[libc++][chrono] implements TAI clock.

libcxx/docs/Status/FormatPaper.csv

@@ -3,7 +3,7 @@ Section,Description,Dependencies,Assignee,Status,First released version
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::duration<Rep, Period>``",,Mark de Wever,|Complete|,16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::sys_time<Duration>``",,Mark de Wever,|Complete|,17
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::utc_time<Duration>``",A ``<chrono>`` implementation,Mark de Wever,|Complete|,20
-`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::tai_time<Duration>``",A ``<chrono>`` implementation,Mark de Wever,,,
++`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::tai_time<Duration>``",,Mark de Wever,|Complete|,21
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::gps_time<Duration>``",A ``<chrono>`` implementation,Mark de Wever,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::file_time<Duration>``",,Mark de Wever,|Complete|,17
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local_time<Duration>``",,Mark de Wever,|Complete|,17

libcxx/include/CMakeLists.txt

@@ -270,6 +270,7 @@ set(files
   __chrono/steady_clock.h
   __chrono/sys_info.h
   __chrono/system_clock.h
+  __chrono/tai_clock.h
   __chrono/time_point.h
   __chrono/time_zone.h
   __chrono/time_zone_link.h

libcxx/include/__chrono/convert_to_tm.h

@@ -23,6 +23,7 @@
 #include <__chrono/statically_widen.h>
 #include <__chrono/sys_info.h>
 #include <__chrono/system_clock.h>
+#include <__chrono/tai_clock.h>
 #include <__chrono/time_point.h>
 #include <__chrono/utc_clock.h>
 #include <__chrono/weekday.h>
@@ -35,6 +36,7 @@
 #include <__config>
 #include <__format/format_error.h>
 #include <__memory/addressof.h>
+#include <__type_traits/common_type.h>
 #include <__type_traits/is_convertible.h>
 #include <__type_traits/is_specialization.h>
 #include <cstdint>
@@ -112,6 +114,16 @@ _LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(chrono::utc_time<_Duration> __tp) {
   return __result;
 }
 
+template <class _Tm, class _Duration>
+_LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(chrono::tai_time<_Duration> __tp) {
+  using _Rp = common_type_t<_Duration, chrono::seconds>;
+  // The time between the TAI epoch (1958-01-01) and UNIX epoch (1970-01-01).
+  // This avoids leap second conversion when going from TAI to UTC.
+  // (It also avoids issues when the date is before the UTC epoch.)
+  constexpr chrono::seconds __offset{4383 * 24 * 60 * 60};
+  return std::__convert_to_tm<_Tm>(chrono::sys_time<_Rp>{__tp.time_since_epoch() - __offset});
+}
+
 #    endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
 #  endif   // _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
 
@@ -131,6 +143,8 @@ _LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(const _ChronoT& __value) {
 #    if _LIBCPP_HAS_EXPERIMENTAL_TZDB
     else if constexpr (same_as<typename _ChronoT::clock, chrono::utc_clock>)
       return std::__convert_to_tm<_Tm>(__value);
+    else if constexpr (same_as<typename _ChronoT::clock, chrono::tai_clock>)
+      return std::__convert_to_tm<_Tm>(__value);
 #    endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
 #  endif   // _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
     else if constexpr (same_as<typename _ChronoT::clock, chrono::file_clock>)

libcxx/include/__chrono/formatter.h

@@ -31,6 +31,7 @@
 #  include <__chrono/statically_widen.h>
 #  include <__chrono/sys_info.h>
 #  include <__chrono/system_clock.h>
+#  include <__chrono/tai_clock.h>
 #  include <__chrono/time_point.h>
 #  include <__chrono/utc_clock.h>
 #  include <__chrono/weekday.h>
@@ -232,9 +233,11 @@ _LIBCPP_HIDE_FROM_ABI __time_zone __convert_to_time_zone([[maybe_unused]] const
   if constexpr (same_as<_Tp, chrono::sys_info>)
     return {__value.abbrev, __value.offset};
 #      if _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM
+  else if constexpr (__is_time_point<_Tp> && requires { requires same_as<typename _Tp::clock, chrono::tai_clock>; })
+    return {"TAI", chrono::seconds{0}};
   else if constexpr (__is_specialization_v<_Tp, chrono::zoned_time>)
     return __formatter::__convert_to_time_zone(__value.get_info());
-#      endif
+#      endif // _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM
   else
 #    endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
     return {"UTC", chrono::seconds{0}};
@@ -734,6 +737,17 @@ struct _LIBCPP_TEMPLATE_VIS formatter<chrono::utc_time<_Duration>, _CharT> : pub
   }
 };
 
+template <class _Duration, __fmt_char_type _CharT>
+struct _LIBCPP_TEMPLATE_VIS formatter<chrono::tai_time<_Duration>, _CharT> : public __formatter_chrono<_CharT> {
+public:
+  using _Base _LIBCPP_NODEBUG = __formatter_chrono<_CharT>;
+
+  template <class _ParseContext>
+  _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
+    return _Base::__parse(__ctx, __format_spec::__fields_chrono, __format_spec::__flags::__clock);
+  }
+};
+
 #      endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
 #    endif   // _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM
 

libcxx/include/__chrono/ostream.h

@@ -26,6 +26,7 @@
 #  include <__chrono/statically_widen.h>
 #  include <__chrono/sys_info.h>
 #  include <__chrono/system_clock.h>
+#  include <__chrono/tai_clock.h>
 #  include <__chrono/utc_clock.h>
 #  include <__chrono/weekday.h>
 #  include <__chrono/year.h>
@@ -71,6 +72,12 @@ operator<<(basic_ostream<_CharT, _Traits>& __os, const utc_time<_Duration>& __tp
   return __os << std::format(__os.getloc(), _LIBCPP_STATICALLY_WIDEN(_CharT, "{:L%F %T}"), __tp);
 }
 
+template <class _CharT, class _Traits, class _Duration>
+_LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, const tai_time<_Duration>& __tp) {
+  return __os << std::format(__os.getloc(), _LIBCPP_STATICALLY_WIDEN(_CharT, "{:L%F %T}"), __tp);
+}
+
 #      endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
 #    endif   // _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM
 

libcxx/include/__chrono/tai_clock.h

@@ -0,0 +1,99 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___CHRONO_TAI_CLOCK_H
+#define _LIBCPP___CHRONO_TAI_CLOCK_H
+
+#include <version>
+// Enable the contents of the header only when libc++ was built with experimental features enabled.
+#if _LIBCPP_HAS_EXPERIMENTAL_TZDB
+
+#  include <__assert>
+#  include <__chrono/duration.h>
+#  include <__chrono/time_point.h>
+#  include <__chrono/utc_clock.h>
+#  include <__config>
+#  include <__type_traits/common_type.h>
+
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#  if _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
+
+namespace chrono {
+
+class tai_clock;
+
+template <class _Duration>
+using tai_time    = time_point<tai_clock, _Duration>;
+using tai_seconds = tai_time<seconds>;
+
+// [time.clock.tai.overview]/1
+//    The clock tai_clock measures seconds since 1958-01-01 00:00:00 and is
+//    offset 10s ahead of UTC at this date. That is, 1958-01-01 00:00:00 TAI is
+//    equivalent to 1957-12-31 23:59:50 UTC. Leap seconds are not inserted into
+//    TAI. Therefore every time a leap second is inserted into UTC, UTC shifts
+//    another second with respect to TAI. For example by 2000-01-01 there had
+//    been 22 positive and 0 negative leap seconds inserted so 2000-01-01
+//    00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI (22s plus the
+//    initial 10s offset).
+//
+// Note this does not specify what the UTC offset before 1958-01-01 00:00:00
+// TAI is. However the member functions are fully specified in the standard.
+// https://koka-lang.github.io/koka/doc/std_time_utc.html contains more
+// information and references.
+class tai_clock {
+public:
+  using rep                       = utc_clock::rep;
+  using period                    = utc_clock::period;
+  using duration                  = chrono::duration<rep, period>;
+  using time_point                = chrono::time_point<tai_clock>;
+  static constexpr bool is_steady = false; // The utc_clock is not steady.
+
+  // The static difference between UTC and TAI time.
+  static constexpr chrono::seconds __offset{378691210};
+
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static time_point now() { return from_utc(utc_clock::now()); }
+
+  template <class _Duration>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static utc_time<common_type_t<_Duration, seconds>>
+  to_utc(const tai_time<_Duration>& __time) noexcept {
+    using _Rp                    = common_type_t<_Duration, seconds>;
+    _Duration __time_since_epoch = __time.time_since_epoch();
+    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__time_since_epoch >= utc_time<_Rp>::min().time_since_epoch() + __offset,
+                                          "the TAI to UTC conversion would underflow");
+
+    return utc_time<_Rp>{__time_since_epoch - __offset};
+  }
+
+  template <class _Duration>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static tai_time<common_type_t<_Duration, seconds>>
+  from_utc(const utc_time<_Duration>& __time) noexcept {
+    using _Rp                    = common_type_t<_Duration, seconds>;
+    _Duration __time_since_epoch = __time.time_since_epoch();
+    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__time_since_epoch <= utc_time<_Rp>::max().time_since_epoch() - __offset,
+                                          "the UTC to TAI conversion would overflow");
+
+    return tai_time<_Rp>{__time_since_epoch + __offset};
+  }
+};
+
+} // namespace chrono
+
+#  endif // _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM &&
+         // _LIBCPP_HAS_LOCALIZATION
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_HAS_EXPERIMENTAL_TZDB
+
+#endif // _LIBCPP___CHRONO_TAI_CLOCK_H

libcxx/include/chrono

@@ -335,6 +335,34 @@ struct leap_second_info {                               // C++20
 template<class Duration>                                // C++20
   leap_second_info get_leap_second_info(const utc_time<Duration>& ut);
 
+
+// [time.clock.tai], class tai_clock
+class tai_clock {                                      // C++20
+public:
+    using rep                       = a signed arithmetic type;
+    using period                    = ratio<unspecified, unspecified>;
+    using duration                  = chrono::duration<rep, period>;
+    using time_point                = chrono::time_point<tai_clock>;
+    static constexpr bool is_steady = unspecified;
+
+    static time_point now();
+
+    template<class Duration>
+      static utc_time<common_type_t<Duration, seconds>>
+        to_utc(const tai_time<Duration>& t);
+    template<class Duration>
+      static tai_time<common_type_t<Duration, seconds>>
+        from_utc(const utc_time<Duration>& t);
+};
+
+template<class Duration>
+using tai_time  = time_point<tai_clock, Duration>;      // C++20
+using tai_seconds = tai_time<seconds>;                  // C++20
+
+template<class charT, class traits, class Duration>     // C++20
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os, const tai_time<Duration>& t);
+
 class file_clock                                        // C++20
 {
 public:
@@ -898,6 +926,8 @@ namespace std {
     struct formatter<chrono::sys_time<Duration>, charT>;                          // C++20
   template<class Duration, class charT>
     struct formatter<chrono::utc_time<Duration>, charT>;                          // C++20
+  template<class Duration, class charT>
+    struct formatter<chrono::tai_time<Duration>, charT>;                          // C++20
   template<class Duration, class charT>
     struct formatter<chrono::filetime<Duration>, charT>;                          // C++20
   template<class Duration, class charT>
@@ -1014,6 +1044,7 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 
 #    if _LIBCPP_HAS_TIME_ZONE_DATABASE && _LIBCPP_HAS_FILESYSTEM && _LIBCPP_HAS_LOCALIZATION
 #      include <__chrono/leap_second.h>
+#      include <__chrono/tai_clock.h>
 #      include <__chrono/time_zone.h>
 #      include <__chrono/time_zone_link.h>
 #      include <__chrono/tzdb.h>

libcxx/include/module.modulemap

@@ -967,6 +967,10 @@ module std [system] {
       header "__chrono/system_clock.h"
       export std.chrono.time_point
     }
+    module tai_clock {
+      header "__chrono/tai_clock.h"
+      export std.chrono.time_point
+    }
     module time_point                 { header "__chrono/time_point.h" }
     module time_zone_link             { header "__chrono/time_zone_link.h" }
     module time_zone                  { header "__chrono/time_zone.h" }

libcxx/modules/std/chrono.inc

@@ -97,13 +97,13 @@ export namespace std {
 
     using std::chrono::get_leap_second_info;
 
-#    if 0
     // [time.clock.tai], class tai_clock
     using std::chrono::tai_clock;
 
     using std::chrono::tai_seconds;
     using std::chrono::tai_time;
 
+#    if 0
     // [time.clock.gps], class gps_clock
     using std::chrono::gps_clock;
 

libcxx/test/libcxx/diagnostics/chrono.nodiscard.verify.cpp

@@ -102,4 +102,15 @@ void test(std::chrono::time_zone tz, std::chrono::time_zone_link link, std::chro
     zt.get_sys_time();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
     zt.get_info();       // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   }
+
+  { // [time.clock.tai]
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::chrono::tai_clock::now();
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::chrono::tai_clock::to_utc(std::chrono::tai_seconds{});
+
+    // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::chrono::tai_clock::from_utc(std::chrono::utc_seconds{});
+  }
 }