[libc++][TZDB] Improves time zone format specifiers. (#85797)

Per [tab:time.format.spec]
%z  The offset from UTC as specified in ISO 8601-1:2019, subclause
    5.3.4.1. For example -0430 refers to 4 hours 30 minutes behind UTC.
    If the offset is zero, +0000 is used. The modified commands %Ez and
    %Oz insert a : between the hours and minutes: -04:30. If the offset
    information is not available, an exception of type format_error is
    thrown.

Typically the modified versions Oz or Ez would have wording like

  The modified command %OS produces the locale's alternative
  representation.

In this case the modified version does not depend on the locale.

This change is a preparation for formatting sys_info which has time zone
information. The function time_put<_CharT>::put() does not have proper
time zone support, therefore it's a manual implementation.

Fixes #78184

Author: mordante

libcxx/include/__chrono/formatter.h

@@ -10,6 +10,7 @@
 #ifndef _LIBCPP___CHRONO_FORMATTER_H
 #define _LIBCPP___CHRONO_FORMATTER_H
 
+#include <__algorithm/ranges_copy.h>
 #include <__chrono/calendar.h>
 #include <__chrono/concepts.h>
 #include <__chrono/convert_to_tm.h>
@@ -170,10 +171,45 @@ _LIBCPP_HIDE_FROM_ABI void __format_century(basic_stringstream<_CharT>& __sstr,
   __sstr << std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:02}"), __century);
 }
 
+// Implements the %z format specifier according to [tab:time.format.spec], where
+// '__modifier' signals %Oz or %Ez were used. (Both modifiers behave the same,
+// so there is no need to distinguish between them.)
+template <class _CharT>
+_LIBCPP_HIDE_FROM_ABI void
+__format_zone_offset(basic_stringstream<_CharT>& __sstr, chrono::seconds __offset, bool __modifier) {
+  if (__offset < 0s) {
+    __sstr << _CharT('-');
+    __offset = -__offset;
+  } else {
+    __sstr << _CharT('+');
+  }
+
+  chrono::hh_mm_ss __hms{__offset};
+  std::ostreambuf_iterator<_CharT> __out_it{__sstr};
+  if (__modifier)
+    std::format_to(__out_it, _LIBCPP_STATICALLY_WIDEN(_CharT, "{:%H:%M}"), __hms);
+  else
+    std::format_to(__out_it, _LIBCPP_STATICALLY_WIDEN(_CharT, "{:%H%M}"), __hms);
+}
+
+// Helper to store the time zone information needed for formatting.
+struct _LIBCPP_HIDE_FROM_ABI __time_zone {
+  // Typically these abbreviations are short and fit in the string's internal
+  // buffer.
+  string __abbrev;
+  chrono::seconds __offset;
+};
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI __time_zone __convert_to_time_zone([[maybe_unused]] const _Tp& __value) {
+  return {"UTC", chrono::seconds{0}};
+}
+
 template <class _CharT, class _Tp>
 _LIBCPP_HIDE_FROM_ABI void __format_chrono_using_chrono_specs(
     basic_stringstream<_CharT>& __sstr, const _Tp& __value, basic_string_view<_CharT> __chrono_specs) {
   tm __t              = std::__convert_to_tm<tm>(__value);
+  __time_zone __z     = __formatter::__convert_to_time_zone(__value);
   const auto& __facet = std::use_facet<time_put<_CharT>>(__sstr.getloc());
   for (auto __it = __chrono_specs.begin(); __it != __chrono_specs.end(); ++__it) {
     if (*__it == _CharT('%')) {
@@ -296,9 +332,13 @@ _LIBCPP_HIDE_FROM_ABI void __format_chrono_using_chrono_specs(
               {__sstr}, __sstr, _CharT(' '), std::addressof(__t), std::to_address(__s), std::to_address(__it + 1));
       } break;
 
+      case _CharT('z'):
+        __formatter::__format_zone_offset(__sstr, __z.__offset, false);
+        break;
+
       case _CharT('Z'):
-        // TODO FMT Add proper timezone support.
-        __sstr << _LIBCPP_STATICALLY_WIDEN(_CharT, "UTC");
+        // __abbrev is always a char so the copy may convert.
+        ranges::copy(__z.__abbrev, std::ostreambuf_iterator<_CharT>{__sstr});
         break;
 
       case _CharT('O'):
@@ -314,9 +354,15 @@ _LIBCPP_HIDE_FROM_ABI void __format_chrono_using_chrono_specs(
             break;
           }
         }
+
+        // Oz produces the same output as Ez below.
         [[fallthrough]];
       case _CharT('E'):
         ++__it;
+        if (*__it == 'z') {
+          __formatter::__format_zone_offset(__sstr, __z.__offset, true);
+          break;
+        }
         [[fallthrough]];
       default:
         __facet.put(

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

@@ -904,12 +904,6 @@ static void test_valid_values_date_time() {
 
 template <class CharT>
 static void test_valid_values_time_zone() {
-// The Apple CI gives %z='-0700'	%Ez='-0700'	%Oz='-0700'	%Z='UTC'
-// -0700 looks like the local time where the CI happens to reside, therefore
-// omit this test on Apple.
-// The Windows CI gives %z='-0000', but on local machines set to a different
-// timezone, it gives e.g. %z='+0200'.
-#if !defined(__APPLE__) && !defined(_WIN32)
   using namespace std::literals::chrono_literals;
 
   constexpr std::basic_string_view<CharT> fmt  = SV("{:%%z='%z'%t%%Ez='%Ez'%t%%Oz='%Oz'%t%%Z='%Z'%n}");
@@ -918,48 +912,23 @@ static void test_valid_values_time_zone() {
   const std::locale loc(LOCALE_ja_JP_UTF_8);
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
 
-#  if defined(_AIX)
   // Non localized output using C-locale
-  check(SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
+  check(SV("%z='+0000'\t%Ez='+00:00'\t%Oz='+00:00'\t%Z='UTC'\n"),
         fmt,
         file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 
   // Use the global locale (fr_FR)
-  check(SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
+  check(SV("%z='+0000'\t%Ez='+00:00'\t%Oz='+00:00'\t%Z='UTC'\n"),
         lfmt,
         file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 
-  // Use supplied locale (ja_JP). This locale has a different alternate.a
+  // Use supplied locale (ja_JP).
   check(loc,
-        SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
-        lfmt,
-        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-#  else                    // defined(_AIX)
-  // Non localized output using C-locale
-  check(SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
-        fmt,
-        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-
-  // Use the global locale (fr_FR)
-  check(SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
+        SV("%z='+0000'\t%Ez='+00:00'\t%Oz='+00:00'\t%Z='UTC'\n"),
         lfmt,
         file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 
-  // Use supplied locale (ja_JP). This locale has a different alternate.a
-#    if defined(__FreeBSD__)
-  check(loc,
-        SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
-        lfmt,
-        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-#    else
-  check(loc,
-        SV("%z='+0000'\t%Ez='+0000'\t%Oz='+〇'\t%Z='UTC'\n"),
-        lfmt,
-        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-#    endif
-#  endif                   // defined(_AIX)
   std::locale::global(std::locale::classic());
-#endif // !defined(__APPLE__) && !defined(_WIN32)
 }
 
 template <class CharT>

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

@@ -900,12 +900,6 @@ static void test_valid_values_date_time() {
 
 template <class CharT>
 static void test_valid_values_time_zone() {
-// The Apple CI gives %z='-0700'	%Ez='-0700'	%Oz='-0700'	%Z='UTC'
-// -0700 looks like the local time where the CI happens to reside, therefore
-// omit this test on Apple.
-// The Windows CI gives %z='-0000', but on local machines set to a different
-// timezone, it gives e.g. %z='+0200'.
-#if !defined(__APPLE__) && !defined(_WIN32)
   using namespace std::literals::chrono_literals;
 
   constexpr std::basic_string_view<CharT> fmt  = SV("{:%%z='%z'%t%%Ez='%Ez'%t%%Oz='%Oz'%t%%Z='%Z'%n}");
@@ -914,48 +908,23 @@ static void test_valid_values_time_zone() {
   const std::locale loc(LOCALE_ja_JP_UTF_8);
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
 
-#  if defined(_AIX)
   // Non localized output using C-locale
-  check(SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
+  check(SV("%z='+0000'\t%Ez='+00:00'\t%Oz='+00:00'\t%Z='UTC'\n"),
         fmt,
         std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 
   // Use the global locale (fr_FR)
-  check(SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
+  check(SV("%z='+0000'\t%Ez='+00:00'\t%Oz='+00:00'\t%Z='UTC'\n"),
         lfmt,
         std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 
-  // Use supplied locale (ja_JP). This locale has a different alternate.a
+  // Use supplied locale (ja_JP).
   check(loc,
-        SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
-        lfmt,
-        std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-#  else                                // defined(_AIX)
-  // Non localized output using C-locale
-  check(SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
-        fmt,
-        std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-
-  // Use the global locale (fr_FR)
-  check(SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
+        SV("%z='+0000'\t%Ez='+00:00'\t%Oz='+00:00'\t%Z='UTC'\n"),
         lfmt,
         std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 
-  // Use supplied locale (ja_JP). This locale has a different alternate.a
-#    if defined(__FreeBSD__)
-  check(loc,
-        SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
-        lfmt,
-        std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-#    else
-  check(loc,
-        SV("%z='+0000'\t%Ez='+0000'\t%Oz='+〇'\t%Z='UTC'\n"),
-        lfmt,
-        std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
-#    endif
-#  endif                               // defined(_AIX)
   std::locale::global(std::locale::classic());
-#endif // !defined(__APPLE__) && !defined(_WIN32)
 }
 
 template <class CharT>