[libc++][TZDB] Implements zoned_time formatters.

Implements parts of:
- P0355 Extending to chrono Calendars and Time Zones
- P1361 Integration of chrono with text formatting
- P2372 Fixing locale handling in chrono formatters

libcxx/docs/Status/FormatPaper.csv

@@ -7,7 +7,7 @@ Section,Description,Dependencies,Assignee,Status,First released version
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::gps_time<Duration>``",A ``<chrono>`` implementation,Mark de Wever,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::file_time<Duration>``",,Mark de Wever,|Complete|,17.0
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local_time<Duration>``",,Mark de Wever,|Complete|,17.0
-`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local-time-format-t<Duration>``",A ``<chrono>`` implementation,Mark de Wever,,,
+`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local-time-format-t<Duration>``",,,|Nothing To Do|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::day``",,Mark de Wever,|Complete|,16.0
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month``",,Mark de Wever,|Complete|,16.0
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year``",,Mark de Wever,|Complete|,16.0
@@ -26,7 +26,7 @@ Section,Description,Dependencies,Assignee,Status,First released version
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::hh_mm_ss<duration<Rep, Period>>``",,Mark de Wever,|Complete|,17.0
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::sys_info``",,Mark de Wever,|Complete|,19.0
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local_info``",,Mark de Wever,|Complete|,19.0
-`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::zoned_time<Duration, TimeZonePtr>``",A ``<chrono>`` implementation,Mark de Wever,,
+`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::zoned_time<Duration, TimeZonePtr>``",,Mark de Wever,|Complete|,19.0
 
 "`P2693R1 <https://wg21.link/P2693R1>`__","Formatting ``thread::id`` and ``stacktrace``"
 `[thread.thread.id] <https://wg21.link/thread.thread.id>`_,"Formatting ``thread::id``",,Mark de Wever,|Complete|,17.0

libcxx/include/__chrono/convert_to_tm.h

@@ -29,11 +29,13 @@
 #include <__chrono/year_month.h>
 #include <__chrono/year_month_day.h>
 #include <__chrono/year_month_weekday.h>
+#include <__chrono/zoned_time.h>
 #include <__concepts/same_as.h>
 #include <__config>
 #include <__format/format_error.h>
 #include <__memory/addressof.h>
 #include <__type_traits/is_convertible.h>
+#include <__type_traits/is_specialization.h>
 #include <cstdint>
 #include <ctime>
 #include <limits>
@@ -178,7 +180,10 @@ _LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(const _ChronoT& __value) {
     // Has no time information.
   } else if constexpr (same_as<_ChronoT, chrono::local_info>) {
     // Has no time information.
-#  endif
+  } else if constexpr (__is_specialization_v<_ChronoT, chrono::zoned_time>) {
+    return std::__convert_to_tm<_Tm>(
+        chrono::sys_time<typename _ChronoT::duration>{__value.get_local_time().time_since_epoch()});
+#  endif // !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB)
   } else
     static_assert(sizeof(_ChronoT) == 0, "Add the missing type specialization");
 

libcxx/include/__chrono/formatter.h

@@ -33,6 +33,7 @@
 #include <__chrono/year_month.h>
 #include <__chrono/year_month_day.h>
 #include <__chrono/year_month_weekday.h>
+#include <__chrono/zoned_time.h>
 #include <__concepts/arithmetic.h>
 #include <__concepts/same_as.h>
 #include <__config>
@@ -44,6 +45,7 @@
 #include <__format/parser_std_format_spec.h>
 #include <__format/write_escaped.h>
 #include <__memory/addressof.h>
+#include <__type_traits/is_specialization.h>
 #include <cmath>
 #include <ctime>
 #include <sstream>
@@ -136,10 +138,22 @@ __format_sub_seconds(basic_stringstream<_CharT>& __sstr, const chrono::hh_mm_ss<
                    __value.fractional_width);
 }
 
+#  if !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB)
+template <class _CharT, class _Duration, class _TimeZonePtr>
+_LIBCPP_HIDE_FROM_ABI void
+__format_sub_seconds(basic_stringstream<_CharT>& __sstr, const chrono::zoned_time<_Duration, _TimeZonePtr>& __value) {
+  __formatter::__format_sub_seconds(__sstr, __value.get_local_time().time_since_epoch());
+}
+#  endif // !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB)
+
 template <class _Tp>
 consteval bool __use_fraction() {
   if constexpr (__is_time_point<_Tp>)
     return chrono::hh_mm_ss<typename _Tp::duration>::fractional_width;
+#  if !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB)
+  else if constexpr (__is_specialization_v<_Tp, chrono::zoned_time>)
+    return chrono::hh_mm_ss<typename _Tp::duration>::fractional_width;
+#  endif
   else if constexpr (chrono::__is_duration<_Tp>::value)
     return chrono::hh_mm_ss<_Tp>::fractional_width;
   else if constexpr (__is_hh_mm_ss<_Tp>)
@@ -211,6 +225,8 @@ _LIBCPP_HIDE_FROM_ABI __time_zone __convert_to_time_zone([[maybe_unused]] const
 #  if !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB)
   if constexpr (same_as<_Tp, chrono::sys_info>)
     return {__value.abbrev, __value.offset};
+  else if constexpr (__is_specialization_v<_Tp, chrono::zoned_time>)
+    return __formatter::__convert_to_time_zone(__value.get_info());
   else
 #  endif
     return {"UTC", chrono::seconds{0}};
@@ -425,6 +441,8 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_ok(const _Tp& __value) {
     return true;
   else if constexpr (same_as<_Tp, chrono::local_info>)
     return true;
+  else if constexpr (__is_specialization_v<_Tp, chrono::zoned_time>)
+    return true;
 #  endif
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
@@ -471,6 +489,8 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_name_ok(const _Tp& __value) {
     return true;
   else if constexpr (same_as<_Tp, chrono::local_info>)
     return true;
+  else if constexpr (__is_specialization_v<_Tp, chrono::zoned_time>)
+    return true;
 #  endif
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
@@ -517,6 +537,8 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool __date_ok(const _Tp& __value) {
     return true;
   else if constexpr (same_as<_Tp, chrono::local_info>)
     return true;
+  else if constexpr (__is_specialization_v<_Tp, chrono::zoned_time>)
+    return true;
 #  endif
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
@@ -563,6 +585,8 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool __month_name_ok(const _Tp& __value) {
     return true;
   else if constexpr (same_as<_Tp, chrono::local_info>)
     return true;
+  else if constexpr (__is_specialization_v<_Tp, chrono::zoned_time>)
+    return true;
 #  endif
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
@@ -916,6 +940,19 @@ struct formatter<chrono::local_info, _CharT> : public __formatter_chrono<_CharT>
     return _Base::__parse(__ctx, __format_spec::__fields_chrono, __format_spec::__flags{});
   }
 };
+
+// Note due to how libc++'s formatters are implemented there is no need to add
+// exposition only local-time-format-t abstraction.
+template <class _Duration, class _TimeZonePtr, __fmt_char_type _CharT>
+struct formatter< chrono::zoned_time<_Duration, _TimeZonePtr>, _CharT> : public __formatter_chrono<_CharT> {
+public:
+  using _Base = __formatter_chrono<_CharT>;
+
+  template <class _ParseContext>
+  _LIBCPP_HIDE_FROM_ABI constexpr typename _ParseContext::iterator parse(_ParseContext& __ctx) {
+    return _Base::__parse(__ctx, __format_spec::__fields_chrono, __format_spec::__flags::__clock);
+  }
+};
 #  endif // !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB)
 
 #endif // if _LIBCPP_STD_VER >= 20

libcxx/include/__chrono/ostream.h

@@ -27,6 +27,7 @@
 #include <__chrono/year_month.h>
 #include <__chrono/year_month_day.h>
 #include <__chrono/year_month_weekday.h>
+#include <__chrono/zoned_time.h>
 #include <__concepts/same_as.h>
 #include <__config>
 #include <__format/format_functions.h>
@@ -302,6 +303,12 @@ operator<<(basic_ostream<_CharT, _Traits>& __os, const local_info& __info) {
              _LIBCPP_STATICALLY_WIDEN(_CharT, "{}: {{{}, {}}}"), __result(), __info.first, __info.second);
 }
 
+template <class _CharT, class _Traits, class _Duration, class _TimeZonePtr>
+_LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, const zoned_time<_Duration, _TimeZonePtr>& __tp) {
+  return __os << std::format(__os.getloc(), _LIBCPP_STATICALLY_WIDEN(_CharT, "{:L%F %T %Z}"), __tp);
+}
+
 #  endif // !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_TZDB)
 
 } // namespace chrono

libcxx/include/chrono

@@ -799,6 +799,11 @@ template<class Duration1, class Duration2, class TimeZonePtr>
   bool operator==(const zoned_time<Duration1, TimeZonePtr>& x,
                   const zoned_time<Duration2, TimeZonePtr>& y);
 
+template<class charT, class traits, class Duration, class TimeZonePtr>           // C++20
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os,
+               const zoned_time<Duration, TimeZonePtr>& t);
+
 // [time.zone.leap], leap second support
 class leap_second {                                                              // C++20
 public:
@@ -881,6 +886,8 @@ namespace std {
     struct formatter<chrono::hh_mm_ss<duration<Rep, Period>>, charT>;             // C++20
   template<class charT> struct formatter<chrono::sys_info, charT>;                // C++20
   template<class charT> struct formatter<chrono::local_info, charT>;              // C++20
+  template<class Duration, class TimeZonePtr, class charT>                        // C++20
+    struct formatter<chrono::zoned_time<Duration, TimeZonePtr>, charT>;
 } // namespace std
 
 namespace chrono {