[libc++][chrono] Add hh_mm_ss formatter.

[Mordante]

Summary

Partially implements:

• P1361 Integration of chrono with text formatting
• P2372 Fixing locale handling in chrono formatters
• P1466 Miscellaneous minor fixes for chrono

Depends on D137022

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

Fix CI.

[Mordante]

CI fixes

[Mordante]

Enable entire CI.

[Mordante]

Rebased and some minor improvements.

[Mordante]

Adds missing includes.

[Mordante]

CI fixes.

[Mordante]

Rebased to test CI.

[Mordante]

Disable GCC tests.

[Mordante]

Reviewed and CI fixes.

[h-vetinari]

libcxx/include/__chrono/formatter.h

526–527

For leap seconds, it would be legal to have a duration of a day take 24:00:01.

[Mordante]

libcxx/include/__chrono/formatter.h

526–527

But that should be displayed as 23:59:60, which probably doesn't work either. The formatting is done using a tm struct, there 24 is not a valid value for tm_hour https://en.cppreference.com/w/cpp/chrono/c/tm.

Looking at http://eel.is/c++draft/time.hms#members-3 a duration of 24*3600 + 1 seconds will be initialized to 24 hours and one second; but without any calendar information and a leapseconds database it's unknown whether it's a leap second or not.

Also see the comment above; it's a bit underspecified what the code should do. Originally this class was a time of day, but that has been changed.
Note this issue exception will only be thrown when you try to format the time using a 12 or 24 hour clock. Formatting the hours itself will work for values > 23.

[ldionne]

libcxx/include/__chrono/convert_to_tm.h

129

I think this is closer to what you actually mean.

libcxx/include/__chrono/formatter.h

79–81

Same here -- we should consider using chrono::duration<...> here instead of __is_duration.

126–127

Any reason for not doing this way instead?

520

libcxx/test/std/time/time.syn/formatter.hh_mm_ss.pass.cpp

194

Isn't that completely broken? Do you have a short reproducer? I could probably get that fixed.

[Mordante]

Thanks for the review!

libcxx/include/__chrono/convert_to_tm.h

129

As mentioned in the 1:1 that part is tested on the line below, I think this looks slightly cleaner.

libcxx/include/__chrono/formatter.h

126–127

Good point, not sure why I picked this solution, the patch is quite old. The followup patch doesn't require that either.

[Mordante]

Addresses review comments.

libcxx/docs/Status/FormatPaper.csv

 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_day_last``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_weekday``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_weekday_last``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year_month``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year_month_day``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year_month_day_last``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year_month_weekday``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year_month_weekday_last``",,Mark de Wever,|Complete|, Clang 16
-`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::hh_mm_ss<duration<Rep, Period>>``",,Mark de Wever,|In Progress|,
+`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::hh_mm_ss<duration<Rep, Period>>``",,Mark de Wever,|Complete|, Clang 17
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::sys_info``",A ``<chrono>`` implementation,Mark de Wever,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local_info``",A ``<chrono>`` implementation,Mark de Wever,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::zoned_time<Duration, TimeZonePtr>``",A ``<chrono>`` implementation,Mark de Wever,,
 `P2286R8 <https://wg21.link/P2286R8>`__,"Formatting ranges"
 `[format.syn] <https://wg21.link/format.syn>`_,"Concept ``formattable``",,Mark de Wever,|Complete|, Clang 16
 `[format.string.std] <https://wg21.link/format.string.std>`_,"std-format-spec ``type`` debug",,Mark de Wever,|Complete|,Clang 16
 `[format.range] <https://wg21.link/format.range>`_,"Formatting for ranges: sequences",,Mark de Wever,|Complete|,Clang 16
 `[format.range.fmtmap] <https://wg21.link/format.range.fmtmap>`_,"Formatting for ranges: map",,Mark de Wever,|Complete|,Clang 16

libcxx/include/CMakeLists.txt

set(files
   __bsd_locale_defaults.h
   __bsd_locale_fallbacks.h
   __charconv/chars_format.h
   __charconv/from_chars_result.h
   __charconv/tables.h
   __charconv/to_chars_base_10.h
   __charconv/to_chars_result.h
   __chrono/calendar.h
+  __chrono/concepts.h
   __chrono/convert_to_timespec.h
   __chrono/convert_to_tm.h
   __chrono/day.h
   __chrono/duration.h
   __chrono/file_clock.h
   __chrono/formatter.h
   __chrono/hh_mm_ss.h
   __chrono/high_resolution_clock.h

libcxx/include/__chrono/concepts.h

+// -*- 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_CONCEPTS_H
+#define _LIBCPP___CHRONO_CONCEPTS_H
+
+#include <__chrono/hh_mm_ss.h>
+#include <__config>
+#include <__type_traits/is_specialization.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+template <class _Tp>
+concept __is_hh_mm_ss = __is_specialization_v<_Tp, chrono::hh_mm_ss>;
+
+#endif // _LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CHRONO_CONCEPTS_H

libcxx/include/__chrono/convert_to_tm.h

 // -*- 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_CONVERT_TO_TM_H
 #define _LIBCPP___CHRONO_CONVERT_TO_TM_H
 
+#include <__chrono/concepts.h>
 #include <__chrono/day.h>
 #include <__chrono/duration.h>
 #include <__chrono/hh_mm_ss.h>
 #include <__chrono/month.h>
 #include <__chrono/month_weekday.h>
 #include <__chrono/monthday.h>
 #include <__chrono/statically_widen.h>
 #include <__chrono/system_clock.h>
 #include <__chrono/time_point.h>
 #include <__chrono/weekday.h>
 #include <__chrono/year.h>
 #include <__chrono/year_month.h>
 #include <__chrono/year_month_day.h>
 #include <__chrono/year_month_weekday.h>
 #include <__concepts/same_as.h>
 #include <__config>
+#include <__format/format_error.h>
 #include <__memory/addressof.h>
 #include <cstdint>
 #include <ctime>
+#include <limits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
 // Conerts a chrono date and weekday to a given _Tm type.
 //
 // This is an implementation detail for the function
 //   template <class _Tm, class _ChronoT>
else if constexpr (same_as<_ChronoT, chrono::month_day>) {
     __result.tm_year = static_cast<int>(__value.year()) - 1900;
     __result.tm_mon  = static_cast<unsigned>(__value.month()) - 1;
   } else if constexpr (same_as<_ChronoT, chrono::year_month_day> || same_as<_ChronoT, chrono::year_month_day_last>) {
     return std::__convert_to_tm<_Tm>(
         chrono::year_month_day{__value}, chrono::weekday{static_cast<chrono::sys_days>(__value)});
   } else if constexpr (same_as<_ChronoT, chrono::year_month_weekday> ||
                        same_as<_ChronoT, chrono::year_month_weekday_last>) {
     return std::__convert_to_tm<_Tm>(chrono::year_month_day{static_cast<chrono::sys_days>(__value)}, __value.weekday());
+  } else if constexpr (__is_hh_mm_ss<_ChronoT>) {
+    __result.tm_sec = __value.seconds().count();
+    __result.tm_min = __value.minutes().count();
+    // In libc++ hours is stored as a long. The type in std::tm is an int. So
+    // the overflow can only occur when hour uses more bits than an int
+    // provides.
+    if constexpr (sizeof(std::chrono::hours::rep) > sizeof(__result.tm_hour))
+      if (__value.hours().count() > std::numeric_limits<decltype(__result.tm_hour)>::max())
+        std::__throw_format_error("Formatting hh_mm_ss, encountered an hour overflow");
+    __result.tm_hour = __value.hours().count();
   } else
     static_assert(sizeof(_ChronoT) == 0, "Add the missing type specialization");
 
   return __result;
 }
 
 #endif //if _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
+_LIBCPP_POP_MACROS
+
 #endif // _LIBCPP___CHRONO_CONVERT_TO_TM_H

libcxx/include/__chrono/formatter.h

 // -*- 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_FORMATTER_H
 #define _LIBCPP___CHRONO_FORMATTER_H
 
 #include <__chrono/calendar.h>
+#include <__chrono/concepts.h>
 #include <__chrono/convert_to_tm.h>
 #include <__chrono/day.h>
 #include <__chrono/duration.h>
 #include <__chrono/hh_mm_ss.h>
 #include <__chrono/month.h>
 #include <__chrono/month_weekday.h>
 #include <__chrono/monthday.h>
 #include <__chrono/ostream.h>

 ///      requirement of Table 97: Meaning of conversion specifiers
 ///      [tab:time.format.spec].
 ///
 /// When no chrono-specs are provided it uses the stream formatter.
 
 // For tiny ratios it's not possible to convert a duration to a hh_mm_ss. This
 // fails compile-time due to the limited precision of the ratio (64-bit is too
 // small). Therefore a duration uses its own conversion.
-template <class _CharT, class _Tp>
-  requires(chrono::__is_duration<_Tp>::value)
-_LIBCPP_HIDE_FROM_ABI void __format_sub_seconds(const _Tp& __value, basic_stringstream<_CharT>& __sstr) {
+template <class _CharT, class _Rep, class _Period>
+_LIBCPP_HIDE_FROM_ABI void
+__format_sub_seconds(const chrono::duration<_Rep, _Period>& __value, basic_stringstream<_CharT>& __sstr) {
   __sstr << std::use_facet<numpunct<_CharT>>(__sstr.getloc()).decimal_point();
 
+  using __duration = chrono::duration<_Rep, _Period>;
+
   auto __fraction = __value - chrono::duration_cast<chrono::seconds>(__value);
-  if constexpr (chrono::treat_as_floating_point_v<typename _Tp::rep>)
+  if constexpr (chrono::treat_as_floating_point_v<_Rep>)
     // When the floating-point value has digits itself they are ignored based
     // on the wording in [tab:time.format.spec]
     //   If the precision of the input cannot be exactly represented with
     //   seconds, then the format is a decimal floating-point number with a
     //   fixed format and a precision matching that of the precision of the
     //   input (or to a microseconds precision if the conversion to
     //   floating-point decimal seconds cannot be made within 18 fractional
     //   digits).
     //
     // This matches the behaviour of MSVC STL, fmtlib interprets this
     // differently and uses 3 decimals.
     // https://godbolt.org/z/6dsbnW8ba
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}.0f}"),
                    __fraction.count(),
-                   chrono::hh_mm_ss<_Tp>::fractional_width);
+                   chrono::hh_mm_ss<__duration>::fractional_width);
   else
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}}"),
                    __fraction.count(),
-                   chrono::hh_mm_ss<_Tp>::fractional_width);
+                   chrono::hh_mm_ss<__duration>::fractional_width);
+}
+
+template <class _CharT, class _Duration>
+_LIBCPP_HIDE_FROM_ABI void
+__format_sub_seconds(const chrono::hh_mm_ss<_Duration>& __value, basic_stringstream<_CharT>& __sstr) {
+  __sstr << std::use_facet<numpunct<_CharT>>(__sstr.getloc()).decimal_point();
+  if constexpr (chrono::treat_as_floating_point_v<typename _Duration::rep>)
+    std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
+                   _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}.0f}"),
+                   __value.subseconds().count(),
+                   __value.fractional_width);
+  else
+    std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
+                   _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}}"),
+                   __value.subseconds().count(),
+                   __value.fractional_width);
 }
 
 template <class _Tp>
 consteval bool __use_fraction() {
   if constexpr (chrono::__is_duration<_Tp>::value)
     return chrono::hh_mm_ss<_Tp>::fractional_width;
+  else if constexpr (__is_hh_mm_ss<_Tp>)
+    return _Tp::fractional_width;
   else
     return false;
 }
 
 template <class _CharT>
 _LIBCPP_HIDE_FROM_ABI void __format_year(int __year, basic_stringstream<_CharT>& __sstr) {
   if (__year < 0) {
     __sstr << _CharT('-');
_LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_ok(const _Tp& __value) {
   else if constexpr (same_as<_Tp, chrono::year_month_day>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year_month_day_last>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday>)
     return __value.weekday().ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday_last>)
     return __value.weekday().ok();
+  else if constexpr (__is_hh_mm_ss<_Tp>)
+    return true;
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
 }
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_name_ok(const _Tp& __value) {
   if constexpr (same_as<_Tp, chrono::day>)
     return true;
_LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_name_ok(const _Tp& __value) {
   else if constexpr (same_as<_Tp, chrono::year_month_day>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year_month_day_last>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday>)
     return __value.weekday().ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday_last>)
     return __value.weekday().ok();
+  else if constexpr (__is_hh_mm_ss<_Tp>)
+    return true;
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
 }
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __date_ok(const _Tp& __value) {
   if constexpr (same_as<_Tp, chrono::day>)
     return true;
_LIBCPP_HIDE_FROM_ABI constexpr bool __date_ok(const _Tp& __value) {
   else if constexpr (same_as<_Tp, chrono::year_month_day>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year_month_day_last>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday_last>)
     return __value.ok();
+  else if constexpr (__is_hh_mm_ss<_Tp>)
+    return true;
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
 }
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __month_name_ok(const _Tp& __value) {
   if constexpr (same_as<_Tp, chrono::day>)
     return true;
_LIBCPP_HIDE_FROM_ABI constexpr bool __month_name_ok(const _Tp& __value) {
   else if constexpr (same_as<_Tp, chrono::year_month_day>)
     return __value.month().ok();
   else if constexpr (same_as<_Tp, chrono::year_month_day_last>)
     return __value.month().ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday>)
     return __value.month().ok();
   else if constexpr (same_as<_Tp, chrono::year_month_weekday_last>)
     return __value.month().ok();
+  else if constexpr (__is_hh_mm_ss<_Tp>)
+    return true;
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
 }
 
 template <class _CharT, class _Tp>
 _LIBCPP_HIDE_FROM_ABI auto
 __format_chrono(const _Tp& __value,
                 auto& __ctx,
if constexpr (chrono::__is_duration<_Tp>::value) {
         std::__throw_format_error("formatting a day of year needs a valid date");
 
       if (__specs.__chrono_.__week_of_year_ && !__formatter::__date_ok(__value))
         std::__throw_format_error("formatting a week of year needs a valid date");
 
       if (__specs.__chrono_.__month_name_ && !__formatter::__month_name_ok(__value))
         std::__throw_format_error("formatting a month name from an invalid month number");
 
+      if constexpr (__is_hh_mm_ss<_Tp>) {
+        // Note this is a pedantic intepretation of the Standard. A hh_mm_ss
+        // is no longer a time_of_day and can store an arbitrary number of
+        // hours. A number of hours in a 12 or 24 hour clock can't represent
+        // 24 hours or more. The functions std::chrono::make12 and
+        // std::chrono::make24 reaffirm this view point.
+        //
+        // Interestingly this will be the only output stream function that
+        // throws.
+        //
+        // TODO FMT The wording probably needs to be adapted to
+        // - The displayed hours is hh_mm_ss.hours() % 24
+        // - It should probably allow %j in the same fashion as duration.
+        // - The stream formatter should change its output when hours >= 24
+        //   - Write it as not valid,
+        //   - or write the number of days.
+        if (__specs.__chrono_.__hour_ && __value.hours().count() > 23)
+          std::__throw_format_error("formatting a hour needs a valid value");
+
+        if (__value.is_negative())
+          __sstr << _CharT('-');
+      }
+
       __formatter::__format_chrono_using_chrono_specs(__value, __sstr, __chrono_specs);
     }
   }
 
   // TODO FMT Use the stringstream's view after P0408R7 has been implemented.
   basic_string<_CharT> __str = __sstr.str();
   return __formatter::__write_string(basic_string_view<_CharT>{__str}, __ctx.out(), __specs);
 }
public:
   using _Base = __formatter_chrono<_CharT>;
 
   _LIBCPP_HIDE_FROM_ABI constexpr auto parse(basic_format_parse_context<_CharT>& __parse_ctx)
       -> decltype(__parse_ctx.begin()) {
     return _Base::__parse(__parse_ctx, __format_spec::__fields_chrono, __format_spec::__flags::__date);
   }
 };
 
+template <class _Duration, __fmt_char_type _CharT>
+struct formatter<chrono::hh_mm_ss<_Duration>, _CharT> : public __formatter_chrono<_CharT> {
+public:
+  using _Base = __formatter_chrono<_CharT>;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto parse(basic_format_parse_context<_CharT>& __parse_ctx)
+      -> decltype(__parse_ctx.begin()) {
+    return _Base::__parse(__parse_ctx, __format_spec::__fields_chrono, __format_spec::__flags::__time);
+  }
+};
 #endif // if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif //  _LIBCPP___CHRONO_FORMATTER_H

libcxx/include/__chrono/hh_mm_ss.h

 
 private:
     bool            __is_neg_;
     chrono::hours   __h_;
     chrono::minutes __m_;
     chrono::seconds __s_;
     precision       __f_;
 };
+_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(hh_mm_ss);
 
 _LIBCPP_HIDE_FROM_ABI constexpr bool is_am(const hours& __h) noexcept { return __h >= hours( 0) && __h < hours(12); }
 _LIBCPP_HIDE_FROM_ABI constexpr bool is_pm(const hours& __h) noexcept { return __h >= hours(12) && __h < hours(24); }
 
 _LIBCPP_HIDE_FROM_ABI constexpr hours make12(const hours& __h) noexcept
 {
     if      (__h == hours( 0)) return hours(12);
     else if (__h <= hours(12)) return __h;

libcxx/include/__chrono/ostream.h

 // -*- 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_OSTREAM_H
 #define _LIBCPP___CHRONO_OSTREAM_H
 
 #include <__chrono/day.h>
 #include <__chrono/duration.h>
+#include <__chrono/hh_mm_ss.h>
 #include <__chrono/month.h>
 #include <__chrono/month_weekday.h>
 #include <__chrono/monthday.h>
 #include <__chrono/statically_widen.h>
 #include <__chrono/weekday.h>
 #include <__chrono/year.h>
 #include <__chrono/year_month.h>
 #include <__chrono/year_month_day.h>
operator<<(basic_ostream<_CharT, _Traits>& __os, const year_month_weekday_last& __ymwdl) {
   return __os << std::format(
              __os.getloc(),
              _LIBCPP_STATICALLY_WIDEN(_CharT, "{}/{:L}/{:L}"),
              __ymwdl.year(),
              __ymwdl.month(),
              __ymwdl.weekday_last());
 }
 
+template <class _CharT, class _Traits, class _Duration>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, const hh_mm_ss<_Duration> __hms) {
+  return __os << std::format(__os.getloc(), _LIBCPP_STATICALLY_WIDEN(_CharT, "{:L%T}"), __hms);
+}
+
 } // namespace chrono
 
 #endif //if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___CHRONO_OSTREAM_H

libcxx/include/__chrono/parser_std_format_spec.h

__parse_conversion_spec(_ConstIterator& __begin, _ConstIterator __end, __flags __flags) {
 
     case _CharT('M'):
       __format_spec::__validate_minute(__flags);
       break;
 
     case _CharT('p'): // TODO FMT does the formater require an hour or a time?
     case _CharT('H'):
     case _CharT('I'):
+      __parser_.__hour_ = true;
       __validate_hour(__flags);
       break;
 
     case _CharT('r'):
     case _CharT('R'):
     case _CharT('T'):
     case _CharT('X'):
+      __parser_.__hour_ = true;
       __format_spec::__validate_time(__flags);
       break;
 
     case _CharT('d'):
     case _CharT('e'):
       __format_spec::__validate_day(__flags);
       break;
 
private:
   _LIBCPP_HIDE_FROM_ABI constexpr void
   __parse_modifier_E(_ConstIterator& __begin, _ConstIterator __end, __flags __flags) {
     ++__begin;
     if (__begin == __end)
       std::__throw_format_error("End of input while parsing the modifier E");
 
     switch (*__begin) {
     case _CharT('X'):
+      __parser_.__hour_ = true;
       __format_spec::__validate_time(__flags);
       break;
 
     case _CharT('y'):
     case _CharT('C'):
     case _CharT('Y'):
       __format_spec::__validate_year(__flags);
       break;
case _CharT('S'):
       break;
 
     case _CharT('M'):
       __format_spec::__validate_minute(__flags);
       break;
 
     case _CharT('I'):
     case _CharT('H'):
+      __parser_.__hour_ = true;
       __format_spec::__validate_hour(__flags);
       break;
 
     case _CharT('d'):
     case _CharT('e'):
       __format_spec::__validate_day(__flags);
       break;
 

libcxx/include/__format/parser_std_format_spec.h

struct __std {
   bool __alternate_form_ : 1;
   bool __locale_specific_form_ : 1;
   __type __type_;
 };
 
 struct __chrono {
   __alignment __alignment_ : 3;
   bool __locale_specific_form_ : 1;
+  bool __hour_                 : 1;
   bool __weekday_name_ : 1;
   bool __weekday_              : 1;
   bool __day_of_year_          : 1;
   bool __week_of_year_         : 1;
   bool __month_name_ : 1;
 };
 
 /// Contains the parsed formatting specifications.
return __parsed_specifications<_CharT>{
         .__fill_{__fill_}};
   }
 
   _LIBCPP_HIDE_FROM_ABI __parsed_specifications<_CharT> __get_parsed_chrono_specifications(auto& __ctx) const {
     return __parsed_specifications<_CharT>{
         .__chrono_ =
             __chrono{.__alignment_            = __alignment_,
                      .__locale_specific_form_ = __locale_specific_form_,
+                     .__hour_                 = __hour_,
                      .__weekday_name_         = __weekday_name_,
                      .__weekday_              = __weekday_,
                      .__day_of_year_          = __day_of_year_,
                      .__week_of_year_         = __week_of_year_,
                      .__month_name_           = __month_name_},
         .__width_{__get_width(__ctx)},
         .__precision_{__get_precision(__ctx)},
         .__fill_{__fill_}};
   }
 
   __alignment __alignment_ : 3 {__alignment::__default};
   __sign __sign_ : 2 {__sign::__default};
   bool __alternate_form_ : 1 {false};
   bool __locale_specific_form_ : 1 {false};
   bool __reserved_0_ : 1 {false};
   __type __type_{__type::__default};
 
   // These flags are only used for formatting chrono. Since the struct has
   // padding space left it's added to this structure.
+  bool __hour_ : 1 {false};
+
   bool __weekday_name_ : 1 {false};
   bool __weekday_      : 1 {false};
 
   bool __day_of_year_  : 1 {false};
   bool __week_of_year_ : 1 {false};
 
   bool __month_name_ : 1 {false};
 
-  uint8_t __reserved_1_ : 3 {0};
+  uint8_t __reserved_1_ : 2 {0};
   uint8_t __reserved_2_ : 6 {0};
   // These two flags are only used internally and not part of the
   // __parsed_specifications. Therefore put them at the end.
   bool __width_as_arg_ : 1 {false};
   bool __precision_as_arg_ : 1 {false};
 
   /// The requested width, either the value or the arg-id.
   int32_t __width_{0};

libcxx/include/chrono

public:
     constexpr chrono::minutes minutes() const noexcept;
     constexpr chrono::seconds seconds() const noexcept;
     constexpr precision subseconds() const noexcept;
 
     constexpr explicit operator  precision()   const noexcept;
     constexpr          precision to_duration() const noexcept;
 };
 
+template<class charT, class traits, class Duration>
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os, const hh_mm_ss<Duration>& hms); // C++20
+
 // 26.10, 12/24 hour functions
 constexpr bool is_am(hours const& h) noexcept;
 constexpr bool is_pm(hours const& h) noexcept;
 constexpr hours make12(const hours& h) noexcept;
 constexpr hours make24(const hours& h, bool is_pm) noexcept;
 
 // 25.10.5, class time_zone    // C++20
 enum class choose {earliest, latest};
namespace std {
   template<class charT> struct formatter<chrono::month_day_last, charT>;          // C++20
   template<class charT> struct formatter<chrono::month_weekday, charT>;           // C++20
   template<class charT> struct formatter<chrono::month_weekday_last, charT>;      // C++20
   template<class charT> struct formatter<chrono::year_month, charT>;              // C++20
   template<class charT> struct formatter<chrono::year_month_day, charT>;          // C++20
   template<class charT> struct formatter<chrono::year_month_day_last, charT>;     // C++20
   template<class charT> struct formatter<chrono::year_month_weekday, charT>;      // C++20
   template<class charT> struct formatter<chrono::year_month_weekday_last, charT>; // C++20
+  template<class Rep, class Period, class charT>
+    struct formatter<chrono::hh_mm_ss<duration<Rep, Period>>, charT>;             // C++20
 } // namespace std
 
 namespace chrono {
 // calendrical constants
 inline constexpr last_spec                              last{};       // C++20
 inline constexpr chrono::weekday                        Sunday{0};    // C++20
 inline constexpr chrono::weekday                        Monday{1};    // C++20
 inline constexpr chrono::weekday                        Tuesday{2};   // C++20

libcxx/include/module.modulemap.in

module std [system] {
 
   }
   module chrono {
     header "chrono"
     export *
 
     module __chrono {
       module calendar               { private header "__chrono/calendar.h" }
+      module concepts               { private header "__chrono/concepts.h" }
       module convert_to_timespec    { private header "__chrono/convert_to_timespec.h" }
       module convert_to_tm          { private header "__chrono/convert_to_tm.h" }
       module day                    { private header "__chrono/day.h" }
       module duration               { private header "__chrono/duration.h" }
       module file_clock             { private header "__chrono/file_clock.h" }
       module formatter              {
         @requires_LIBCXX_ENABLE_LOCALIZATION@
         private header "__chrono/formatter.h"

libcxx/test/libcxx/private_headers.verify.cpp

 #include <__bit/rotate.h> // expected-error@*:* {{use of private header from outside its module: '__bit/rotate.h'}}
 #include <__bit_reference> // expected-error@*:* {{use of private header from outside its module: '__bit_reference'}}
 #include <__charconv/chars_format.h> // expected-error@*:* {{use of private header from outside its module: '__charconv/chars_format.h'}}
 #include <__charconv/from_chars_result.h> // expected-error@*:* {{use of private header from outside its module: '__charconv/from_chars_result.h'}}
 #include <__charconv/tables.h> // expected-error@*:* {{use of private header from outside its module: '__charconv/tables.h'}}
 #include <__charconv/to_chars_base_10.h> // expected-error@*:* {{use of private header from outside its module: '__charconv/to_chars_base_10.h'}}
 #include <__charconv/to_chars_result.h> // expected-error@*:* {{use of private header from outside its module: '__charconv/to_chars_result.h'}}
 #include <__chrono/calendar.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/calendar.h'}}
+#include <__chrono/concepts.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/concepts.h'}}
 #include <__chrono/convert_to_timespec.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/convert_to_timespec.h'}}
 #include <__chrono/convert_to_tm.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/convert_to_tm.h'}}
 #include <__chrono/day.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/day.h'}}
 #include <__chrono/duration.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/duration.h'}}
 #include <__chrono/file_clock.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/file_clock.h'}}
 #include <__chrono/formatter.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/formatter.h'}}
 #include <__chrono/hh_mm_ss.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/hh_mm_ss.h'}}
 #include <__chrono/high_resolution_clock.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/high_resolution_clock.h'}}

libcxx/test/libcxx/time/convert_to_tm.pass.cpp

+//===----------------------------------------------------------------------===//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// <chrono>
+
+// template <class _Tm, class _ChronoT>
+// _LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(const _ChronoT& __value)
+
+// Most of the code is tested indirectly in the chrono formatters. This only
+// tests the hour overflow.
+
+#include <chrono>
+#include <cassert>
+#include <format>
+#include <string_view>
+
+#include "test_macros.h"
+
+// libc++ uses a long as representation in std::chrono::hours.
+// std::tm uses an int for its integral members. The overflow in the hour
+// conversion can only occur on platforms where sizeof(long) > sizeof(int).
+// Instead emulate this error by using a "tm" with shorts.
+// (The function is already templated to this is quite easy to do,)
+struct minimal_short_tm {
+  short tm_sec;
+  short tm_min;
+  short tm_hour;
+  const char* tm_zone;
+};
+
+int main(int, char**) {
+  { // Test with the maximum number of hours that fit in a short.
+    std::chrono::hh_mm_ss time{std::chrono::hours{32767}};
+    minimal_short_tm result = std::__convert_to_tm<minimal_short_tm>(time);
+    assert(result.tm_sec == 0);
+    assert(result.tm_min == 0);
+    assert(result.tm_hour == 32767);
+  }
+
+#ifndef TEST_HAS_NO_EXCEPTIONS
+  { // Test above the maximum number of hours that fit in a short.
+    std::chrono::hh_mm_ss time{std::chrono::hours{32768}};
+    try {
+      TEST_IGNORE_NODISCARD std::__convert_to_tm<minimal_short_tm>(time);
+      assert(false);
+    } catch ([[maybe_unused]] const std::format_error& e) {
+      LIBCPP_ASSERT(e.what() == std::string_view("Formatting hh_mm_ss, encountered an hour overflow"));
+      return 0;
+    }
+    assert(false);
+  }
+#endif // TEST_HAS_NO_EXCEPTIONS
+
+  return 0;
+}

libcxx/test/std/time/time.hms/time.hms.nonmembers/ostream.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// TODO FMT Evaluate gcc-12 status
+// UNSUPPORTED: gcc-12
+
+// TODO FMT Investigate Windows issues.
+// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+
+// class hh_mm_ss;
+
+// template<class charT, class traits, class Duration>
+//   basic_ostream<charT, traits>&
+//     operator<<(basic_ostream<charT, traits>& os, const hh_mm_ss<Duration>& hms);
+
+#include <chrono>
+#include <cassert>
+#include <sstream>
+
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "test_macros.h"
+
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT, class Duration>
+static std::basic_string<CharT> stream_c_locale(std::chrono::hh_mm_ss<Duration> hms) {
+  std::basic_stringstream<CharT> sstr;
+  sstr << hms;
+  return sstr.str();
+}
+
+template <class CharT, class Duration>
+static std::basic_string<CharT> stream_fr_FR_locale(std::chrono::hh_mm_ss<Duration> hms) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_fr_FR_UTF_8);
+  sstr.imbue(locale);
+  sstr << hms;
+  return sstr.str();
+}
+
+template <class CharT, class Duration>
+static std::basic_string<CharT> stream_ja_JP_locale(std::chrono::hh_mm_ss<Duration> hms) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_ja_JP_UTF_8);
+  sstr.imbue(locale);
+  sstr << hms;
+  return sstr.str();
+}
+
+template <class CharT>
+static void test() {
+  // Note std::atto can't be tested since the ratio conversion from std::atto
+  // std::chrono::seconds to std::chrono::hours overflows when intmax_t is a
+  // 64-bit type. This is a limitiation in the constructor of
+  // std::chrono::hh_mm_ss.
+
+  // C locale - integral power of 10 ratios
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::femto>{1'234'567'890}}) ==
+         SV("00:00:00.000001234567890"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::pico>{1'234'567'890}}) ==
+         SV("00:00:00.001234567890"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::nano>{1'234'567'890}}) ==
+         SV("00:00:01.234567890"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::micro>{1'234'567}}) ==
+         SV("00:00:01.234567"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::milli>{123'456}}) ==
+         SV("00:02:03.456"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::centi>{12'345}}) ==
+         SV("00:02:03.45"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::deci>{1'234}}) ==
+         SV("00:02:03.4"));
+
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t>{123}}) == SV("00:02:03"));
+
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::deca>{-366}}) ==
+         SV("-01:01:00"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::hecto>{-72}}) ==
+         SV("-02:00:00"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::kilo>{-86}}) ==
+         SV("-23:53:20"));
+
+  // Starting at mega it will pass one day
+
+  // fr_FR locale - integral power of not 10 ratios
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{
+             std::chrono::duration<intmax_t, std::ratio<1, 5'000'000>>{5'000}}) == SV("00:00:00,0010000"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 8'000>>{3}}) ==
+         SV("00:00:00,000375"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 4'000>>{1}}) ==
+         SV("00:00:00,00025"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 5'000>>{5}}) ==
+         SV("00:00:00,0010"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 8>>{-4}}) ==
+         SV("-00:00:00,500"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 4>>{-8}}) ==
+         SV("-00:00:02,00"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 5>>{-5}}) ==
+         SV("-00:00:01,0"));
+
+  // TODO FMT Note there's no wording on the rounding
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 9>>{5}}) ==
+         SV("00:00:00,555555"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 7>>{7}}) ==
+         SV("00:00:01,000000"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 6>>{1}}) ==
+         SV("00:00:00,166666"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<intmax_t, std::ratio<1, 3>>{2}}) ==
+         SV("00:00:00,666666"));
+
+  // ja_JP locale - floating points
+
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{
+             std::chrono::duration<long double, std::femto>{1'234'567'890.123}}) == SV("00:00:00.000001234567890"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{
+             std::chrono::duration<long double, std::pico>{1'234'567'890.123}}) == SV("00:00:00.001234567890"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{
+             std::chrono::duration<long double, std::nano>{1'234'567'890.123}}) == SV("00:00:01.234567890"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<double, std::micro>{1'234'567.123}}) ==
+         SV("00:00:01.234567"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<double, std::milli>{123'456.123}}) ==
+         SV("00:02:03.456"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<double, std::centi>{12'345.123}}) ==
+         SV("00:02:03.45"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<float, std::deci>{1'234.123}}) ==
+         SV("00:02:03.4"));
+
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<float>{123.123}}) == SV("00:02:03"));
+
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<double, std::deca>{-366.5}}) ==
+         SV("-01:01:05"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<double, std::hecto>{-72.64}}) ==
+         SV("-02:01:04"));
+  assert(stream_c_locale<CharT>(std::chrono::hh_mm_ss{std::chrono::duration<double, std::kilo>{-86}}) ==
+         SV("-23:53:20"));
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}

libcxx/test/std/time/time.syn/formatter.hh_mm_ss.pass.cpp

+//===----------------------------------------------------------------------===//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// TODO FMT Evaluate gcc-12 status
+// UNSUPPORTED: gcc-12
+
+// TODO FMT Investigate Windows issues.
+// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+
+// template<class Rep, class Period, class charT>
+//   struct formatter<chrono::hh_mm_ss<duration<Rep, Period>>, charT>;
+
+#include <chrono>
+#include <format>
+
+#include <cassert>
+#include <concepts>
+#include <locale>
+#include <iostream>
+#include <type_traits>
+
+#include "formatter_tests.h"
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "string_literal.h"
+#include "test_macros.h"
+
+template <class CharT>
+static void test_no_chrono_specs() {
+  using namespace std::literals::chrono_literals;
+
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output
+  check(SV("00:00:00.000"), SV("{}"), std::chrono::hh_mm_ss{0ms});
+  check(SV("*00:00:00.000*"), SV("{:*^14}"), std::chrono::hh_mm_ss{0ms});
+  check(SV("*00:00:00.000"), SV("{:*>13}"), std::chrono::hh_mm_ss{0ms});
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt = SV(
+      "{:"
+      "%%H='%H'%t"
+      "%%OH='%OH'%t"
+      "%%I='%I'%t"
+      "%%OI='%OI'%t"
+      "%%M='%M'%t"
+      "%%OM='%OM'%t"
+      "%%S='%S'%t"
+      "%%OS='%OS'%t"
+      "%%p='%p'%t"
+      "%%R='%R'%t"
+      "%%T='%T'%t"
+      "%%r='%r'%t"
+      "%%X='%X'%t"
+      "%%EX='%EX'%t"
+      "%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV(
+      "{:L"
+      "%%H='%H'%t"
+      "%%OH='%OH'%t"
+      "%%I='%I'%t"
+      "%%OI='%OI'%t"
+      "%%M='%M'%t"
+      "%%OM='%OM'%t"
+      "%%S='%S'%t"
+      "%%OS='%OS'%t"
+      "%%p='%p'%t"
+      "%%R='%R'%t"
+      "%%T='%T'%t"
+      "%%r='%r'%t"
+      "%%X='%X'%t"
+      "%%EX='%EX'%t"
+      "%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%H='00'\t"
+           "%OH='00'\t"
+           "%I='12'\t"
+           "%OI='12'\t"
+           "%M='00'\t"
+           "%OM='00'\t"
+           "%S='00'\t"
+           "%OS='00'\t"
+           "%p='AM'\t"
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+           "%r='12:00:00 AM'\t"
+           "%X='00:00:00'\t"
+           "%EX='00:00:00'\t"
+           "\n"),
+        fmt,
+        std::chrono::hh_mm_ss(0s));
+
+  check(SV("%H='23'\t"
+           "%OH='23'\t"
+           "%I='11'\t"
+           "%OI='11'\t"
+           "%M='31'\t"
+           "%OM='31'\t"
+           "%S='30.123'\t"
+           "%OS='30.123'\t"
+           "%p='PM'\t"
+           "%R='23:31'\t"
+           "%T='23:31:30.123'\t"
+           "%r='11:31:30 PM'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
+           "\n"),
+        fmt,
+        std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
+
+  check(SV("-%H='03'\t"
+           "%OH='03'\t"
+           "%I='03'\t"
+           "%OI='03'\t"
+           "%M='02'\t"
+           "%OM='02'\t"
+           "%S='01.123456789012'\t"
+           "%OS='01.123456789012'\t"
+           "%p='AM'\t"
+           "%R='03:02'\t"
+           "%T='03:02:01.123456789012'\t"
+           "%r='03:02:01 AM'\t"
+           "%X='03:02:01'\t"
+           "%EX='03:02:01'\t"
+           "\n"),
+        fmt,
+        std::chrono::hh_mm_ss(-(3h + 2min + 1s + std::chrono::duration<int64_t, std::pico>(123456789012))));
+
+  // The number of fractional seconds is 0 according to the Standard
+  // TODO FMT Determine what to do.
+  check(SV("%H='01'\t"
+           "%OH='01'\t"
+           "%I='01'\t"
+           "%OI='01'\t"
+           "%M='01'\t"
+           "%OM='01'\t"
+           "%S='01'\t"
+           "%OS='01'\t"
+           "%p='AM'\t"
+           "%R='01:01'\t"
+           "%T='01:01:01'\t"
+           "%r='01:01:01 AM'\t"
+           "%X='01:01:01'\t"
+           "%EX='01:01:01'\t"
+           "\n"),
+        fmt,
+        std::chrono::hh_mm_ss(std::chrono::duration<double>(3661.123456)));
+
+  // Use the global locale (fr_FR)
+  check(SV("%H='00'\t"
+           "%OH='00'\t"
+           "%I='12'\t"
+           "%OI='12'\t"
+           "%M='00'\t"
+           "%OM='00'\t"
+           "%S='00'\t"
+           "%OS='00'\t"
+#if defined(_AIX)
+           "%p='AM'\t"
+#else
+           "%p=''\t"
+#endif
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+#ifdef _WIN32
+           "%r='00:00:00'\t"
+#elif defined(_AIX)
+           "%r='12:00:00 AM'\t"
+#elif defined(__APPLE__)
+           "%r=''\t"
+#else
+           "%r='12:00:00 '\t"
+#endif
+           "%X='00:00:00'\t"
+           "%EX='00:00:00'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(0s));
+
+  check(SV("%H='23'\t"
+           "%OH='23'\t"
+           "%I='11'\t"
+           "%OI='11'\t"
+           "%M='31'\t"
+           "%OM='31'\t"
+           "%S='30,123'\t"
+           "%OS='30,123'\t"
+#if defined(_AIX)
+           "%p='PM'\t"
+#else
+           "%p=''\t"
+#endif
+           "%R='23:31'\t"
+           "%T='23:31:30,123'\t"
+#ifdef _WIN32
+           "%r='23:31:30'\t"
+#elif defined(_AIX)
+           "%r='11:31:30 PM'\t"
+#elif defined(__APPLE__)
+           "%r=''\t"
+#else
+           "%r='11:31:30 '\t"
+#endif
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
+
+  check(SV("-%H='03'\t"
+           "%OH='03'\t"
+           "%I='03'\t"
+           "%OI='03'\t"
+           "%M='02'\t"
+           "%OM='02'\t"
+           "%S='01,123456789012'\t"
+           "%OS='01,123456789012'\t"
+#if defined(_AIX)
+           "%p='AM'\t"
+#else
+           "%p=''\t"
+#endif
+           "%R='03:02'\t"
+           "%T='03:02:01,123456789012'\t"
+#ifdef _WIN32
+           "%r='03:02:01'\t"
+#elif defined(_AIX)
+           "%r='03:02:01 AM'\t"
+#elif defined(__APPLE__)
+           "%r=''\t"
+#else
+           "%r='03:02:01 '\t"
+#endif
+           "%X='03:02:01'\t"
+           "%EX='03:02:01'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(-(3h + 2min + 1s + std::chrono::duration<int64_t, std::pico>(123456789012))));
+
+  check(SV("%H='01'\t"
+           "%OH='01'\t"
+           "%I='01'\t"
+           "%OI='01'\t"
+           "%M='01'\t"
+           "%OM='01'\t"
+           "%S='01'\t"
+           "%OS='01'\t"
+#if defined(_AIX)
+           "%p='AM'\t"
+#else
+           "%p=''\t"
+#endif
+           "%R='01:01'\t"
+           "%T='01:01:01'\t"
+#ifdef _WIN32
+           "%r='01:01:01'\t"
+#elif defined(_AIX)
+           "%r='01:01:01 AM'\t"
+#elif defined(__APPLE__)
+           "%r=''\t"
+#else
+           "%r='01:01:01 '\t"
+#endif
+           "%X='01:01:01'\t"
+           "%EX='01:01:01'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(std::chrono::duration<double>(3661.123456)));
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#if defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%H='00'\t"
+           "%OH='00'\t"
+           "%I='12'\t"
+           "%OI='12'\t"
+           "%M='00'\t"
+           "%OM='00'\t"
+           "%S='00'\t"
+           "%OS='00'\t"
+#  if defined(__APPLE__)
+           "%p='AM'\t"
+#  else
+           "%p='午前'\t"
+#  endif
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+#  if defined(__APPLE__)
+           "%r='12:00:00 AM'\t"
+           "%X='00時00分00秒'\t"
+           "%EX='00時00分00秒'\t"
+#  else
+           "%r='午前12:00:00'\t"
+           "%X='00:00:00'\t"
+           "%EX='00:00:00'\t"
+#  endif
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(0s));
+
+  check(loc,
+        SV("%H='23'\t"
+           "%OH='23'\t"
+           "%I='11'\t"
+           "%OI='11'\t"
+           "%M='31'\t"
+           "%OM='31'\t"
+           "%S='30.123'\t"
+           "%OS='30.123'\t"
+#  if defined(__APPLE__)
+           "%p='PM'\t"
+#  else
+           "%p='午後'\t"
+#  endif
+           "%R='23:31'\t"
+           "%T='23:31:30.123'\t"
+#  if defined(__APPLE__)
+           "%r='11:31:30 PM'\t"
+           "%X='23時31分30秒'\t"
+           "%EX='23時31分30秒'\t"
+#  else
+           "%r='午後11:31:30'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
+#  endif
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
+
+  check(loc,
+        SV("-%H='03'\t"
+           "%OH='03'\t"
+           "%I='03'\t"
+           "%OI='03'\t"
+           "%M='02'\t"
+           "%OM='02'\t"
+           "%S='01.123456789012'\t"
+           "%OS='01.123456789012'\t"
+#  if defined(__APPLE__)
+           "%p='AM'\t"
+#  else
+           "%p='午前'\t"
+#  endif
+           "%R='03:02'\t"
+           "%T='03:02:01.123456789012'\t"
+#  if defined(__APPLE__)
+           "%r='03:02:01 AM'\t"
+           "%X='03時02分01秒'\t"
+           "%EX='03時02分01秒'\t"
+#  else
+           "%r='午前03:02:01'\t"
+           "%X='03:02:01'\t"
+           "%EX='03:02:01'\t"
+#  endif
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(-(3h + 2min + 1s + std::chrono::duration<int64_t, std::pico>(123456789012))));
+
+  check(loc,
+        SV("%H='01'\t"
+           "%OH='01'\t"
+           "%I='01'\t"
+           "%OI='01'\t"
+           "%M='01'\t"
+           "%OM='01'\t"
+           "%S='01'\t"
+           "%OS='01'\t"
+#  if defined(__APPLE__)
+           "%p='AM'\t"
+#  else
+           "%p='午前'\t"
+#  endif
+           "%R='01:01'\t"
+           "%T='01:01:01'\t"
+#  if defined(__APPLE__)
+           "%r='01:01:01 AM'\t"
+           "%X='01時01分01秒'\t"
+           "%EX='01時01分01秒'\t"
+#  else
+           "%r='午前01:01:01'\t"
+           "%X='01:01:01'\t"
+           "%EX='01:01:01'\t"
+#  endif
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(std::chrono::duration<double>(3661.123456)));
+#else  // defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%H='00'\t"
+           "%OH='〇'\t"
+           "%I='12'\t"
+           "%OI='十二'\t"
+           "%M='00'\t"
+           "%OM='〇'\t"
+           "%S='00'\t"
+           "%OS='〇'\t"
+           "%p='午前'\t"
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+           "%r='午前12時00分00秒'\t"
+           "%X='00時00分00秒'\t"
+           "%EX='00時00分00秒'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(0s));
+
+  // TODO FMT What should fractions be in alternate display mode?
+  check(loc,
+        SV("%H='23'\t"
+           "%OH='二十三'\t"
+           "%I='11'\t"
+           "%OI='十一'\t"
+           "%M='31'\t"
+           "%OM='三十一'\t"
+           "%S='30.123'\t"
+           "%OS='三十.123'\t"
+           "%p='午後'\t"
+           "%R='23:31'\t"
+           "%T='23:31:30.123'\t"
+           "%r='午後11時31分30秒'\t"
+           "%X='23時31分30秒'\t"
+           "%EX='23時31分30秒'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
+
+  check(loc,
+        SV("-%H='03'\t"
+           "%OH='三'\t"
+           "%I='03'\t"
+           "%OI='三'\t"
+           "%M='02'\t"
+           "%OM='二'\t"
+           "%S='01.123456789012'\t"
+           "%OS='一.123456789012'\t"
+           "%p='午前'\t"
+           "%R='03:02'\t"
+           "%T='03:02:01.123456789012'\t"
+           "%r='午前03時02分01秒'\t"
+           "%X='03時02分01秒'\t"
+           "%EX='03時02分01秒'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(-(3h + 2min + 1s + std::chrono::duration<int64_t, std::pico>(123456789012))));
+
+  check(loc,
+        SV("%H='01'\t"
+           "%OH='一'\t"
+           "%I='01'\t"
+           "%OI='一'\t"
+           "%M='01'\t"
+           "%OM='一'\t"
+           "%S='01'\t"
+           "%OS='一'\t"
+           "%p='午前'\t"
+           "%R='01:01'\t"
+           "%T='01:01:01'\t"
+           "%r='午前01時01分01秒'\t"
+           "%X='01時01分01秒'\t"
+           "%EX='01時01分01秒'\t"
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(std::chrono::duration<double>(3661.123456)));
+#endif // defined(__APPLE__) || defined(_AIX)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_invalid_values() {
+  using namespace std::literals::chrono_literals;
+
+  // This looks odd, however the 24 hours is not valid for a 24 hour clock.
+  // TODO FMT discuss what the "proper" behaviour is.
+  check_exception("formatting a hour needs a valid value", SV("{:%H"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%OH"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%I"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%OI"), std::chrono::hh_mm_ss{24h});
+  check(SV("00"), SV("{:%M}"), std::chrono::hh_mm_ss{24h});
+  check(SV("00"), SV("{:%OM}"), std::chrono::hh_mm_ss{24h});
+  check(SV("00"), SV("{:%S}"), std::chrono::hh_mm_ss{24h});
+  check(SV("00"), SV("{:%OS}"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%p"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%R"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%T"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%r"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%X"), std::chrono::hh_mm_ss{24h});
+  check_exception("formatting a hour needs a valid value", SV("{:%EX"), std::chrono::hh_mm_ss{24h});
+}
+
+template <class CharT>
+static void test() {
+  using namespace std::literals::chrono_literals;
+
+  test_no_chrono_specs<CharT>();
+  test_valid_values<CharT>();
+  test_invalid_values<CharT>();
+  check_invalid_types<CharT>(
+      {SV("H"),
+       SV("I"),
+       SV("M"),
+       SV("S"),
+       SV("p"),
+       SV("r"),
+       SV("R"),
+       SV("T"),
+       SV("X"),
+       SV("OH"),
+       SV("OI"),
+       SV("OM"),
+       SV("OS"),
+       SV("EX")},
+      std::chrono::hh_mm_ss{0ms});
+
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:A"), std::chrono::hh_mm_ss{0ms});
+  check_exception("The chrono-specs contains a '{'", SV("{:%%{"), std::chrono::hh_mm_ss{0ms});
+  check_exception(
+      "End of input while parsing the modifier chrono conversion-spec", SV("{:%"), std::chrono::hh_mm_ss{0ms});
+  check_exception("End of input while parsing the modifier E", SV("{:%E"), std::chrono::hh_mm_ss{0ms});
+  check_exception("End of input while parsing the modifier O", SV("{:%O"), std::chrono::hh_mm_ss{0ms});
+
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:.3}"), std::chrono::hh_mm_ss{0ms});
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formattable/concept.formattable.compile.pass.cpp

#ifndef TEST_HAS_NO_LOCALIZATION
   assert_is_formattable<std::chrono::month_weekday_last, CharT>();
 
   assert_is_formattable<std::chrono::year_month, CharT>();
   assert_is_formattable<std::chrono::year_month_day, CharT>();
   assert_is_formattable<std::chrono::year_month_day_last, CharT>();
   assert_is_formattable<std::chrono::year_month_weekday, CharT>();
   assert_is_formattable<std::chrono::year_month_weekday_last, CharT>();
 
-  assert_is_not_formattable<std::chrono::hh_mm_ss<std::chrono::microseconds>, CharT>();
+  assert_is_formattable<std::chrono::hh_mm_ss<std::chrono::microseconds>, CharT>();
 
   //assert_is_formattable<std::chrono::sys_info, CharT>();
   //assert_is_formattable<std::chrono::local_info, CharT>();
 
   //assert_is_formattable<std::chrono::zoned_time, CharT>();
 
 #endif // TEST_HAS_NO_LOCALIZATION
 }