[libc++][chrono] Implements formatter month.

[Mordante]

Summary

Partially implements:

• P1361 Integration of chrono with text formatting

[Mordante]

Test CI,

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

Fixes CI.

[Mordante]

Rebased and enabled full CI.

[Mordante]

Fixes modular build.

[Mordante]

Fixes Ci.

[ldionne]

libcxx/include/__chrono/formatter.h

179

For watchers, this looks quite weird right now but Mark explained that this was going to be extended for other values, and the logic will become non-trivial. So I think this is mandated indeed.

211

libcxx/include/__chrono/ostream.h

51

Can you somehow indicate that this is likely a bug in the spec, not in our implementation? As currently worded, the comment seems to imply that we're doing something wrong.

libcxx/test/std/time/time.cal/time.cal.month/time.cal.month.nonmembers/ostream.pass.cpp

11

Here and elsewhere.

17–18

Can you double-check that these tests are getting run at all in our CI? It seems rather suspicious to me that the no-localization CI job didn't fail despite using the incorrect // UNSUPPORTED: libcpp-has-no-localization formulation above.

20–21

[Mordante]

Rebased and addresses review comments.

[Mordante]

Rebased.

[Mordante]

libcxx/test/std/time/time.cal/time.cal.month/time.cal.month.nonmembers/ostream.pass.cpp

17–18

I verified and these tests are executed in the CI.
(I would be surprised if they weren't since a lot of the platform specific output have been found by the CI.)

libcxx/docs/Status/FormatPaper.csv

 Section,Description,Dependencies,Assignee,Status,First released version
 `P1361 <https://wg21.link/P1361>`__ `P2372 <https://wg21.link/P2372>`__,"Formatting chrono"
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::duration<Rep, Period>``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::sys_time<Duration>``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::utc_time<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::tai_time<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::gps_time<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::file_time<Duration>``",,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local_time<Duration>``",,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local-time-format-t<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::day``",,Mark de Wever,|Complete|, Clang 16
-`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month``",,Mark de Wever,|In Progress|,
+`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday_indexed``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday_last``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_day``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_day_last``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_weekday``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_weekday_last``",,Mark de Wever,|In Progress|,

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/day.h>
+#include <__chrono/month.h>
 #include <__chrono/year.h>
 #include <__concepts/same_as.h>
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
 // Convert a chrono calendar time point to the given tm type,
 // which must have the same properties as std::tm.
 template <class _Tm, class _ChronoCalendarTimePoint>
 _LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(const _ChronoCalendarTimePoint& __value) {
   _Tm __result = {};
 #  ifdef __GLIBC__
   __result.tm_zone = "UTC";
 #  endif
 
   if constexpr (same_as<_ChronoCalendarTimePoint, chrono::day>)
     __result.tm_mday = static_cast<unsigned>(__value);
+  else if constexpr (same_as<_ChronoCalendarTimePoint, chrono::month>)
+    __result.tm_mon = static_cast<unsigned>(__value) - 1;
   else if constexpr (same_as<_ChronoCalendarTimePoint, chrono::year>)
     __result.tm_year = static_cast<int>(__value) - 1900;
   else
     static_assert(sizeof(_ChronoCalendarTimePoint) == 0, "Add the missing type specialization");
 
   return __result;
 }
 
 #endif //if _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 #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/convert_to_tm.h>
 #include <__chrono/day.h>
+#include <__chrono/month.h>
 #include <__chrono/parser_std_format_spec.h>
 #include <__chrono/statically_widen.h>
 #include <__chrono/year.h>
+#include <__concepts/same_as.h>
 #include <__config>
 #include <__format/concepts.h>
+#include <__format/format_error.h>
 #include <__format/format_functions.h>
 #include <__format/format_parse_context.h>
 #include <__format/formatter.h>
 #include <__format/formatter_output.h>
 #include <__format/parser_std_format_spec.h>
 #include <__memory/addressof.h>
 #include <cmath>
 #include <ctime>
#  endif // defined(__GLIBC__) || defined(_AIX)
         break;
       }
     } else {
       __sstr << *__it;
     }
   }
 }
 
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr bool __month_name_ok(const _Tp& __value) {
+  if constexpr (same_as<_Tp, chrono::day>)
+    return true;
+  else if constexpr (same_as<_Tp, chrono::month>)
+    return __value.ok();
+  else if constexpr (same_as<_Tp, chrono::year>)
+    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,
                 __format_spec::__parsed_specifications<_CharT> __specs,
                 basic_string_view<_CharT> __chrono_specs) -> decltype(__ctx.out()) {
   basic_stringstream<_CharT> __sstr;
   // [time.format]/2
   // 2.1 - the "C" locale if the L option is not present in chrono-format-spec, otherwise
   // 2.2 - the locale passed to the formatting function if any, otherwise
   // 2.3 - the global locale.
   // Note that the __ctx's locale() call does 2.2 and 2.3.
   if (__specs.__chrono_.__locale_specific_form_)
     __sstr.imbue(__ctx.locale());
   else
     __sstr.imbue(locale::classic());
 
   if (__chrono_specs.empty())
     __sstr << __value;
-  else
+  else {
+    if (__specs.__chrono_.__month_name_ && !__formatter::__month_name_ok(__value))
+      std::__throw_format_error("formatting a month name from an invalid month number");
+
     __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);
 }
 
 } // namespace __formatter
 
public:
 
   _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::__day);
   }
 };
 
 template <__fmt_char_type _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<chrono::month, _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::__month);
+  }
+};
+
+template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<chrono::year, _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::__year);
   }
 };
 
 #endif // if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif //  _LIBCPP___CHRONO_FORMATTER_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/month.h>
 #include <__chrono/statically_widen.h>
 #include <__chrono/year.h>
 #include <__config>
 #include <__format/format_functions.h>
 #include <ostream>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
return __os
               // the formatted day seems to be either modulo 100 or completely
               // omitted. Judging by the wording this is valid.
               // TODO FMT Write a paper of file an LWG issue.
               : std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:02} is not a valid day"), static_cast<unsigned>(__d)));
 }
 
 template <class _CharT, class _Traits>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, const month& __m) {
+  return __os << (__m.ok() ? std::format(__os.getloc(), _LIBCPP_STATICALLY_WIDEN(_CharT, "{:L%b}"), __m)
+                           : std::format(__os.getloc(),
+                                         _LIBCPP_STATICALLY_WIDEN(_CharT, "{} is not a valid month"),
+                                         static_cast<unsigned>(__m))); // TODO FMT Standard mandated locale isn't used.
+}
+
+template <class _CharT, class _Traits>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os, const year& __y) {
   return __os << (__y.ok() ? std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:%Y}"), __y)
                            : std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:%Y} is not a valid year"), __y));
 }
 
 } // 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

 class month;
 constexpr bool operator==(const month& x, const month& y) noexcept;
 constexpr strong_ordering operator<=>(const month& x, const month& y) noexcept;
 
 constexpr month  operator+(const month&  x, const months& y) noexcept;
 constexpr month  operator+(const months& x,  const month& y) noexcept;
 constexpr month  operator-(const month&  x, const months& y) noexcept;
 constexpr months operator-(const month&  x,  const month& y) noexcept;
+template<class charT, class traits>
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os, const month& m);
 
 // 25.8.5, class year    // C++20
 class year;
 constexpr bool operator==(const year& x, const year& y) noexcept;
 constexpr strong_ordering operator<=>(const year& x, const year& y) noexcept;
 
 constexpr year  operator+(const year&  x, const years& y) noexcept;
 constexpr year  operator+(const years& x, const year&  y) noexcept;

 bool operator<(const time_zone& x, const time_zone& y) noexcept;
 bool operator>(const time_zone& x, const time_zone& y) noexcept;
 bool operator<=(const time_zone& x, const time_zone& y) noexcept;
 bool operator>=(const time_zone& x, const time_zone& y) noexcept;
 }  // chrono
 
 namespace std {
   template<class charT> struct formatter<chrono::day, charT>;                     // C++20
+  template<class charT> struct formatter<chrono::month, charT>;                   // C++20
   template<class charT> struct formatter<chrono::year, 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

libcxx/test/libcxx/utilities/format/format.formatter/format.formatter.spec/formattable.compile.pass.cpp

#ifndef TEST_HAS_NO_LOCALIZATION
   assert_is_not_formattable<std::chrono::sys_time<std::chrono::microseconds>, CharT>();
   //assert_is_formattable<std::chrono::utc_time<std::chrono::microseconds>, CharT>();
   //assert_is_formattable<std::chrono::tai_time<std::chrono::microseconds>, CharT>();
   //assert_is_formattable<std::chrono::gps_time<std::chrono::microseconds>, CharT>();
   assert_is_not_formattable<std::chrono::file_time<std::chrono::microseconds>, CharT>();
   assert_is_not_formattable<std::chrono::local_time<std::chrono::microseconds>, CharT>();
 
   assert_is_formattable<std::chrono::day, CharT>();
-  assert_is_not_formattable<std::chrono::month, CharT>();
+  assert_is_formattable<std::chrono::month, CharT>();
   assert_is_formattable<std::chrono::year, CharT>();
 
   assert_is_not_formattable<std::chrono::weekday, CharT>();
   assert_is_not_formattable<std::chrono::weekday_indexed, CharT>();
   assert_is_not_formattable<std::chrono::weekday_last, CharT>();
 
   assert_is_not_formattable<std::chrono::month_day, CharT>();
   assert_is_not_formattable<std::chrono::month_day_last, CharT>();

libcxx/test/std/time/time.cal/time.cal.month/time.cal.month.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 Investigate Windows issues.
+// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+
+// class month;
+
+// template<class charT, class traits>
+//   basic_ostream<charT, traits>&
+//     operator<<(basic_ostream<charT, traits>& os, const month& month);
+
+#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>
+static std::basic_string<CharT> stream_c_locale(std::chrono::month month) {
+  std::basic_stringstream<CharT> sstr;
+  sstr << month;
+  return sstr.str();
+}
+
+template <class CharT>
+static std::basic_string<CharT> stream_fr_FR_locale(std::chrono::month month) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_fr_FR_UTF_8);
+  sstr.imbue(locale);
+  sstr << month;
+  return sstr.str();
+}
+
+template <class CharT>
+static std::basic_string<CharT> stream_ja_JP_locale(std::chrono::month month) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_ja_JP_UTF_8);
+  sstr.imbue(locale);
+  sstr << month;
+  return sstr.str();
+}
+
+template <class CharT>
+static void test() {
+  assert(stream_c_locale<CharT>(std::chrono::month{0}) == SV("0 is not a valid month"));
+  assert(stream_c_locale<CharT>(std::chrono::month{1}) == SV("Jan"));
+  assert(stream_c_locale<CharT>(std::chrono::month{2}) == SV("Feb"));
+  assert(stream_c_locale<CharT>(std::chrono::month{3}) == SV("Mar"));
+  assert(stream_c_locale<CharT>(std::chrono::month{4}) == SV("Apr"));
+  assert(stream_c_locale<CharT>(std::chrono::month{5}) == SV("May"));
+  assert(stream_c_locale<CharT>(std::chrono::month{6}) == SV("Jun"));
+  assert(stream_c_locale<CharT>(std::chrono::month{7}) == SV("Jul"));
+  assert(stream_c_locale<CharT>(std::chrono::month{8}) == SV("Aug"));
+  assert(stream_c_locale<CharT>(std::chrono::month{9}) == SV("Sep"));
+  assert(stream_c_locale<CharT>(std::chrono::month{10}) == SV("Oct"));
+  assert(stream_c_locale<CharT>(std::chrono::month{11}) == SV("Nov"));
+  assert(stream_c_locale<CharT>(std::chrono::month{12}) == SV("Dec"));
+  assert(stream_c_locale<CharT>(std::chrono::month{13}) == SV("13 is not a valid month"));
+  assert(stream_c_locale<CharT>(std::chrono::month{255}) == SV("255 is not a valid month"));
+
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{0}) == SV("0 is not a valid month"));
+#if defined(__APPLE__)
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{1}) == SV("jan"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{2}) == SV("fév"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{3}) == SV("mar"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{4}) == SV("avr"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{5}) == SV("mai"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{6}) == SV("jui"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{7}) == SV("jul"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{8}) == SV("aoû"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{9}) == SV("sep"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{10}) == SV("oct"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{11}) == SV("nov"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{12}) == SV("déc"));
+#else //  defined(__APPLE__)
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{1}) == SV("janv."));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{2}) == SV("févr."));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{3}) == SV("mars"));
+#  if defined(_WIN32) || defined(_AIX)
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{4}) == SV("avr."));
+#  else
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{4}) == SV("avril"));
+#  endif
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{5}) == SV("mai"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{6}) == SV("juin"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{7}) == SV("juil."));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{8}) == SV("août"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{9}) == SV("sept."));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{10}) == SV("oct."));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{11}) == SV("nov."));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{12}) == SV("déc."));
+#endif //  defined(__APPLE__)
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{13}) == SV("13 is not a valid month"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::month{255}) == SV("255 is not a valid month"));
+
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{0}) == SV("0 is not a valid month"));
+#if defined(__APPLE__)
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{1}) == SV(" 1"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{2}) == SV(" 2"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{3}) == SV(" 3"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{4}) == SV(" 4"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{5}) == SV(" 5"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{6}) == SV(" 6"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{7}) == SV(" 7"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{8}) == SV(" 8"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{9}) == SV(" 9"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{10}) == SV("10"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{11}) == SV("11"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{12}) == SV("12"));
+#else // defined(__APPLE__)
+#  if defined(_WIN32) || defined(_AIX)
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{1}) == SV("1月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{2}) == SV("2月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{3}) == SV("3月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{4}) == SV("4月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{5}) == SV("5月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{6}) == SV("6月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{7}) == SV("7月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{8}) == SV("8月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{9}) == SV("9月"));
+#  else  // defined(_WIN32) || defined(_AIX)
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{1}) == SV(" 1月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{2}) == SV(" 2月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{3}) == SV(" 3月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{4}) == SV(" 4月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{5}) == SV(" 5月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{6}) == SV(" 6月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{7}) == SV(" 7月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{8}) == SV(" 8月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{9}) == SV(" 9月"));
+#  endif // defined(_WIN32) || defined(_AIX)
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{10}) == SV("10月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{11}) == SV("11月"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{12}) == SV("12月"));
+#endif   // defined(__APPLE__)
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{13}) == SV("13 is not a valid month"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::month{255}) == SV("255 is not a valid month"));
+}
+
+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.month.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
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+
+// template<class charT> struct formatter<chrono::month, 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() {
+  // Valid month
+  check(SV("Jan"), SV("{}"), std::chrono::month{1});
+  check(SV("*Jan*"), SV("{:*^5}"), std::chrono::month{1});
+  check(SV("*Jan"), SV("{:*>4}"), std::chrono::month{1});
+
+  // Invalid month
+  check(SV("0 is not a valid month"), SV("{}"), std::chrono::month{0});
+  check(SV("*0 is not a valid month*"), SV("{:*^24}"), std::chrono::month{0});
+}
+
+template <class CharT>
+static void test_valid_values() {
+  // Test that %b, %h, and %B throw an exception.
+  check_exception("formatting a month name from an invalid month number", SV("{:%b}"), std::chrono::month{200});
+  check_exception("formatting a month name from an invalid month number", SV("{:%b}"), std::chrono::month{13});
+  check_exception("formatting a month name from an invalid month number", SV("{:%b}"), std::chrono::month{255});
+
+  check_exception("formatting a month name from an invalid month number", SV("{:%h}"), std::chrono::month{0});
+  check_exception("formatting a month name from an invalid month number", SV("{:%h}"), std::chrono::month{13});
+  check_exception("formatting a month name from an invalid month number", SV("{:%h}"), std::chrono::month{255});
+
+  check_exception("formatting a month name from an invalid month number", SV("{:%B}"), std::chrono::month{0});
+  check_exception("formatting a month name from an invalid month number", SV("{:%B}"), std::chrono::month{13});
+  check_exception("formatting a month name from an invalid month number", SV("{:%B}"), std::chrono::month{255});
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%b='%b'%t%%B='%B'%t%%h='%h'%t%%m='%m'%t%%Om='%Om'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%b='%b'%t%%B='%B'%t%%h='%h'%t%%m='%m'%t%%Om='%Om'%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("%b='Jan'\t%B='January'\t%h='Jan'\t%m='01'\t%Om='01'\n"), fmt, std::chrono::January);
+  check(SV("%b='Feb'\t%B='February'\t%h='Feb'\t%m='02'\t%Om='02'\n"), fmt, std::chrono::February);
+  check(SV("%b='Mar'\t%B='March'\t%h='Mar'\t%m='03'\t%Om='03'\n"), fmt, std::chrono::March);
+  check(SV("%b='Apr'\t%B='April'\t%h='Apr'\t%m='04'\t%Om='04'\n"), fmt, std::chrono::April);
+  check(SV("%b='May'\t%B='May'\t%h='May'\t%m='05'\t%Om='05'\n"), fmt, std::chrono::May);
+  check(SV("%b='Jun'\t%B='June'\t%h='Jun'\t%m='06'\t%Om='06'\n"), fmt, std::chrono::June);
+  check(SV("%b='Jul'\t%B='July'\t%h='Jul'\t%m='07'\t%Om='07'\n"), fmt, std::chrono::July);
+  check(SV("%b='Aug'\t%B='August'\t%h='Aug'\t%m='08'\t%Om='08'\n"), fmt, std::chrono::August);
+  check(SV("%b='Sep'\t%B='September'\t%h='Sep'\t%m='09'\t%Om='09'\n"), fmt, std::chrono::September);
+  check(SV("%b='Oct'\t%B='October'\t%h='Oct'\t%m='10'\t%Om='10'\n"), fmt, std::chrono::October);
+  check(SV("%b='Nov'\t%B='November'\t%h='Nov'\t%m='11'\t%Om='11'\n"), fmt, std::chrono::November);
+  check(SV("%b='Dec'\t%B='December'\t%h='Dec'\t%m='12'\t%Om='12'\n"), fmt, std::chrono::December);
+
+  // Use the global locale (fr_FR)
+#if defined(__APPLE__)
+  check(SV("%b='jan'\t%B='janvier'\t%h='jan'\t%m='01'\t%Om='01'\n"), lfmt, std::chrono::January);
+  check(SV("%b='fév'\t%B='février'\t%h='fév'\t%m='02'\t%Om='02'\n"), lfmt, std::chrono::February);
+  check(SV("%b='mar'\t%B='mars'\t%h='mar'\t%m='03'\t%Om='03'\n"), lfmt, std::chrono::March);
+  check(SV("%b='avr'\t%B='avril'\t%h='avr'\t%m='04'\t%Om='04'\n"), lfmt, std::chrono::April);
+  check(SV("%b='mai'\t%B='mai'\t%h='mai'\t%m='05'\t%Om='05'\n"), lfmt, std::chrono::May);
+  check(SV("%b='jui'\t%B='juin'\t%h='jui'\t%m='06'\t%Om='06'\n"), lfmt, std::chrono::June);
+  check(SV("%b='jul'\t%B='juillet'\t%h='jul'\t%m='07'\t%Om='07'\n"), lfmt, std::chrono::July);
+  check(SV("%b='aoû'\t%B='août'\t%h='aoû'\t%m='08'\t%Om='08'\n"), lfmt, std::chrono::August);
+  check(SV("%b='sep'\t%B='septembre'\t%h='sep'\t%m='09'\t%Om='09'\n"), lfmt, std::chrono::September);
+  check(SV("%b='oct'\t%B='octobre'\t%h='oct'\t%m='10'\t%Om='10'\n"), lfmt, std::chrono::October);
+  check(SV("%b='nov'\t%B='novembre'\t%h='nov'\t%m='11'\t%Om='11'\n"), lfmt, std::chrono::November);
+  check(SV("%b='déc'\t%B='décembre'\t%h='déc'\t%m='12'\t%Om='12'\n"), lfmt, std::chrono::December);
+#else // defined(__APPLE__)
+  check(SV("%b='janv.'\t%B='janvier'\t%h='janv.'\t%m='01'\t%Om='01'\n"), lfmt, std::chrono::January);
+  check(SV("%b='févr.'\t%B='février'\t%h='févr.'\t%m='02'\t%Om='02'\n"), lfmt, std::chrono::February);
+  check(SV("%b='mars'\t%B='mars'\t%h='mars'\t%m='03'\t%Om='03'\n"), lfmt, std::chrono::March);
+#  if defined(_WIN32) || defined(_AIX)
+  check(SV("%b='avr.'\t%B='avril'\t%h='avr.'\t%m='04'\t%Om='04'\n"), lfmt, std::chrono::April);
+#  else
+  check(SV("%b='avril'\t%B='avril'\t%h='avril'\t%m='04'\t%Om='04'\n"), lfmt, std::chrono::April);
+#  endif
+  check(SV("%b='mai'\t%B='mai'\t%h='mai'\t%m='05'\t%Om='05'\n"), lfmt, std::chrono::May);
+  check(SV("%b='juin'\t%B='juin'\t%h='juin'\t%m='06'\t%Om='06'\n"), lfmt, std::chrono::June);
+  check(SV("%b='juil.'\t%B='juillet'\t%h='juil.'\t%m='07'\t%Om='07'\n"), lfmt, std::chrono::July);
+  check(SV("%b='août'\t%B='août'\t%h='août'\t%m='08'\t%Om='08'\n"), lfmt, std::chrono::August);
+  check(SV("%b='sept.'\t%B='septembre'\t%h='sept.'\t%m='09'\t%Om='09'\n"), lfmt, std::chrono::September);
+  check(SV("%b='oct.'\t%B='octobre'\t%h='oct.'\t%m='10'\t%Om='10'\n"), lfmt, std::chrono::October);
+  check(SV("%b='nov.'\t%B='novembre'\t%h='nov.'\t%m='11'\t%Om='11'\n"), lfmt, std::chrono::November);
+  check(SV("%b='déc.'\t%B='décembre'\t%h='déc.'\t%m='12'\t%Om='12'\n"), lfmt, std::chrono::December);
+#endif // defined(__APPLE__)
+
+  // Use supplied locale (ja_JP)
+#if defined(_WIN32)
+  check(loc, SV("%b='1'\t%B='1月'\t%h='1'\t%m='01'\t%Om='01'\n"), lfmt, std::chrono::January);
+  check(loc, SV("%b='2'\t%B='2月'\t%h='2'\t%m='02'\t%Om='02'\n"), lfmt, std::chrono::February);
+  check(loc, SV("%b='3'\t%B='3月'\t%h='3'\t%m='03'\t%Om='03'\n"), lfmt, std::chrono::March);
+  check(loc, SV("%b='4'\t%B='4月'\t%h='4'\t%m='04'\t%Om='04'\n"), lfmt, std::chrono::April);
+  check(loc, SV("%b='5'\t%B='5月'\t%h='5'\t%m='05'\t%Om='05'\n"), lfmt, std::chrono::May);
+  check(loc, SV("%b='6'\t%B='6月'\t%h='6'\t%m='06'\t%Om='06'\n"), lfmt, std::chrono::June);
+  check(loc, SV("%b='7'\t%B='7月'\t%h='7'\t%m='07'\t%Om='07'\n"), lfmt, std::chrono::July);
+  check(loc, SV("%b='8'\t%B='8月'\t%h='8'\t%m='08'\t%Om='08'\n"), lfmt, std::chrono::August);
+  check(loc, SV("%b='9'\t%B='9月'\t%h='9'\t%m='09'\t%Om='09'\n"), lfmt, std::chrono::September);
+  check(loc, SV("%b='10'\t%B='10月'\t%h='10'\t%m='10'\t%Om='10'\n"), lfmt, std::chrono::October);
+  check(loc, SV("%b='11'\t%B='11月'\t%h='11'\t%m='11'\t%Om='11'\n"), lfmt, std::chrono::November);
+  check(loc, SV("%b='12'\t%B='12月'\t%h='12'\t%m='12'\t%Om='12'\n"), lfmt, std::chrono::December);
+#elif defined(_AIX)      // defined(_WIN32)
+  check(loc, SV("%b='1月'\t%B='1月'\t%h='1月'\t%m='01'\t%Om='01'\n"), lfmt, std::chrono::January);
+  check(loc, SV("%b='2月'\t%B='2月'\t%h='2月'\t%m='02'\t%Om='02'\n"), lfmt, std::chrono::February);
+  check(loc, SV("%b='3月'\t%B='3月'\t%h='3月'\t%m='03'\t%Om='03'\n"), lfmt, std::chrono::March);
+  check(loc, SV("%b='4月'\t%B='4月'\t%h='4月'\t%m='04'\t%Om='04'\n"), lfmt, std::chrono::April);
+  check(loc, SV("%b='5月'\t%B='5月'\t%h='5月'\t%m='05'\t%Om='05'\n"), lfmt, std::chrono::May);
+  check(loc, SV("%b='6月'\t%B='6月'\t%h='6月'\t%m='06'\t%Om='06'\n"), lfmt, std::chrono::June);
+  check(loc, SV("%b='7月'\t%B='7月'\t%h='7月'\t%m='07'\t%Om='07'\n"), lfmt, std::chrono::July);
+  check(loc, SV("%b='8月'\t%B='8月'\t%h='8月'\t%m='08'\t%Om='08'\n"), lfmt, std::chrono::August);
+  check(loc, SV("%b='9月'\t%B='9月'\t%h='9月'\t%m='09'\t%Om='09'\n"), lfmt, std::chrono::September);
+  check(loc, SV("%b='10月'\t%B='10月'\t%h='10月'\t%m='10'\t%Om='10'\n"), lfmt, std::chrono::October);
+  check(loc, SV("%b='11月'\t%B='11月'\t%h='11月'\t%m='11'\t%Om='11'\n"), lfmt, std::chrono::November);
+  check(loc, SV("%b='12月'\t%B='12月'\t%h='12月'\t%m='12'\t%Om='12'\n"), lfmt, std::chrono::December);
+#elif defined(__APPLE__) // defined(_WIN32)
+  check(loc, SV("%b=' 1'\t%B='1月'\t%h=' 1'\t%m='01'\t%Om='01'\n"), lfmt, std::chrono::January);
+  check(loc, SV("%b=' 2'\t%B='2月'\t%h=' 2'\t%m='02'\t%Om='02'\n"), lfmt, std::chrono::February);
+  check(loc, SV("%b=' 3'\t%B='3月'\t%h=' 3'\t%m='03'\t%Om='03'\n"), lfmt, std::chrono::March);
+  check(loc, SV("%b=' 4'\t%B='4月'\t%h=' 4'\t%m='04'\t%Om='04'\n"), lfmt, std::chrono::April);
+  check(loc, SV("%b=' 5'\t%B='5月'\t%h=' 5'\t%m='05'\t%Om='05'\n"), lfmt, std::chrono::May);
+  check(loc, SV("%b=' 6'\t%B='6月'\t%h=' 6'\t%m='06'\t%Om='06'\n"), lfmt, std::chrono::June);
+  check(loc, SV("%b=' 7'\t%B='7月'\t%h=' 7'\t%m='07'\t%Om='07'\n"), lfmt, std::chrono::July);
+  check(loc, SV("%b=' 8'\t%B='8月'\t%h=' 8'\t%m='08'\t%Om='08'\n"), lfmt, std::chrono::August);
+  check(loc, SV("%b=' 9'\t%B='9月'\t%h=' 9'\t%m='09'\t%Om='09'\n"), lfmt, std::chrono::September);
+  check(loc, SV("%b='10'\t%B='10月'\t%h='10'\t%m='10'\t%Om='10'\n"), lfmt, std::chrono::October);
+  check(loc, SV("%b='11'\t%B='11月'\t%h='11'\t%m='11'\t%Om='11'\n"), lfmt, std::chrono::November);
+  check(loc, SV("%b='12'\t%B='12月'\t%h='12'\t%m='12'\t%Om='12'\n"), lfmt, std::chrono::December);
+#else                    // defined(_WIN32)
+  check(loc, SV("%b=' 1月'\t%B='1月'\t%h=' 1月'\t%m='01'\t%Om='一'\n"), lfmt, std::chrono::January);
+  check(loc, SV("%b=' 2月'\t%B='2月'\t%h=' 2月'\t%m='02'\t%Om='二'\n"), lfmt, std::chrono::February);
+  check(loc, SV("%b=' 3月'\t%B='3月'\t%h=' 3月'\t%m='03'\t%Om='三'\n"), lfmt, std::chrono::March);
+  check(loc, SV("%b=' 4月'\t%B='4月'\t%h=' 4月'\t%m='04'\t%Om='四'\n"), lfmt, std::chrono::April);
+  check(loc, SV("%b=' 5月'\t%B='5月'\t%h=' 5月'\t%m='05'\t%Om='五'\n"), lfmt, std::chrono::May);
+  check(loc, SV("%b=' 6月'\t%B='6月'\t%h=' 6月'\t%m='06'\t%Om='六'\n"), lfmt, std::chrono::June);
+  check(loc, SV("%b=' 7月'\t%B='7月'\t%h=' 7月'\t%m='07'\t%Om='七'\n"), lfmt, std::chrono::July);
+  check(loc, SV("%b=' 8月'\t%B='8月'\t%h=' 8月'\t%m='08'\t%Om='八'\n"), lfmt, std::chrono::August);
+  check(loc, SV("%b=' 9月'\t%B='9月'\t%h=' 9月'\t%m='09'\t%Om='九'\n"), lfmt, std::chrono::September);
+  check(loc, SV("%b='10月'\t%B='10月'\t%h='10月'\t%m='10'\t%Om='十'\n"), lfmt, std::chrono::October);
+  check(loc, SV("%b='11月'\t%B='11月'\t%h='11月'\t%m='11'\t%Om='十一'\n"), lfmt, std::chrono::November);
+  check(loc, SV("%b='12月'\t%B='12月'\t%h='12月'\t%m='12'\t%Om='十二'\n"), lfmt, std::chrono::December);
+#endif                   //  defined(_WIN32)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test() {
+  test_no_chrono_specs<CharT>();
+  test_valid_values<CharT>();
+  check_invalid_types<CharT>({SV("b"), SV("B"), SV("h"), SV("m"), SV("Om")}, std::chrono::January);
+
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:A"), std::chrono::January);
+  check_exception("The chrono-specs contains a '{'", SV("{:%%{"), std::chrono::January);
+  check_exception("End of input while parsing the modifier chrono conversion-spec", SV("{:%"), std::chrono::January);
+  check_exception("End of input while parsing the modifier E", SV("{:%E"), std::chrono::January);
+  check_exception("End of input while parsing the modifier O", SV("{:%O"), std::chrono::January);
+
+  // Precision not allowed
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:.3}"), std::chrono::January);
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/types.compile.pass.cpp

#ifndef TEST_HAS_NO_LOCALIZATION
   assert_formatter_is_disabled<std::chrono::sys_time<std::chrono::microseconds>, CharT>();
   //assert_formatter_is_enabled<std::chrono::utc_time<std::chrono::microseconds>, CharT>();
   //assert_formatter_is_enabled<std::chrono::tai_time<std::chrono::microseconds>, CharT>();
   //assert_formatter_is_enabled<std::chrono::gps_time<std::chrono::microseconds>, CharT>();
   assert_formatter_is_disabled<std::chrono::file_time<std::chrono::microseconds>, CharT>();
   assert_formatter_is_disabled<std::chrono::local_time<std::chrono::microseconds>, CharT>();
 
   assert_formatter_is_enabled<std::chrono::day, CharT>();
-  assert_formatter_is_disabled<std::chrono::month, CharT>();
+  assert_formatter_is_enabled<std::chrono::month, CharT>();
   assert_formatter_is_enabled<std::chrono::year, CharT>();
 
   assert_formatter_is_disabled<std::chrono::weekday, CharT>();
   assert_formatter_is_disabled<std::chrono::weekday_indexed, CharT>();
   assert_formatter_is_disabled<std::chrono::weekday_last, CharT>();
 
   assert_formatter_is_disabled<std::chrono::month_day, CharT>();
   assert_formatter_is_disabled<std::chrono::month_day_last, CharT>();

libcxx/utils/ci/run-buildbot

check-generated-output)
     # Reject patches that introduce non-ASCII characters or hard tabs.
     # Depends on LC_COLLATE set at the top of this script.
     ! grep -rn '[^ -~]' libcxx/include libcxx/src libcxx/test libcxx/benchmarks \
            --exclude '*.dat' \
            --exclude 'format_tests.h' \
            --exclude 'formatter.*.pass.cpp' \
            --exclude 'grep.pass.cpp' \
            --exclude 'locale-specific_form.pass.cpp' \
+           --exclude 'ostream.pass.cpp' \
            --exclude 'std_format_spec_string_unicode.bench.cpp' \
            || false
 
     # Reject code with trailing whitespace
     ! grep -rn '[[:blank:]]$' libcxx/include libcxx/src libcxx/test libcxx/benchmarks || false
 ;;
 generic-cxx03)
     clean