dm) noexcept
        { return (__ymd.year() / __ymd.month() + __dm) / __ymd.day(); }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_day
        operator+(const months& __dm, const year_month_day& __ymd) noexcept
        { return __ymd + __dm; }
 
      friend constexpr year_month_day
      operator+(const year_month_day& __ymd, const years& __dy) noexcept
      { return (__ymd.year() + __dy) / __ymd.month() / __ymd.day(); }
 
      friend constexpr year_month_day
      operator+(const years& __dy, const year_month_day& __ymd) noexcept
      { return __ymd + __dy; }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_day
        operator-(const year_month_day& __ymd, const months& __dm) noexcept
        { return __ymd + -__dm; }
 
      friend constexpr year_month_day
      operator-(const year_month_day& __ymd, const years& __dy) noexcept
      { return __ymd + -__dy; }
 
      friend constexpr year_month_day
      operator/(const year_month& __ym, const chrono::day& __d) noexcept
      { return {__ym.year(), __ym.month(), __d}; }
 
      friend constexpr year_month_day
      operator/(const year_month& __ym, int __d) noexcept
      { return __ym / chrono::day{unsigned(__d)}; }
 
      friend constexpr year_month_day
      operator/(const chrono::year& __y, const month_day& __md) noexcept
      { return __y / __md.month() / __md.day(); }
 
      friend constexpr year_month_day
      operator/(int __y, const month_day& __md) noexcept
      { return chrono::year{__y} / __md; }
 
      friend constexpr year_month_day
      operator/(const month_day& __md, const chrono::year& __y) noexcept
      { return __y / __md; }
 
      friend constexpr year_month_day
      operator/(const month_day& __md, int __y) noexcept
      { return chrono::year(__y) / __md; }
    };
 
    // Construct from days since 1970/01/01.
    // Proposition 6.3 of Neri and Schneider,
    // "Euclidean Affine Functions and Applications to Calendar Algorithms".
    // https://arxiv.org/abs/2102.06959
    constexpr year_month_day
    year_month_day::_S_from_days(const days& __dp) noexcept
    {
      constexpr auto __z2    = static_cast<uint32_t>(-1468000);
      constexpr auto __r2_e3 = static_cast<uint32_t>(536895458);
 
      const auto __r0 = static_cast<uint32_t>(__dp.count()) + __r2_e3;
 
      const auto __n1 = 4 * __r0 + 3;
      const auto __q1 = __n1 / 146097;
      const auto __r1 = __n1 % 146097 / 4;
 
      constexpr auto __p32 = static_cast<uint64_t>(1) << 32;
      const auto __n2 = 4 * __r1 + 3;
      const auto __u2 = static_cast<uint64_t>(2939745) * __n2;
      const auto __q2 = static_cast<uint32_t>(__u2 / __p32);
      const auto __r2 = static_cast<uint32_t>(__u2 % __p32) / 2939745 / 4;
 
      constexpr auto __p16 = static_cast<uint32_t>(1) << 16;
      const auto __n3 = 2141 * __r2 + 197913;
      const auto __q3 = __n3 / __p16;
      const auto __r3 = __n3 % __p16 / 2141;
 
      const auto __y0 = 100 * __q1 + __q2;
      const auto __m0 = __q3;
      const auto __d0 = __r3;
 
      const auto __j  = __r2 >= 306;
      const auto __y1 = __y0 + __j;
      const auto __m1 = __j ? __m0 - 12 : __m0;
      const auto __d1 = __d0 + 1;
 
      return year_month_day{chrono::year{static_cast<int>(__y1 + __z2)},
                            chrono::month{__m1}, chrono::day{__d1}};
    }
 
    // Days since 1970/01/01.
    // Proposition 6.2 of Neri and Schneider,
    // "Euclidean Affine Functions and Applications to Calendar Algorithms".
    // https://arxiv.org/abs/2102.06959
    constexpr days
    year_month_day::_M_days_since_epoch() const noexcept
    {
      auto constexpr __z2    = static_cast<uint32_t>(-1468000);
      auto constexpr __r2_e3 = static_cast<uint32_t>(536895458);
 
      const auto __y1 = static_cast<uint32_t>(static_cast<int>(_M_y)) - __z2;
      const auto __m1 = static_cast<uint32_t>(static_cast<unsigned>(_M_m));
      const auto __d1 = static_cast<uint32_t>(static_cast<unsigned>(_M_d));
 
      const auto __j  = static_cast<uint32_t>(__m1 < 3);
      const auto __y0 = __y1 - __j;
      const auto __m0 = __j ? __m1 + 12 : __m1;
      const auto __d0 = __d1 - 1;
 
      const auto __q1 = __y0 / 100;
      const auto __yc = 1461 * __y0 / 4 - __q1 + __q1 / 4;
      const auto __mc = (979 *__m0 - 2919) / 32;
      const auto __dc = __d0;
 
      return days{static_cast<int32_t>(__yc + __mc + __dc - __r2_e3)};
    }
 
    // YEAR_MONTH_DAY_LAST
 
    class year_month_day_last
    {
    private:
      chrono::year _M_y;
      chrono::month_day_last _M_mdl;
 
    public:
      constexpr
      year_month_day_last(const chrono::year& __y,
                          const chrono::month_day_last& __mdl) noexcept
      : _M_y{__y}, _M_mdl{__mdl}
      { }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_day_last&
        operator+=(const months& __m) noexcept
        {
          *this = *this + __m;
          return *this;
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_day_last&
        operator-=(const months& __m) noexcept
        {
          *this = *this - __m;
          return *this;
        }
 
      constexpr year_month_day_last&
      operator+=(const years& __y)  noexcept
      {
        *this = *this + __y;
        return *this;
      }
 
      constexpr year_month_day_last&
      operator-=(const years& __y)  noexcept
      {
        *this = *this - __y;
        return *this;
      }
 
      constexpr chrono::year
      year() const noexcept
      { return _M_y; }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_mdl.month(); }
 
      constexpr chrono::month_day_last
      month_day_last() const noexcept
      { return _M_mdl; }
 
      // Return A day representing the last day of this year, month pair.
      constexpr chrono::day
      day() const noexcept
      {
        const auto __m = static_cast<unsigned>(month());
 
        // The result is unspecified if __m < 1 or __m > 12.  Hence, assume
        // 1 <= __m <= 12.  For __m != 2, day() == 30 or day() == 31 or, in
        // other words, day () == 30 | b, where b is in {0, 1}.
 
        // If __m in {1, 3, 4, 5, 6, 7}, then b is 1 if, and only if, __m is
        // odd.  Hence, b = __m & 1 = (__m ^ 0) & 1.
 
        // If __m in {8, 9, 10, 11, 12}, then b is 1 if, and only if, __m is
        // even.  Hence, b = (__m ^ 1) & 1.
 
        // Therefore, b = (__m ^ c) & 1, where c = 0, if __m < 8, or c = 1 if
        // __m >= 8, that is, c = __m >> 3.
 
        // Since 30 = (11110)_2 and __m <= 31 = (11111)_2, the "& 1" in b's
        // calculation is unnecessary.
 
        // The performance of this implementation does not depend on look-up
        // tables being on the L1 cache.
        return chrono::day{__m != 2 ? (__m ^ (__m >> 3)) | 30
          : _M_y.is_leap() ? 29 : 28};
      }
 
      constexpr
      operator sys_days() const noexcept
      { return sys_days{year() / month() / day()}; }
 
      explicit constexpr
      operator local_days() const noexcept
      { return local_days{sys_days{*this}.time_since_epoch()}; }
 
      constexpr bool
      ok() const noexcept
      { return _M_y.ok() && _M_mdl.ok(); }
 
      friend constexpr bool
      operator==(const year_month_day_last& __x,
                 const year_month_day_last& __y) noexcept
      {
        return __x.year() == __y.year()
          && __x.month_day_last() == __y.month_day_last();
      }
 
      friend constexpr strong_ordering
      operator<=>(const year_month_day_last& __x,
                  const year_month_day_last& __y) noexcept
        = default;
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_day_last
        operator+(const year_month_day_last& __ymdl,
                  const months& __dm) noexcept
        { return (__ymdl.year() / __ymdl.month() + __dm) / last; }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_day_last
        operator+(const months& __dm,
                  const year_month_day_last& __ymdl) noexcept
        { return __ymdl + __dm; }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_day_last
        operator-(const year_month_day_last& __ymdl,
                  const months& __dm) noexcept
        { return __ymdl + -__dm; }
 
      friend constexpr year_month_day_last
      operator+(const year_month_day_last& __ymdl,
                const years& __dy) noexcept
      { return {__ymdl.year() + __dy, __ymdl.month_day_last()}; }
 
      friend constexpr year_month_day_last
      operator+(const years& __dy,
                const year_month_day_last& __ymdl) noexcept
      { return __ymdl + __dy; }
 
      friend constexpr year_month_day_last
      operator-(const year_month_day_last& __ymdl,
                const years& __dy) noexcept
      { return __ymdl + -__dy; }
 
      friend constexpr year_month_day_last
      operator/(const year_month& __ym, last_spec) noexcept
      { return {__ym.year(), chrono::month_day_last{__ym.month()}}; }
 
      friend constexpr year_month_day_last
      operator/(const chrono::year& __y,
                const chrono::month_day_last& __mdl) noexcept
      { return {__y, __mdl}; }
 
      friend constexpr year_month_day_last
      operator/(int __y, const chrono::month_day_last& __mdl) noexcept
      { return chrono::year(__y) / __mdl; }
 
      friend constexpr year_month_day_last
      operator/(const chrono::month_day_last& __mdl,
                const chrono::year& __y) noexcept
      { return __y / __mdl; }
 
      friend constexpr year_month_day_last
      operator/(const chrono::month_day_last& __mdl, int __y) noexcept
      { return chrono::year(__y) / __mdl; }
    };
 
    // year_month_day ctor from year_month_day_last
    constexpr
    year_month_day::year_month_day(const year_month_day_last& __ymdl) noexcept
    : _M_y{__ymdl.year()}, _M_m{__ymdl.month()}, _M_d{__ymdl.day()}
    { }
 
    constexpr bool
    year_month_day::ok() const noexcept
    {
      if (!_M_y.ok() || !_M_m.ok())
        return false;
      return chrono::day{1} <= _M_d && _M_d <= (_M_y / _M_m / last).day();
    }
 
    // YEAR_MONTH_WEEKDAY
 
    class year_month_weekday
    {
    private:
      chrono::year _M_y;
      chrono::month _M_m;
      chrono::weekday_indexed _M_wdi;
 
      static constexpr year_month_weekday
      _S_from_sys_days(const sys_days& __dp)
      {
        year_month_day __ymd{__dp};
        chrono::weekday __wd{__dp};
        auto __index = __wd[(unsigned{__ymd.day()} - 1) / 7 + 1];
        return {__ymd.year(), __ymd.month(), __index};
      }
 
    public:
      year_month_weekday() = default;
 
      constexpr
      year_month_weekday(const chrono::year& __y, const chrono::month& __m,
                         const chrono::weekday_indexed& __wdi) noexcept
      : _M_y{__y}, _M_m{__m}, _M_wdi{__wdi}
      { }
 
      constexpr
      year_month_weekday(const sys_days& __dp) noexcept
      : year_month_weekday{_S_from_sys_days(__dp)}
      { }
 
      explicit constexpr
      year_month_weekday(const local_days& __dp) noexcept
      : year_month_weekday{sys_days{__dp.time_since_epoch()}}
      { }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_weekday&
        operator+=(const months& __m) noexcept
        {
          *this = *this + __m;
          return *this;
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_weekday&
        operator-=(const months& __m) noexcept
        {
          *this = *this - __m;
          return *this;
        }
 
      constexpr year_month_weekday&
      operator+=(const years& __y) noexcept
      {
        *this = *this + __y;
        return *this;
      }
 
      constexpr year_month_weekday&
      operator-=(const years& __y) noexcept
      {
        *this = *this - __y;
        return *this;
      }
 
      constexpr chrono::year
      year() const noexcept
      { return _M_y; }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      constexpr chrono::weekday
      weekday() const noexcept
      { return _M_wdi.weekday(); }
 
      constexpr unsigned
      index() const noexcept
      { return _M_wdi.index(); }
 
      constexpr chrono::weekday_indexed
      weekday_indexed() const noexcept
      { return _M_wdi; }
 
      constexpr
      operator sys_days() const noexcept
      {
        auto __d = sys_days{year() / month() / 1};
        return __d + (weekday() - chrono::weekday(__d)
                      + days{(static_cast<int>(index())-1)*7});
      }
 
      explicit constexpr
      operator local_days() const noexcept
      { return local_days{sys_days{*this}.time_since_epoch()}; }
 
      constexpr bool
      ok() const noexcept
      {
        if (!_M_y.ok() || !_M_m.ok() || !_M_wdi.ok())
          return false;
        if (_M_wdi.index() <= 4)
          return true;
        days __d = (_M_wdi.weekday()
                    - chrono::weekday{sys_days{_M_y / _M_m / 1}}
                    + days((_M_wdi.index()-1)*7 + 1));
        __glibcxx_assert(__d.count() >= 1);
        return (unsigned)__d.count() <= (unsigned)(_M_y / _M_m / last).day();
      }
 
      friend constexpr bool
      operator==(const year_month_weekday& __x,
                 const year_month_weekday& __y) noexcept
      {
        return __x.year() == __y.year()
          && __x.month() == __y.month()
          && __x.weekday_indexed() == __y.weekday_indexed();
      }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_weekday
        operator+(const year_month_weekday& __ymwd, const months& __dm) noexcept
        {
          return ((__ymwd.year() / __ymwd.month() + __dm)
                  / __ymwd.weekday_indexed());
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_weekday
        operator+(const months& __dm, const year_month_weekday& __ymwd) noexcept
        { return __ymwd + __dm; }
 
      friend constexpr year_month_weekday
      operator+(const year_month_weekday& __ymwd, const years& __dy) noexcept
      { return {__ymwd.year() + __dy, __ymwd.month(), __ymwd.weekday_indexed()}; }
 
      friend constexpr year_month_weekday
      operator+(const years& __dy, const year_month_weekday& __ymwd) noexcept
      { return __ymwd + __dy; }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_weekday
        operator-(const year_month_weekday& __ymwd, const months& __dm) noexcept
        { return __ymwd + -__dm; }
 
      friend constexpr year_month_weekday
      operator-(const year_month_weekday& __ymwd, const years& __dy) noexcept
      { return __ymwd + -__dy; }
 
      friend constexpr year_month_weekday
      operator/(const year_month& __ym,
                const chrono::weekday_indexed& __wdi) noexcept
      { return {__ym.year(), __ym.month(), __wdi}; }
 
      friend constexpr year_month_weekday
      operator/(const chrono::year& __y, const month_weekday& __mwd) noexcept
      { return {__y, __mwd.month(), __mwd.weekday_indexed()}; }
 
      friend constexpr year_month_weekday
      operator/(int __y, const month_weekday& __mwd) noexcept
      { return chrono::year(__y) / __mwd; }
 
      friend constexpr year_month_weekday
      operator/(const month_weekday& __mwd, const chrono::year& __y) noexcept
      { return __y / __mwd; }
 
      friend constexpr year_month_weekday
      operator/(const month_weekday& __mwd, int __y) noexcept
      { return chrono::year(__y) / __mwd; }
    };
 
    // YEAR_MONTH_WEEKDAY_LAST
 
    class year_month_weekday_last
    {
    private:
      chrono::year _M_y;
      chrono::month _M_m;
      chrono::weekday_last _M_wdl;
 
    public:
      constexpr
      year_month_weekday_last(const chrono::year& __y, const chrono::month& __m,
                              const chrono::weekday_last& __wdl) noexcept
      : _M_y{__y}, _M_m{__m}, _M_wdl{__wdl}
      { }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_weekday_last&
        operator+=(const months& __m) noexcept
        {
          *this = *this + __m;
          return *this;
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        constexpr year_month_weekday_last&
        operator-=(const months& __m) noexcept
        {
          *this = *this - __m;
          return *this;
        }
 
      constexpr year_month_weekday_last&
      operator+=(const years& __y)  noexcept
      {
        *this = *this + __y;
        return *this;
      }
 
      constexpr year_month_weekday_last&
      operator-=(const years& __y)  noexcept
      {
        *this = *this - __y;
        return *this;
      }
 
      constexpr chrono::year
      year() const noexcept
      { return _M_y; }
 
      constexpr chrono::month
      month() const noexcept
      { return _M_m; }
 
      constexpr chrono::weekday
      weekday() const noexcept
      { return _M_wdl.weekday(); }
 
      constexpr chrono::weekday_last
      weekday_last() const noexcept
      { return _M_wdl; }
 
      constexpr
      operator sys_days() const noexcept
      {
        const auto __d = sys_days{_M_y / _M_m / last};
        return sys_days{(__d - (chrono::weekday{__d}
                                - _M_wdl.weekday())).time_since_epoch()};
      }
 
      explicit constexpr
      operator local_days() const noexcept
      { return local_days{sys_days{*this}.time_since_epoch()}; }
 
      constexpr bool
      ok() const noexcept
      { return _M_y.ok() && _M_m.ok() && _M_wdl.ok(); }
 
      friend constexpr bool
      operator==(const year_month_weekday_last& __x,
                 const year_month_weekday_last& __y) noexcept
      {
        return __x.year() == __y.year()
          && __x.month() == __y.month()
          && __x.weekday_last() == __y.weekday_last();
      }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_weekday_last
        operator+(const year_month_weekday_last& __ymwdl,
                  const months& __dm) noexcept
        {
          return ((__ymwdl.year() / __ymwdl.month() + __dm)
                  / __ymwdl.weekday_last());
        }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_weekday_last
        operator+(const months& __dm,
                  const year_month_weekday_last& __ymwdl) noexcept
        { return __ymwdl + __dm; }
 
      friend constexpr year_month_weekday_last
      operator+(const year_month_weekday_last& __ymwdl,
                const years& __dy) noexcept
      { return {__ymwdl.year() + __dy, __ymwdl.month(), __ymwdl.weekday_last()}; }
 
      friend constexpr year_month_weekday_last
      operator+(const years& __dy,
                const year_month_weekday_last& __ymwdl) noexcept
      { return __ymwdl + __dy; }
 
      template<typename = __detail::__months_years_conversion_disambiguator>
        friend constexpr year_month_weekday_last
        operator-(const year_month_weekday_last& __ymwdl,
                  const months& __dm) noexcept
        { return __ymwdl + -__dm; }
 
      friend constexpr year_month_weekday_last
      operator-(const year_month_weekday_last& __ymwdl,
                const years& __dy) noexcept
      { return __ymwdl + -__dy; }
 
      friend constexpr year_month_weekday_last
      operator/(const year_month& __ym,
                const chrono::weekday_last& __wdl) noexcept
      { return {__ym.year(), __ym.month(), __wdl}; }
 
      friend constexpr year_month_weekday_last
      operator/(const chrono::year& __y,
                const chrono::month_weekday_last& __mwdl) noexcept
      { return {__y, __mwdl.month(), __mwdl.weekday_last()}; }
 
      friend constexpr year_month_weekday_last
      operator/(int __y, const chrono::month_weekday_last& __mwdl) noexcept
      { return chrono::year(__y) / __mwdl; }
 
      friend constexpr year_month_weekday_last
      operator/(const chrono::month_weekday_last& __mwdl,
                const chrono::year& __y) noexcept
      { return __y / __mwdl; }
 
      friend constexpr year_month_weekday_last
      operator/(const chrono::month_weekday_last& __mwdl, int __y) noexcept
      { return chrono::year(__y) / __mwdl; }
    };
 
    // HH_MM_SS
 
    /// @cond undocumented
    namespace __detail
    {
      consteval long long
      __pow10(unsigned __n)
      {
        long long __r = 1;
        while (__n-- > 0)
          __r *= 10;
        return __r;
      }
    }
    /// @endcond
 
    /** Utility for splitting a duration into hours, minutes, and seconds
     *
     * This is a convenience type that provides accessors for the constituent
     * parts (hours, minutes, seconds and subseconds) of a duration.
     *
     * @since C++20
     */
    template<typename _Duration>
      class hh_mm_ss
      {
        static_assert( __is_duration<_Duration>::value );
 
      private:
        static consteval int
        _S_fractional_width()
        {
          auto __den = _Duration::period::den;
          const int __multiplicity_2 = std::__countr_zero((uintmax_t)__den);
          __den >>= __multiplicity_2;
          int __multiplicity_5 = 0;
          while ((__den % 5) == 0)
            {
              ++__multiplicity_5;
              __den /= 5;
            }
          if (__den != 1)
            return 6;
 
          int __width = (__multiplicity_2 > __multiplicity_5
                         ? __multiplicity_2 : __multiplicity_5);
          if (__width > 18)
            __width = 18;
          return __width;
        }
 
        constexpr
        hh_mm_ss(_Duration __d, bool __is_neg)
        : _M_h (duration_cast<chrono::hours>(__d)),
          _M_m (duration_cast<chrono::minutes>(__d - hours())),
          _M_s (duration_cast<chrono::seconds>(__d - hours() - minutes())),
          _M_is_neg(__is_neg)
        {
          auto __ss = __d - hours() - minutes() - seconds();
          if constexpr (treat_as_floating_point_v<typename precision::rep>)
            _M_ss._M_r = __ss.count();
          else if constexpr (precision::period::den != 1)
            _M_ss._M_r = duration_cast<precision>(__ss).count();
        }
 
        static constexpr _Duration
        _S_abs(_Duration __d)
        {
          if constexpr (numeric_limits<typename _Duration::rep>::is_signed)
            return chrono::abs(__d);
          else
            return __d;
        }
 
      public:
        static constexpr unsigned fractional_width = {_S_fractional_width()};
 
        using precision
          = duration<common_type_t<typename _Duration::rep,
                                   chrono::seconds::rep>,
                     ratio<1, __detail::__pow10(fractional_width)>>;
 
        constexpr hh_mm_ss() noexcept = default;
 
        constexpr explicit
        hh_mm_ss(_Duration __d)
        : hh_mm_ss(_S_abs(__d), __d < _Duration::zero())
        { }
 
        constexpr bool
        is_negative() const noexcept
        {
          if constexpr (!_S_is_unsigned)
            return _M_is_neg;
          else
            return false;
        }
 
        constexpr chrono::hours
        hours() const noexcept
        { return _M_h; }
 
        constexpr chrono::minutes
        minutes() const noexcept
        { return _M_m; }
 
        constexpr chrono::seconds
        seconds() const noexcept
        { return _M_s; }
 
        constexpr precision
        subseconds() const noexcept
        { return static_cast<precision>(_M_ss); }
 
        constexpr explicit
        operator precision() const noexcept
        { return to_duration(); }
 
        constexpr precision
        to_duration() const noexcept
        {
          if constexpr (!_S_is_unsigned)
            if (_M_is_neg)
              return -(_M_h + _M_m + _M_s + subseconds());
          return _M_h + _M_m + _M_s + subseconds();
        }
 
      private:
        static constexpr bool _S_is_unsigned
          = __and_v<is_integral<typename _Duration::rep>,
                    is_unsigned<typename _Duration::rep>>;
 
        template<typename _Ratio>
          using __byte_duration = duration<unsigned char, _Ratio>;
 
        // The type of the _M_ss member that holds the subsecond precision.
        template<typename _Dur>
          struct __subseconds
          {
            typename _Dur::rep _M_r{};
 
            constexpr explicit
            operator _Dur() const noexcept
            { return _Dur(_M_r); }
          };
 
        // An empty class if this precision doesn't need subseconds.
        template<typename _Rep>
          requires (!treat_as_floating_point_v<_Rep>)
          struct __subseconds<duration<_Rep, ratio<1>>>
          {
            constexpr explicit
            operator duration<_Rep, ratio<1>>() const noexcept
            { return {}; }
          };
 
        template<typename _Rep, typename _Period>
          requires (!treat_as_floating_point_v<_Rep>)
            && ratio_less_v<_Period, ratio<1, 1>>
            && ratio_greater_equal_v<_Period, ratio<1, 250>>
          struct __subseconds<duration<_Rep, _Period>>
          {
            unsigned char _M_r{};
 
            constexpr explicit
            operator duration<_Rep, _Period>() const noexcept
            { return duration<_Rep, _Period>(_M_r); }
          };
 
        template<typename _Rep, typename _Period>
          requires (!treat_as_floating_point_v<_Rep>)
            && ratio_less_v<_Period, ratio<1, 250>>
            && ratio_greater_equal_v<_Period, ratio<1, 4000000000>>
          struct __subseconds<duration<_Rep, _Period>>
          {
            uint_least32_t _M_r{};
 
            constexpr explicit
            operator duration<_Rep, _Period>() const noexcept
            { return duration<_Rep, _Period>(_M_r); }
          };
 
        chrono::hours                  _M_h{};
        __byte_duration<ratio<60>>  _M_m{};
        __byte_duration<ratio<1>>   _M_s{};
        bool                        _M_is_neg{};
        __subseconds<precision>     _M_ss{};
      };
 
    // 12/24 HOURS FUNCTIONS
 
    constexpr bool
    is_am(const hours& __h) noexcept
    { return 0h <= __h && __h <= 11h; }
 
    constexpr bool
    is_pm(const hours& __h) noexcept
    { return 12h <= __h && __h <= 23h; }
 
    constexpr hours
    make12(const hours& __h) noexcept
    {
      if (__h == 0h)
        return 12h;
      else if (__h > 12h)
        return __h - 12h;
      return __h;
    }
 
    constexpr hours
    make24(const hours& __h, bool __is_pm) noexcept
    {
      if (!__is_pm)
        {
          if (__h == 12h)
            return 0h;
          else
            return __h;
        }
      else
        {
          if (__h == 12h)
            return __h;
          else
            return __h + 12h;
        }
    }
 
#if _GLIBCXX_HOSTED
#if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
    // C++20 [time.zones] Time zones
 
    struct tzdb;
 
    struct sys_info
    {
      sys_seconds begin;
      sys_seconds end;
      seconds offset;
      minutes save;
      string abbrev;
    };
 
    struct local_info
    {
      static constexpr int unique      = 0;
      static constexpr int nonexistent = 1;
      static constexpr int ambiguous   = 2;
 
      int result;
      sys_info first;
      sys_info second;
    };
 
    class nonexistent_local_time : public runtime_error
    {
    public:
      template<typename _Duration>
        nonexistent_local_time(const local_time<_Duration>& __tp,
                               const local_info& __i)
        : runtime_error(_S_make_what_str(__tp, __i))
        { __glibcxx_assert(__i.result == local_info::nonexistent); }
 
    private:
      template<typename _Duration>
        static string
        _S_make_what_str(const local_time<_Duration>& __tp,
                         const local_info& __i)
        {
          std::ostringstream __os;
          __os << __tp << " is in a gap between\n"
               << local_seconds(__i.first.end.time_since_epoch())
               + __i.first.offset << ' ' << __i.first.abbrev << " and\n"
               << local_seconds(__i.second.begin.time_since_epoch())
               + __i.second.offset << ' ' << __i.second.abbrev
               << " which are both equivalent to\n"
               << __i.first.end << " UTC";
          return std::move(__os).str();
        }
    };
 
    class ambiguous_local_time : public runtime_error
    {
    public:
      template<typename _Duration>
        ambiguous_local_time(const local_time<_Duration>& __tp,
                             const local_info& __i)
        : runtime_error(_S_make_what_str(__tp, __i))
        { __glibcxx_assert(__i.result == local_info::ambiguous); }
 
    private:
      template<typename _Duration>
        static string
        _S_make_what_str(const local_time<_Duration>& __tp,
                         const local_info& __i)
        {
          std::ostringstream __os;
          __os << __tp << " is ambiguous.  It could be\n"
               << __tp << ' ' << __i.first.abbrev << " == "
               << __tp - __i.first.offset << " UTC or\n"
               << __tp << ' ' << __i.second.abbrev << " == "
               << __tp - __i.second.offset << " UTC";
          return std::move(__os).str();
        }
    };
 
    template<typename _Duration>
      [[noreturn]] void
      __throw_bad_local_time(const local_time<_Duration>& __tp,
                             const local_info& __i)
      {
#if __cpp_exceptions
        if (__i.result == local_info::nonexistent)
          throw nonexistent_local_time(__tp, __i);
        throw ambiguous_local_time(__tp, __i);
#else
        __builtin_abort();
#endif
      }
 
    enum class choose { earliest, latest };
 
    class time_zone
    {
    public:
      time_zone(time_zone&&) = default;
      time_zone& operator=(time_zone&&) = default;
 
      ~time_zone();
 
      [[nodiscard]]
      string_view name() const noexcept { return _M_name; }
 
      template<typename _Duration>
        sys_info
        get_info(const sys_time<_Duration>& __st) const
        { return _M_get_sys_info(chrono::floor<seconds>(__st)); }
 
      template<typename _Duration>
        local_info
        get_info(const local_time<_Duration>& __tp) const
        { return _M_get_local_info(chrono::floor<seconds>(__tp)); }
 
      template<typename _Duration>
        sys_time<common_type_t<_Duration, seconds>>
        to_sys(const local_time<_Duration>& __tp) const
        {
          local_info __info = get_info(__tp);
 
          if (__info.result != local_info::unique)
            __throw_bad_local_time(__tp, __info);
 
          return sys_time<_Duration>(__tp.time_since_epoch())
                   - __info.first.offset;
        }
 
      template<typename _Duration>
        sys_time<common_type_t<_Duration, seconds>>
        to_sys(const local_time<_Duration>& __tp, choose __z) const
        {
          local_info __info = get_info(__tp);
 
          if (__info.result == local_info::nonexistent)
            return __info.first.end; // Last second of the previous sys_info.
 
          sys_time<_Duration> __st(__tp.time_since_epoch());
 
          if (__info.result == local_info::ambiguous && __z == choose::latest)
            return __st - __info.second.offset; // Time in the later sys_info.
          // else if __z == earliest, use __info.first.offset as below:
 
          return __st - __info.first.offset;
        }
 
      template<typename _Duration>
        local_time<common_type_t<_Duration, seconds>>
        to_local(const sys_time<_Duration>& __tp) const
        {
          auto __d = (__tp + get_info(__tp).offset).time_since_epoch();
          return local_time<common_type_t<_Duration, seconds>>(__d);
        }
 
      [[nodiscard]] friend bool
      operator==(const time_zone& __x, const time_zone& __y) noexcept
      { return __x._M_name == __y._M_name; }
 
      [[nodiscard]] friend strong_ordering
      operator<=>(const time_zone& __x, const time_zone& __y) noexcept
      { return __x._M_name <=> __y._M_name; }
 
    private:
      sys_info _M_get_sys_info(sys_seconds) const;
      local_info _M_get_local_info(local_seconds) const;
 
      friend const tzdb& reload_tzdb();
      friend struct tzdb;
      friend class tzdb_list;
 
      struct _Impl;
 
      explicit time_zone(unique_ptr<_Impl> __p);
      string _M_name;
      unique_ptr<_Impl> _M_impl;
    };
 
    const time_zone* locate_zone(string_view __tz_name);
    const time_zone* current_zone();
 
    /** The list of `chrono::tzdb` objects
     *
     * A single object of this type is constructed by the C++ runtime,
     * and can be accessed by calling `chrono::get_tzdb_list()`.
     *
     * The front of the list is the current `tzdb` object and can be accessed
     * via `chrono::get_tzdb_list().front()` or `chrono::get_tzdb()` or
     * `*chrono::get_tzdb_list().begin()`.
     *
     * The `chrono::reload_tzdb()` function will check for a newer version
     * and if found, insert it at the front of the list.
     *
     * @since C++20
     */
    class tzdb_list
    {
      struct _Node;
 
    public:
      tzdb_list(const tzdb_list&) = delete;
      tzdb_list& operator=(const tzdb_list&) = delete;
 
      /** An iterator into the `tzdb_list`
       *
       * As a extension, in libstdc++ each `tzdb` is reference-counted
       * and the `const_iterator` type shares ownership of the object it
       * refers to. This ensures that a `tzdb` erased from the list will
       * not be destroyed while there is an iterator that refers to it.
       */
      class const_iterator
      {
      public:
        using value_type        = tzdb;
        using reference         = const tzdb&;
        using pointer           = const tzdb*;
        using difference_type   = ptrdiff_t;
        using iterator_category = forward_iterator_tag;
 
        constexpr const_iterator() = default;
        const_iterator(const const_iterator&) = default;
        const_iterator(const_iterator&&) = default;
        const_iterator& operator=(const const_iterator&) = default;
        const_iterator& operator=(const_iterator&&) = default;
 
        reference operator*() const noexcept;
        pointer operator->() const noexcept { return &**this; }
        const_iterator& operator++();
        const_iterator operator++(int);
 
        bool operator==(const const_iterator&) const noexcept = default;
 
      private:
        explicit const_iterator(const shared_ptr<_Node>&) noexcept;
 
        friend class tzdb_list;
 
        shared_ptr<_Node> _M_node;
        void* _M_reserved = nullptr;
      };
 
      /** Access the current `tzdb` at the front of the list.
       *
       * This returns a reference to the same object as `chrono::get_tzdb()`.
       *
       * @returns A reference to the current tzdb object.
       * @since C++20
       */
      const tzdb& front() const noexcept;
 
      /** Remove the tzdb object _after_ the one the iterator refers to.
       *
       * Calling this function concurrently with any of `front()`, `begin()`,
       * or `end()` does not cause a data race, but in general this function
       * is not thread-safe. The behaviour may be undefined if erasing an
       * element from the list while another thread is calling the same
       * function, or incrementing an iterator into the list, or accessing
       * the element being erased (unless it is accessed through an iterator).
       *
       * @param __p A dereferenceable iterator.
       * @returns An iterator the element after the one that was erased
       *          (or `end()` if there is no such element).
       * @since C++20
       */
      const_iterator erase_after(const_iterator __p);
 
      const_iterator begin() const noexcept;
      const_iterator end() const noexcept { return {}; }
      const_iterator cbegin() const noexcept { return begin(); }
      const_iterator cend() const noexcept { return end(); }
 
    private:
      constexpr explicit tzdb_list(nullptr_t);
 
      friend tzdb_list& get_tzdb_list();
      friend const tzdb& get_tzdb();
      friend const tzdb& reload_tzdb();
      friend struct tzdb;
      friend class leap_second;
      friend struct time_zone::_Impl;
      friend class time_zone_link;
    };
 
    class time_zone_link
    {
    public:
      time_zone_link(time_zone_link&&) = default;
      time_zone_link& operator=(time_zone_link&&) = default;
 
      string_view name() const noexcept { return _M_name; }
      string_view target() const noexcept { return _M_target; }
 
      friend bool
      operator==(const time_zone_link& __x, const time_zone_link& __y) noexcept
      { return __x.name() == __y.name(); }
 
      friend strong_ordering
      operator<=>(const time_zone_link& __x, const time_zone_link& __y) noexcept
      { return __x.name() <=> __y.name(); }
 
    private:
      friend const tzdb& reload_tzdb();
      friend struct tzdb_list::_Node;
 
      explicit time_zone_link(nullptr_t) { }
 
      string _M_name;
      string _M_target;
    };
 
    class leap_second
    {
    public:
      leap_second(const leap_second&) = default;
      leap_second& operator=(const leap_second&) = default;
 
      [[nodiscard]]
      constexpr sys_seconds
      date() const noexcept
      {
        if (_M_s >= _M_s.zero()) [[likely]]
          return sys_seconds(_M_s);
        return sys_seconds(-_M_s);
      }
 
      [[nodiscard]]
      constexpr seconds
      value() const noexcept
      {
        if (_M_s >= _M_s.zero()) [[likely]]
          return seconds(1);
        return seconds(-1);
      }
 
      // This can be defaulted because the database will never contain two
      // leap_second objects with the same date but different signs.
      [[nodiscard]] friend constexpr bool
      operator==(const leap_second&, const leap_second&) noexcept = default;
 
      [[nodiscard]] friend constexpr strong_ordering
      operator<=>(const leap_second& __x, const leap_second& __y) noexcept
      { return __x.date() <=> __y.date(); }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator==(const leap_second& __x,
                   const sys_time<_Duration>& __y) noexcept
        { return __x.date() == __y; }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator<(const leap_second& __x,
                  const sys_time<_Duration>& __y) noexcept
        { return __x.date() < __y; }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator<(const sys_time<_Duration>& __x,
                  const leap_second& __y) noexcept
        { return __x < __y.date(); }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator>(const leap_second& __x,
                  const sys_time<_Duration>& __y) noexcept
        { return __y < __x.date(); }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator>(const sys_time<_Duration>& __x,
                  const leap_second& __y) noexcept
        { return __y.date() < __x; }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator<=(const leap_second& __x,
                   const sys_time<_Duration>& __y) noexcept
        { return !(__y < __x.date()); }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator<=(const sys_time<_Duration>& __x,
                   const leap_second& __y) noexcept
        { return !(__y.date() < __x); }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator>=(const leap_second& __x,
                   const sys_time<_Duration>& __y) noexcept
        { return !(__x.date() < __y); }
 
      template<typename _Duration>
        [[nodiscard]] friend constexpr bool
        operator>=(const sys_time<_Duration>& __x,
                   const leap_second& __y) noexcept
        { return !(__x < __y.date()); }
 
      // This is a simplified form of the constraint specified in the standard,
      // three_way_comparable_with<sys_seconds, sys_time<_Duration>>.
      template<three_way_comparable_with<seconds> _Duration>
        [[nodiscard]] friend constexpr auto
        operator<=>(const leap_second& __x,
                    const sys_time<_Duration>& __y) noexcept
        { return __x.date() <=> __y; }
 
    private:
      explicit leap_second(seconds::rep __s) : _M_s(__s) { }
 
      friend struct tzdb_list::_Node;
 
      friend const tzdb& reload_tzdb();
 
      template<typename _Duration>
        friend leap_second_info
        get_leap_second_info(const utc_time<_Duration>&);
 
      seconds _M_s; // == date().time_since_epoch() * value().count()
    };
 
    template<class _Tp> struct zoned_traits { };
 
    template<>
      struct zoned_traits<const time_zone*>
      {
        static const time_zone*
        default_zone()
        { return std::chrono::locate_zone("UTC"); }
 
        static const time_zone*
        locate_zone(string_view __name)
        { return std::chrono::locate_zone(__name); }
      };
 
    struct tzdb
    {
      string version;
      _GLIBCXX_STD_C::vector<time_zone> zones;
      _GLIBCXX_STD_C::vector<time_zone_link> links;
      _GLIBCXX_STD_C::vector<leap_second> leap_seconds;
 
      const time_zone*
      locate_zone(string_view __tz_name) const;
 
      const time_zone*
      current_zone() const;
 
    private:
      friend const tzdb& reload_tzdb();
      friend class time_zone;
      friend struct tzdb_list::_Node;
    };
 
    tzdb_list& get_tzdb_list();
    const tzdb& get_tzdb();
 
    const tzdb& reload_tzdb();
    string remote_version();
 
    template<typename _Duration, typename _TimeZonePtr = const time_zone*>
      class zoned_time
      {
        static_assert(__is_duration_v<_Duration>);
 
        using _Traits = zoned_traits<_TimeZonePtr>;
 
        // Every constructor that accepts a string_view as its first parameter
        // does not participate in class template argument deduction.
        using string_view = type_identity_t<std::string_view>;
 
      public:
        using duration = common_type_t<_Duration, seconds>;
 
        zoned_time() requires requires { _Traits::default_zone(); }
        { }
 
        zoned_time(const zoned_time&) = default;
        zoned_time& operator=(const zoned_time&) = default;
 
        zoned_time(const sys_time<_Duration>& __st)
          requires requires { _Traits::default_zone(); }
        : _M_tp(__st)
        { }
 
        explicit
        zoned_time(_TimeZonePtr __z) : _M_zone(std::move(__z)) { }
 
        explicit
        zoned_time(string_view __name)
          requires requires {
            _TimeZonePtr{_Traits::locate_zone(std::string_view{})};
          }
        : _M_zone(_Traits::locate_zone(__name))
        { }
 
        template<typename _Duration2>
          zoned_time(const zoned_time<_Duration2, _TimeZonePtr>& __zt)
          requires is_convertible_v<sys_time<_Duration2>, sys_time<_Duration>>
          : _M_zone(__zt._M_zone), _M_tp(__zt._M_tp)
          { }
 
        zoned_time(_TimeZonePtr __z, const sys_time<_Duration>& __st)
        : _M_zone(std::move(__z)), _M_tp(__st)
        { }
 
        zoned_time(string_view __name, const sys_time<_Duration>& __st)
        : zoned_time(_Traits::locate_zone(__name), __st)
        { }
 
        zoned_time(_TimeZonePtr __z, const local_time<_Duration>& __tp)
        requires requires {
          { __z->to_sys(__tp) } -> convertible_to<sys_time<_Duration>>;
        }
        : _M_zone(std::move(__z)), _M_tp(_M_zone->to_sys(__tp))
        { }
 
        zoned_time(string_view __name, const local_time<_Duration>& __tp)
        requires requires (_TimeZonePtr __z) {
          { _Traits::locate_zone(__name) } -> convertible_to<_TimeZonePtr>;
          { __z->to_sys(__tp) } -> convertible_to<sys_time<_Duration>>;
        }
        : zoned_time(_Traits::locate_zone(__name), __tp)
        { }
 
        zoned_time(_TimeZonePtr __z, const local_time<_Duration>& __tp,
                   choose __c)
        requires requires {
          { __z->to_sys(__tp, __c) } -> convertible_to<sys_time<_Duration>>;
        }
        : _M_zone(std::move(__z)), _M_tp(_M_zone->to_sys(__tp, __c))
        { }
 
        zoned_time(string_view __name, const local_time<_Duration>& __tp,
                   choose __c)
        requires requires (_TimeZonePtr __z) {
          { _Traits::locate_zone(__name) } -> convertible_to<_TimeZonePtr>;
          { __z->to_sys(__tp, __c) } -> convertible_to<sys_time<_Duration>>;
        }
        : _M_zone(_Traits::locate_zone(__name)),
          _M_tp(_M_zone->to_sys(__tp, __c))
        { }
 
        template<typename _Duration2, typename _TimeZonePtr2>
          zoned_time(_TimeZonePtr __z,
                     const zoned_time<_Duration2, _TimeZonePtr2>& __zt)
          requires is_convertible_v<sys_time<_Duration2>, sys_time<_Duration>>
          : _M_zone(__z), _M_tp(__zt._M_tp)
          { }
 
        template<typename _Duration2, typename _TimeZonePtr2>
          zoned_time(_TimeZonePtr __z,
                     const zoned_time<_Duration2, _TimeZonePtr2>& __zt,
                     choose)
          requires is_convertible_v<sys_time<_Duration2>, sys_time<_Duration>>
          : _M_zone(__z), _M_tp(__zt._M_tp)
          { }
 
        template<typename _Duration2, typename _TimeZonePtr2>
          zoned_time(string_view __name,
                     const zoned_time<_Duration2, _TimeZonePtr2>& __zt)
          requires is_convertible_v<sys_time<_Duration2>, sys_time<_Duration>>
          && requires {
            { _Traits::locate_zone(__name) } -> convertible_to<_TimeZonePtr>;
          }
          : _M_zone(_Traits::locate_zone(__name)), _M_tp(__zt._M_tp)
          { }
 
        template<typename _Duration2, typename _TimeZonePtr2>
          zoned_time(string_view __name,
                     const zoned_time<_Duration2, _TimeZonePtr2>& __zt,
                     choose)
          requires is_convertible_v<sys_time<_Duration2>, sys_time<_Duration>>
          && requires {
            { _Traits::locate_zone(__name) } -> convertible_to<_TimeZonePtr>;
          }
          : _M_zone(_Traits::locate_zone(__name)), _M_tp(__zt._M_tp)
          { }
 
        zoned_time&
        operator=(const sys_time<_Duration>& __st)
        {
          _M_tp = __st;
          return *this;
        }
 
        zoned_time&
        operator=(const local_time<_Duration>& __lt)
        {
          _M_tp = _M_zone->to_sys(__lt);
          return *this;
        }
 
        [[nodiscard]]
        operator sys_time<duration>() const { return _M_tp; }
 
        [[nodiscard]]
        explicit operator local_time<duration>() const
        { return get_local_time(); }
 
        [[nodiscard]]
        _TimeZonePtr
        get_time_zone() const
        { return _M_zone; }
 
        [[nodiscard]]
        local_time<duration>
        get_local_time() const
        { return _M_zone->to_local(_M_tp); }
 
        [[nodiscard]]
        sys_time<duration>
        get_sys_time() const
        { return _M_tp; }
 
        [[nodiscard]]
        sys_info
        get_info() const
        { return _M_zone->get_info(_M_tp); }
 
        [[nodiscard]] friend bool
        operator==(const zoned_time&, const zoned_time&) = default;
 
      private:
        _TimeZonePtr       _M_zone{ _Traits::default_zone() };
        sys_time<duration> _M_tp{};
 
        template<typename _Duration2, typename _TimeZonePtr2>
          friend class zoned_time;
      };
 
    zoned_time() -> zoned_time<seconds>;
 
    template<typename _Duration>
    zoned_time(sys_time<_Duration>)
      -> zoned_time<common_type_t<_Duration, seconds>>;
 
  /// @cond undocumented
  template<typename _TimeZonePtrOrName>
    using __time_zone_representation
      = __conditional_t<is_convertible_v<_TimeZonePtrOrName, string_view>,
                        const time_zone*,
                        remove_cvref_t<_TimeZonePtrOrName>>;
  /// @endcond
 
  template<typename _TimeZonePtrOrName>
    zoned_time(_TimeZonePtrOrName&&)
      -> zoned_time<seconds, __time_zone_representation<_TimeZonePtrOrName>>;
 
  template<typename _TimeZonePtrOrName, typename _Duration>
    zoned_time(_TimeZonePtrOrName&&, sys_time<_Duration>)
      -> zoned_time<common_type_t<_Duration, seconds>,
                    __time_zone_representation<_TimeZonePtrOrName>>;
 
  template<typename _TimeZonePtrOrName, typename _Duration>
    zoned_time(_TimeZonePtrOrName&&, local_time<_Duration>,
               choose = choose::earliest)
      -> zoned_time<common_type_t<_Duration, seconds>,
                    __time_zone_representation<_TimeZonePtrOrName>>;
 
  template<typename _Duration, typename _TimeZonePtrOrName,
           typename _TimeZonePtr2>
    zoned_time(_TimeZonePtrOrName&&, zoned_time<_Duration, _TimeZonePtr2>,
               choose = choose::earliest)
      -> zoned_time<common_type_t<_Duration, seconds>,
                    __time_zone_representation<_TimeZonePtrOrName>>;
 
  template<typename _Dur1, typename _TZPtr1, typename _Dur2, typename _TZPtr2>
    [[nodiscard]]
    inline bool
    operator==(const zoned_time<_Dur1, _TZPtr1>& __x,
               const zoned_time<_Dur2, _TZPtr2>& __y)
    {
      return __x.get_time_zone() == __y.get_time_zone()
               && __x.get_sys_time() == __y.get_sys_time();
    }
 
    using zoned_seconds = zoned_time<seconds>;
#endif // _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
 
namespace __detail
{
    inline leap_second_info
    __get_leap_second_info(sys_seconds __ss, bool __is_utc)
    {
      if (__ss < sys_seconds{}) [[unlikely]]
        return {};
 
      const seconds::rep __leaps[] {
          78796800, // 1 Jul 1972
          94694400, // 1 Jan 1973
         126230400, // 1 Jan 1974
         157766400, // 1 Jan 1975
         189302400, // 1 Jan 1976
         220924800, // 1 Jan 1977
         252460800, // 1 Jan 1978
         283996800, // 1 Jan 1979
         315532800, // 1 Jan 1980
         362793600, // 1 Jul 1981
         394329600, // 1 Jul 1982
         425865600, // 1 Jul 1983
         489024000, // 1 Jul 1985
         567993600, // 1 Jan 1988
         631152000, // 1 Jan 1990
         662688000, // 1 Jan 1991
         709948800, // 1 Jul 1992
         741484800, // 1 Jul 1993
         773020800, // 1 Jul 1994
         820454400, // 1 Jan 1996
         867715200, // 1 Jul 1997
         915148800, // 1 Jan 1999
        1136073600, // 1 Jan 2006
        1230768000, // 1 Jan 2009
        1341100800, // 1 Jul 2012
        1435708800, // 1 Jul 2015
        1483228800, // 1 Jan 2017
      };
      // The list above is known to be valid until (at least) this date
      // and only contains positive leap seconds.
      const sys_seconds __expires(1766880000s); // 2025-12-28 00:00:00 UTC
 
#if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
      if (__ss > __expires)
        {
          // Use updated leap_seconds from tzdb.
          size_t __n = std::size(__leaps);
 
          auto __db = get_tzdb_list().begin();
          auto __first = __db->leap_seconds.begin() + __n;
          auto __last = __db->leap_seconds.end();
          auto __pos = std::upper_bound(__first, __last, __ss);
          seconds __elapsed(__n);
          for (auto __i = __first; __i != __pos; ++__i)
            __elapsed += __i->value();
 
          if (__is_utc)
            {
              // Convert utc_time to sys_time:
              __ss -= __elapsed;
              // See if that sys_time is before (or during) previous leap sec:
              if (__pos != __first && __ss < __pos[-1])
                {
                  if ((__ss + 1s) >= __pos[-1])
                    return {true, __elapsed};
                  __elapsed -= __pos[-1].value();
                }
            }
          return {false, __elapsed};
        }
      else
#endif
        {
          seconds::rep __s = __ss.time_since_epoch().count();
          const seconds::rep* __first = std::begin(__leaps);
          const seconds::rep* __last = std::end(__leaps);
 
          // Don't bother searching the list if we're after the last one.
          if (__s > (__last[-1] + (__last - __first) + 1))
            return { false, seconds(__last - __first) };
 
          auto __pos = std::upper_bound(__first, __last, __s);
          seconds __elapsed{__pos - __first};
          if (__is_utc)
            {
              // Convert utc_time to sys_time:
              __s -= __elapsed.count();
              // See if that sys_time is before (or during) previous leap sec:
              if (__pos != __first && __s < __pos[-1])
                {
                  if ((__s + 1) >= __pos[-1])
                    return {true, __elapsed};
                  --__elapsed;
                }
            }
          return {false, __elapsed};
        }
    }
} // namespace __detail
 
    template<typename _Duration>
      [[nodiscard]]
      inline leap_second_info
      get_leap_second_info(const utc_time<_Duration>& __ut)
      {
        auto __s = chrono::duration_cast<seconds>(__ut.time_since_epoch());
        return __detail::__get_leap_second_info(sys_seconds(__s), true);
      }
 
    template<typename _Duration>
      [[nodiscard]]
      inline utc_time<common_type_t<_Duration, seconds>>
      utc_clock::from_sys(const sys_time<_Duration>& __t)
      {
        using _CDur = common_type_t<_Duration, seconds>;
        auto __s = chrono::time_point_cast<seconds>(__t);
        const auto __li = __detail::__get_leap_second_info(__s, false);
        return utc_time<_CDur>{__t.time_since_epoch()} + __li.elapsed;
      }
#endif // _GLIBCXX_HOSTED
 
    /// @} group chrono
#endif // C++20
  } // namespace chrono
 
#if __glibcxx_chrono_cxx20 >= 202306 // C++26
  // Hash support [time.hash]
 
  template<typename _Tp>
    concept __is_nothrow_copy_hashable = requires(const _Tp& __t) {
      { hash<_Tp>{}(_Tp(__t)) } noexcept -> same_as<size_t>;
    };
 
 namespace chrono {
 
   template<typename _T1, typename... _Ts>
     [[__gnu__::__always_inline__]]
     constexpr auto
     __pack_ints(_T1 __v1, _Ts... __vs)
     {
       using _ResT = decltype([] {
         constexpr size_t __tsize = (sizeof(_T1) + ... + sizeof(_Ts));
         if constexpr (__tsize <= 1)
           return static_cast<unsigned char>(0);
         else if constexpr (__tsize <= 2)
           return static_cast<__UINT16_TYPE__>(0);
         else if constexpr (__tsize <= 4)
           return static_cast<__UINT32_TYPE__>(0);
         else if constexpr (__tsize <= 8)
           return static_cast<__UINT64_TYPE__>(0);
         else
           static_assert(__tsize <= 8);
       }());
 
       _ResT __res = __v1;
       ((__res = (__res << (sizeof(_Ts) * __CHAR_BIT__) | _ResT(__vs))), ...);
       return __res;
     }
    
   template<typename _Tp>
     [[__gnu__::__always_inline__]]
     constexpr auto
     __as_int(_Tp __val)
     {
       if constexpr (is_same_v<_Tp, year>)
         return static_cast<unsigned short>(static_cast<int>(__val));
       else if constexpr (is_same_v<_Tp, month> || is_same_v<_Tp, day>)
         return static_cast<unsigned char>(static_cast<unsigned>(__val));
       else if constexpr (is_same_v<_Tp, weekday>)
         return static_cast<unsigned char>(__val.c_encoding());
       else if constexpr (is_same_v<_Tp, weekday_indexed>)
         return __pack_ints(chrono::__as_int(__val.weekday()),
                            static_cast<unsigned char>(__val.index()));
       else if constexpr (is_same_v<_Tp, weekday_last>)
         return chrono::__as_int(__val.weekday());
       else
         static_assert(false);
     }
 
   template<typename _Arg, typename... _Args>
     size_t
     __int_hash(_Arg __arg, _Args... __args)
     {
       static_assert((is_integral_v<_Arg> && ... && is_integral_v<_Args>));
 
       // TODO consider using a better quality hasher
       using _Hasher = _Hash_impl;
       size_t __result = _Hasher::hash(__arg);
       ((__result = _Hasher::__hash_combine(__args, __result)), ...);
       return __result;
     }
 
   template<typename... _Tps>
     [[__gnu__::__always_inline__]]
     inline size_t
     __hash(_Tps... __vals)
     {
       if constexpr (sizeof...(_Tps) == 1)
         return chrono::__int_hash(chrono::__as_int(__vals)...);
       else
         {
           auto __res = chrono::__pack_ints(chrono::__as_int(__vals)...);
           return chrono::__int_hash(__res);
         }
     }
 } // namespace chrono
 
  // duration
  template<typename _Rep, typename _Period>
    requires __is_hash_enabled_for<_Rep>
    struct hash<chrono::duration<_Rep, _Period>>
    {
      size_t
      operator()(const chrono::duration<_Rep, _Period>& __val) const
      noexcept(__is_nothrow_copy_hashable<_Rep>)
      { 
        if constexpr (is_integral_v<_Rep>)
          return chrono::__int_hash(__val.count());
        else
          return hash<_Rep>{}(__val.count());
      }
    };
 
  template<typename _Rep, typename _Period>
    struct __is_fast_hash<hash<chrono::duration<_Rep, _Period>>>
    : __is_fast_hash<hash<_Rep>>
    {};
 
  // time_point
  template<typename _Clock, typename _Dur>
    requires __is_hash_enabled_for<_Dur>
    struct hash<chrono::time_point<_Clock, _Dur>>
    {
      size_t
      operator()(const chrono::time_point<_Clock, _Dur>& __val) const
      noexcept(__is_nothrow_copy_hashable<_Dur>)
      { return hash<_Dur>{}(__val.time_since_epoch()); }
    };
 
  template<typename _Clock, typename _Dur>
    struct __is_fast_hash<hash<chrono::time_point<_Clock, _Dur>>>
    : __is_fast_hash<hash<_Dur>>
    {};
 
  // day
  template<>
    struct hash<chrono::day>
    {
      size_t
      operator()(chrono::day __val) const noexcept
      { return chrono::__hash(__val); }
    };
 
  // month
  template<>
    struct hash<chrono::month>
    {
      size_t
      operator()(chrono::month __val) const noexcept
      { return chrono::__hash(__val); }
    };
 
  // year
  template<>
    struct hash<chrono::year>
    {
      size_t
      operator()(chrono::year __val) const noexcept
      { return chrono::__hash(__val); }
    };
 
  // weekday
  template<>
    struct hash<chrono::weekday>
    {
      size_t
      operator()(chrono::weekday __val) const noexcept
      { return chrono::__hash(__val); }
    };
 
  // weekday_indexed
  template<>
    struct hash<chrono::weekday_indexed>
    {
      size_t
      operator()(chrono::weekday_indexed __val) const noexcept
      { return chrono::__hash(__val); }
    };
 
  // weekday_last
  template<>
    struct hash<chrono::weekday_last>
    {
      size_t
      operator()(chrono::weekday_last __val) const noexcept
      { return chrono::__hash(__val); }
    };
 
  // month_day
  template<>
    struct hash<chrono::month_day>
    {
      size_t
      operator()(chrono::month_day __val) const noexcept
      { return chrono::__hash(__val.month(), __val.day()); }
    };
 
  // month_day_last
  template<>
    struct hash<chrono::month_day_last>
    {
      size_t operator()(chrono::month_day_last __val) const noexcept
      { return chrono::__hash(__val.month()); }
    };
 
  // month_weekday
  template<>
    struct hash<chrono::month_weekday>
    {
      size_t
      operator()(chrono::month_weekday __val) const noexcept
      { return chrono::__hash(__val.month(), __val.weekday_indexed()); }
    };
 
  // month_weekday_last
  template<>
    struct hash<chrono::month_weekday_last>
    {
      size_t
      operator()(chrono::month_weekday_last __val) const noexcept
      { return chrono::__hash(__val.month(), __val.weekday_last()); }
    };
 
  // year_month
  template<>
    struct hash<chrono::year_month>
    {
      size_t
      operator()(chrono::year_month __val) const noexcept
      { return chrono::__hash(__val.year(), __val.month()); }
    };
 
  // year_month_day
  template<>
    struct hash<chrono::year_month_day>
    {
      size_t
      operator()(chrono::year_month_day __val) const noexcept
      { return chrono::__hash(__val.year(), __val.month(), __val.day()); }
    };
 
  // year_month_day_last
  template<>
    struct hash<chrono::year_month_day_last>
    {
      size_t
      operator()(chrono::year_month_day_last __val) const noexcept
      { return chrono::__hash(__val.year(), __val.month()); }
    };
 
  // year_month_weekday
  template<>
    struct hash<chrono::year_month_weekday>
    {
      size_t
      operator()(chrono::year_month_weekday __val) const noexcept
      { 
        return chrono::__hash(__val.year(), __val.month(),
                              __val.weekday_indexed());
      }
    };
 
  // year_month_weekday_last
  template<>
    struct hash<chrono::year_month_weekday_last>
    {
      size_t
      operator()(chrono::year_month_weekday_last __val) const noexcept
      { 
        return chrono::__hash(__val.year(), __val.month(),
                              __val.weekday_last());
      }
    };
 
#if _GLIBCXX_HOSTED
#if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
  // zoned_time
  template<typename _Duration, typename _TimeZonePtr>
    requires __is_hash_enabled_for<
               typename chrono::zoned_time<_Duration, _TimeZonePtr>::duration>
             && __is_hash_enabled_for<_TimeZonePtr>
    struct hash<chrono::zoned_time<_Duration, _TimeZonePtr>>
    {
    private:
      using _ActualDuration =
        typename chrono::zoned_time<_Duration, _TimeZonePtr>::duration;
 
    public:
      size_t
      operator()(const chrono::zoned_time<_Duration, _TimeZonePtr>& __val) const
      noexcept(__is_nothrow_copy_hashable<_ActualDuration>
               && __is_nothrow_copy_hashable<_TimeZonePtr>)
      {
        const auto __iduration = [&] {
          const _ActualDuration __sd = __val.get_sys_time().time_since_epoch();
          if constexpr (is_integral_v<typename _ActualDuration::rep>)
            return __sd.count();
          else
            return hash<_ActualDuration>{}(__sd);
        }();
 
        const auto __izone = [&] {
          const _TimeZonePtr __tz = __val.get_time_zone();
          if constexpr (is_same_v<_TimeZonePtr, const chrono::time_zone*>)
            return reinterpret_cast<uintptr_t>(__tz);
          else
            return hash<_TimeZonePtr>{}(__tz);
        }();
 
        return chrono::__int_hash(__iduration, __izone);
      }
    };
 
  template<typename _Duration, typename _TimeZonePtr>
    struct __is_fast_hash<hash<chrono::zoned_time<_Duration, _TimeZonePtr>>>
    : __and_<__is_fast_hash<hash<
               typename chrono::zoned_time<_Duration, _TimeZonePtr>::duration>>,
             __is_fast_hash<hash<_TimeZonePtr>>>
    {};
 
  // leap_second
  template<>
    struct hash<chrono::leap_second>
    {
      size_t
      operator()(chrono::leap_second __val) const noexcept
      { 
        return chrono::__int_hash(
                 __val.date().time_since_epoch().count(),
                 __val.value().count());
      }
    };
#endif // _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
#endif // _GLIBCXX_HOSTED
#endif // __glibcxx_chrono_cxx20 >= 202306
 
#ifdef __glibcxx_chrono_cxx20
  inline namespace literals
  {
  inline namespace chrono_literals
  {
    /// @addtogroup chrono
    /// @{
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wliteral-suffix"
    /// Literal suffix for creating chrono::day objects.
    /// @since C++20
    constexpr chrono::day
    operator""d(unsigned long long __d) noexcept
    { return chrono::day{static_cast<unsigned>(__d)}; }
 
    /// Literal suffix for creating chrono::year objects.
    /// @since C++20
    constexpr chrono::year
    operator""y(unsigned long long __y) noexcept
    { return chrono::year{static_cast<int>(__y)}; }
#pragma GCC diagnostic pop
    /// @}
  } // inline namespace chrono_literals
  } // inline namespace literals
#endif // C++20
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#if defined __glibcxx_chrono_cxx20 && _GLIBCXX_HOSTED
# include <bits/chrono_io.h>
#endif
 
#endif // C++11
 
#endif //_GLIBCXX_CHRONO