libstdc++: Change names that clash with Win32 or Clang

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

// <chrono> -*- C++ -*-

// Copyright (C) 2008-2022 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/chrono
 *  This is a Standard C++ Library header.
 *  @ingroup chrono
 */

#ifndef _GLIBCXX_CHRONO
#define _GLIBCXX_CHRONO 1

#pragma GCC system_header

#include <bits/requires_hosted.h> // for <ctime> and clocks

#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else

#include <bits/chrono.h>

#if __cplusplus >= 202002L
# include <bit>
# include <sstream>
# include <string>
# include <vector>
# include <bits/charconv.h> // __to_chars_len, __to_chars_10_impl
# include <bits/stl_algo.h> // upper_bound TODO: move leap_second_info to .so
# include <bits/shared_ptr.h>
# include <bits/unique_ptr.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
  {
#if __cplusplus >= 202002L
    /// @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>;

    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;

      static time_point
      now()
      { return from_sys(system_clock::now()); }

      template<typename _Duration>
        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() - seconds{__li.elapsed}};
          if (__li.is_leap_second)
            __s = chrono::floor<seconds>(__s) + seconds{1} - _CDur{1};
          return __s;
        }

      template<typename _Duration>
        static utc_time<common_type_t<_Duration, seconds>>
        from_sys(const sys_time<_Duration>& __t)
        {
          using _CDur = common_type_t<_Duration, seconds>;
          utc_time<_Duration> __u(__t.time_since_epoch());
          const auto __li = chrono::get_leap_second_info(__u);
          return utc_time<_CDur>{__u} + seconds{__li.elapsed};
        }
    };

    /** 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; // XXX true for CLOCK_TAI?

      // TODO move into lib, use CLOCK_TAI on linux, add extension point.
      static time_point
      now()
      { return from_utc(utc_clock::now()); }

      template<typename _Duration>
        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>
        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; // XXX

      // TODO move into lib, add extension point.
      static time_point
      now()
      { return from_utc(utc_clock::now()); }

      template<typename _Duration>
        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>
        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;
        }
    };


    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; }
      };

    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);
          }
      };

    /// @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);
          };

      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)));
          };

    } // namespace __detail
    /// @endcond

    /// Convert a time point to a different clock.
    template<typename _DestClock, typename _SourceClock, typename _Duration>
      inline auto
      clock_cast(const time_point<_SourceClock, _Duration>& __t)
      requires __detail::__clock_convs<_DestClock, _SourceClock, _Duration>
        || __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>
      {
       constexpr bool __direct
         = __detail::__clock_convs<_DestClock, _SourceClock, _Duration>;
       if constexpr (__direct)
         {
           return clock_time_conversion<_DestClock, _SourceClock>{}(__t);
         }
       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)));
                 }
             }
         }
      }

    // 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
    {
      // Compute the remainder of the Euclidean division of __n divided by __d.
      // Euclidean division truncates toward negative infinity and always
      // produces a remainder in the range of [0,__d-1] (whereas standard
      // division truncates toward zero and yields a nonpositive remainder
      // for negative __n).
      constexpr unsigned
      __modulo(long long __n, unsigned __d)
      {
        if (__n >= 0)
          return __n % __d;
        else
          return (__d + (__n % __d)) % __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;

      // TODO: Implement operator<<, to_stream, from_stream.
    };

    // 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
      {
        auto __n = static_cast<long long>(unsigned{__x}) + (__y.count() - 1);
        return month{__detail::__modulo(__n, 12) + 1};
      }

      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
      { return __x + -__y; }

      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;

      // TODO: Implement operator<<, to_stream, from_stream.
    };

    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, that is,
        // return (_M_y % 100 != 0 || _M_y % 400 == 0) && _M_y % 4 == 0;

        // It gets even faster if _M_y is in [-536870800, 536870999]
        // (which is the case here) and _M_y % 100 is replaced by
        // __is_multiple_of_100 below.

        // References:
        // [1] https://github.com/cassioneri/calendar
        // [2] https://accu.org/journals/overload/28/155/overload155.pdf#page=16

        // Furthermore, if y%100 == 0, then y%400==0 is equivalent to y%16==0,
        // so we can simplify it to (!mult_100 && y % 4 == 0) || y % 16 == 0,
        // which is equivalent to (y & (mult_100 ? 15 : 3)) == 0.
        // See https://gcc.gnu.org/pipermail/libstdc++/2021-June/052815.html

        constexpr uint32_t __multiplier   = 42949673;
        constexpr uint32_t __bound        = 42949669;
        constexpr uint32_t __max_dividend = 1073741799;
        constexpr uint32_t __offset       = __max_dividend / 2 / 100 * 100;
        const bool __is_multiple_of_100
          = __multiplier * (_M_y + __offset) < __bound;
        return (_M_y & (__is_multiple_of_100 ? 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;

      // TODO: Implement operator<<, to_stream, from_stream.
    };

    // WEEKDAY

    class weekday
    {
    private:
      unsigned char _M_wd;

      static constexpr weekday
      _S_from_days(const days& __d)
      {
        auto __n = __d.count();
        return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6);
      }

    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
      {
        auto __n = static_cast<long long>(__x._M_wd) + __y.count();
        return weekday{__detail::__modulo(__n, 7)};
      }

      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 __x + -__y; }

      friend constexpr days
      operator-(const weekday& __x, const weekday& __y) noexcept
      {
        auto __n = static_cast<long long>(__x._M_wd) - __y._M_wd;
        return days{__detail::__modulo(__n, 7)};
      }

      // TODO: operator<<, from_stream.
    };

    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;

      // TODO: Implement operator<<.
    };

    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;

      // TODO: Implement operator<<.
    };

    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;

      // TODO: Implement operator<<, from_stream.
    };

    // 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;

      // TODO: Implement operator<<.
    };

    // 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;

      // TODO: Implement operator<<.
    };

    // 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;

      // TODO: Implement operator<<.
    };

    // 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;

      // TODO: Implement operator<<, from_stream.
    };

    // 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; }

      // TODO: Implement operator<<, from_stream.
    };

    // 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());

        // Excluding February, the last day of month __m is either 30 or 31 or,
        // in another words, it is 30 + b = 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.

        // The above mathematically justifies this implementation whose
        // performance does not depend on look-up tables being on the L1 cache.
        return chrono::day{__m != 2 ? ((__m ^ (__m >> 3)) & 1) | 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; }

      // TODO: Implement operator<<.
    };

    // 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 __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; }

      // TODO: Implement operator<<.
    };

    // 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; }

      // TODO: Implement operator<<.
    };

    // 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) noexcept
        : _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();
        }

      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) noexcept
        : hh_mm_ss(chrono::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();
        }

        // TODO: Implement operator<<.

      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 {}; }
          };

        // True if the maximum constructor argument can be represented in _Tp.
        template<typename _Tp>
          static constexpr bool __fits
            = duration_values<typename _Duration::rep>::max()
                <= duration_values<_Tp>::max();

        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>>
                  || __fits<unsigned char>)
          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>>
                  || __fits<uint_least32_t>)
          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;
        }
    }

    // C++20 [time.zones] Time zones

    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> // TODO
        static string
        _S_make_what_str(const local_time<_Duration>&, const local_info&);
    };

    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::nonexistent); }

    private:
      template<typename _Duration> // TODO
        static string
        _S_make_what_str(const local_time<_Duration>&, const local_info&);
    };

    enum class choose { earliest, latest };

    class time_zone
    {
    public:
      time_zone(time_zone&&) = default;
      time_zone& operator=(time_zone&&) = default;

      string_view name() const noexcept { return _M_name; }

      template<typename _Duration>
        sys_info
        get_info(const sys_time<_Duration>& __st) const;

      template<typename _Duration>
        local_info
        get_info(const local_time<_Duration>& __tp) const;

      template<typename _Duration>
        sys_time<common_type_t<_Duration, seconds>>
        to_sys(const local_time<_Duration>& __tp) const;

      template<typename _Duration>
        sys_time<common_type_t<_Duration, seconds>>
        to_sys(const local_time<_Duration>& __tp, choose __z) const;

      template<typename _Duration>
        local_time<common_type_t<_Duration, seconds>>
        to_local(const sys_time<_Duration>& __tp) const;

      friend bool
      operator==(const time_zone& __x, const time_zone& __y) noexcept
      { return __x.name() == __y.name(); }

      friend strong_ordering
      operator<=>(const time_zone& __x, const time_zone& __y) noexcept
      { return __x.name() <=> __y.name(); }

    private:
      string _M_name;
      struct _Impl;
      unique_ptr<_Impl> _M_impl;
    };

    struct tzdb;
    const time_zone* locate_zone(string_view __tz_name);
    const time_zone* current_zone();

    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();
      // TODO unspecified additional constructors
      string _M_name;
      string _M_target;
    };

    class leap_second
    {
    public:
      leap_second(const leap_second&) = default;
      leap_second& operator=(const leap_second&) = default;

      constexpr sys_seconds
      date() const noexcept
      {
        if (_M_s >= _M_s.zero()) [[likely]]
          return sys_seconds(_M_s);
        return sys_seconds(-_M_s);
      }

      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.
      friend constexpr bool
      operator==(const leap_second&, const leap_second&) noexcept = default;

      friend constexpr strong_ordering
      operator<=>(const leap_second& __x, const leap_second& __y) noexcept
      { return __x.date() <=> __y.date(); }

      template<typename _Duration>
        friend constexpr bool
        operator==(const leap_second& __x,
                   const sys_time<_Duration>& __y) noexcept
        { return __x.date() == __y; }

      template<typename _Duration>
        friend constexpr bool
        operator<(const leap_second& __x,
                  const sys_time<_Duration>& __y) noexcept
        { return __x.date() < __y; }

      template<typename _Duration>
        friend constexpr bool
        operator<(const sys_time<_Duration>& __x,
                  const leap_second& __y) noexcept
        { return __x < __y.date(); }

      template<typename _Duration>
        friend constexpr bool
        operator>(const leap_second& __x,
                  const sys_time<_Duration>& __y) noexcept
        { return __y < __x.date(); }

      template<typename _Duration>
        friend constexpr bool
        operator>(const sys_time<_Duration>& __x,
                  const leap_second& __y) noexcept
        { return __y.date() < __x; }

      template<typename _Duration>
        friend constexpr bool
        operator<=(const leap_second& __x,
                  const sys_time<_Duration>& __y) noexcept
        { return !(__y < __x.date()); }

      template<typename _Duration>
        friend constexpr bool
        operator<=(const sys_time<_Duration>& __x,
                  const leap_second& __y) noexcept
        { return !(__y.date() < __x); }

      template<typename _Duration>
        friend constexpr bool
        operator>=(const leap_second& __x,
                  const sys_time<_Duration>& __y) noexcept
        { return !(__x.date() < __y); }

      template<typename _Duration>
        friend constexpr bool
        operator>=(const sys_time<_Duration>& __x,
                  const leap_second& __y) noexcept
        { return !(__x < __y.date()); }

      template<three_way_comparable_with<seconds> _Duration>
        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 const tzdb& reload_tzdb();
      template<typename _Dur>
        friend leap_second_info
        get_leap_second_info(const utc_time<_Dur>&);

      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;
      vector<time_zone> zones;
      vector<time_zone_link> links;
      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();

      struct _Rule;
      vector<_Rule> _M_rules;
    };

    class tzdb_list
    {
      struct _Node;
    public:
      tzdb_list(const tzdb_list&) = delete;
      tzdb_list& operator=(const tzdb_list&) = delete;

      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;

        shared_ptr<_Node> _M_node;
        void* _M_reserved = nullptr;
      };

      // TODO const tzdb& front() const noexcept;

      const_iterator erase_after(const_iterator);

      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 const tzdb_list& get_tzdb_list();
      friend const tzdb& get_tzdb();
      friend const tzdb& reload_tzdb();

      static _Node* _S_head;
      static shared_ptr<_Node> _S_head_owner;
    };

    // TODO
    // const tzdb_list& get_tzdb_list();
    // const tzdb& get_tzdb();

    // const tzdb& reload_tzdb();
    // string remove_version();

    template<typename _Duration, typename _TimeZonePtr = const time_zone*>
      class zoned_time; // TODO

    using zoned_seconds = zoned_time<seconds>;

    template<typename _Duration>
      leap_second_info
      get_leap_second_info(const utc_time<_Duration>& __ut)
      {
        if constexpr (is_same_v<_Duration, seconds>)
          {
            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 2023-06-28 00:00:00 UTC
            const seconds::rep __expires = 1687910400;
            const seconds::rep __s = __ut.time_since_epoch().count();

            const seconds::rep* __first = std::begin(__leaps);
            const seconds::rep* __last = std::end(__leaps);

            if (__s > __expires)
              {
                // TODO: use updated leap_seconds from tzdb
#if 0
                auto __db = get_tzdb_list().begin();
                __first = __db->leap_seconds.data();
                __last = __first + __db->leap_seconds.size();
#endif
              }

            // Don't bother searching the list if we're after the last one.
            if (__s > __last[-1])
              return { false, seconds(__last - __first) };

            auto __pos = std::upper_bound(__first, __last, __s);
            return {
              __pos != begin(__leaps) && __pos[-1] == __s,
              seconds{__pos - __first}
            };
          }
        else
          {
            auto __s = chrono::time_point_cast<seconds>(__ut);
            return chrono::get_leap_second_info(__s);
          }
      }

    /// @} group chrono
#endif // C++20
  } // namespace chrono

#if __cplusplus >= 202002L
  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

  namespace chrono
  {
    /// @addtogroup chrono
    /// @{

    /// @cond undocumented
    namespace __detail
    {
      template<typename _Period>
        const char*
        __units_suffix_misc(char* __buf, size_t __n) noexcept
        {
          namespace __tc = std::__detail;
          char* __p = __buf;
          __p[0] = '[';
          unsigned __nlen = __tc::__to_chars_len((uintmax_t)_Period::num);
          __tc::__to_chars_10_impl(__p + 1, __nlen, (uintmax_t)_Period::num);
          __p += 1 + __nlen;
          if constexpr (_Period::den != 1)
            {
              __p[0] = '/';
              unsigned __dlen = __tc::__to_chars_len((uintmax_t)_Period::den);
              __tc::__to_chars_10_impl(__p + 1, __dlen, (uintmax_t)_Period::den);
              __p += 1 + __dlen;
            }
          __p[0] = ']';
          __p[1] = 's';
          __p[2] = '\0';
          return __buf;
        }

      template<typename _Period, typename _CharT>
        auto
        __units_suffix(char* __buf, size_t __n) noexcept
        {
#define _GLIBCXX_UNITS_SUFFIX(period, suffix) \
        if constexpr (is_same_v<_Period, period>)       \
          {                                             \
            if constexpr (is_same_v<_CharT, wchar_t>)   \
              return L##suffix;                         \
            else                                        \
              return suffix;                            \
          }                                             \
        else

          _GLIBCXX_UNITS_SUFFIX(atto, "as")
          _GLIBCXX_UNITS_SUFFIX(femto, "fs")
          _GLIBCXX_UNITS_SUFFIX(pico, "ps")
          _GLIBCXX_UNITS_SUFFIX(nano, "ns")
          _GLIBCXX_UNITS_SUFFIX(micro, "\u00b5s")
          _GLIBCXX_UNITS_SUFFIX(milli, "ms")
          _GLIBCXX_UNITS_SUFFIX(centi, "cs")
          _GLIBCXX_UNITS_SUFFIX(deci, "ds")
          _GLIBCXX_UNITS_SUFFIX(ratio<1>, "s")
          _GLIBCXX_UNITS_SUFFIX(deca, "das")
          _GLIBCXX_UNITS_SUFFIX(hecto, "hs")
          _GLIBCXX_UNITS_SUFFIX(kilo, "ks")
          _GLIBCXX_UNITS_SUFFIX(mega, "Ms")
          _GLIBCXX_UNITS_SUFFIX(giga, "Gs")
          _GLIBCXX_UNITS_SUFFIX(tera, "Ts")
          _GLIBCXX_UNITS_SUFFIX(tera, "Ts")
          _GLIBCXX_UNITS_SUFFIX(peta, "Ps")
          _GLIBCXX_UNITS_SUFFIX(exa, "Es")
          _GLIBCXX_UNITS_SUFFIX(ratio<60>, "min")
          _GLIBCXX_UNITS_SUFFIX(ratio<3600>, "h")
          _GLIBCXX_UNITS_SUFFIX(ratio<86400>, "d")
#undef _GLIBCXX_UNITS_SUFFIX
          return __detail::__units_suffix_misc<_Period>(__buf, __n);
        }
    } // namespace __detail
    /// @endcond

    template<typename _CharT, typename _Traits,
             typename _Rep, typename _Period>
      inline basic_ostream<_CharT, _Traits>&
      operator<<(std::basic_ostream<_CharT, _Traits>& __os,
                const duration<_Rep, _Period>& __d)
      {
        using period = typename _Period::type;
        char __buf[sizeof("[/]s") + 2 * numeric_limits<intmax_t>::digits10];
        std::basic_ostringstream<_CharT, _Traits> __s;
        __s.flags(__os.flags());
        __s.imbue(__os.getloc());
        __s.precision(__os.precision());
        __s << __d.count();
        __s << __detail::__units_suffix<period, _CharT>(__buf, sizeof(__buf));
        __os << std::move(__s).str();
        return __os;
      }

    // TODO: from_stream for duration

    /// @} group chrono
  } // namespace chrono
#endif // C++20

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++11

#endif //_GLIBCXX_CHRONO