Fix formatting of timezone names

Author: vitaut

include/fmt/chrono.h

@@ -991,16 +991,28 @@ inline auto tm_mon_short_name(int mon) -> const char* {
 }
 
 template <typename T, typename = void>
-struct has_member_data_tm_gmtoff : std::false_type {};
+struct has_tm_gmtoff : std::false_type {};
 template <typename T>
-struct has_member_data_tm_gmtoff<T, void_t<decltype(T::tm_gmtoff)>>
-    : std::true_type {};
+struct has_tm_gmtoff<T, void_t<decltype(T::tm_gmtoff)>> : std::true_type {};
 
-template <typename T, typename = void>
-struct has_member_data_tm_zone : std::false_type {};
+template <typename T, typename = void> struct has_tm_zone : std::false_type {};
 template <typename T>
-struct has_member_data_tm_zone<T, void_t<decltype(T::tm_zone)>>
-    : std::true_type {};
+struct has_tm_zone<T, void_t<decltype(T::tm_zone)>> : std::true_type {};
+
+template <typename T, FMT_ENABLE_IF(has_tm_zone<T>::value)>
+bool set_tm_zone(T& time, char* tz) {
+  time.tm_zone = tz;
+  return true;
+}
+template <typename T, FMT_ENABLE_IF(!has_tm_zone<T>::value)>
+bool set_tm_zone(T&, char*) {
+  return false;
+}
+
+inline char* utc() {
+  static char tz[] = "UTC";
+  return tz;
+}
 
 // Converts value to Int and checks that it's in the range [0, upper).
 template <typename T, typename Int, FMT_ENABLE_IF(std::is_integral<T>::value)>
@@ -1260,24 +1272,24 @@ class tm_writer {
     write2(static_cast<int>(offset % 60));
   }
 
-  template <typename T, FMT_ENABLE_IF(has_member_data_tm_gmtoff<T>::value)>
-  void format_utc_offset_impl(const T& tm, numeric_system ns) {
+  template <typename T, FMT_ENABLE_IF(has_tm_gmtoff<T>::value)>
+  void format_utc_offset(const T& tm, numeric_system ns) {
     write_utc_offset(tm.tm_gmtoff, ns);
   }
-  template <typename T, FMT_ENABLE_IF(!has_member_data_tm_gmtoff<T>::value)>
-  void format_utc_offset_impl(const T&, numeric_system ns) {
+  template <typename T, FMT_ENABLE_IF(!has_tm_gmtoff<T>::value)>
+  void format_utc_offset(const T&, numeric_system ns) {
     write_utc_offset(0, ns);
   }
 
-  template <typename T, FMT_ENABLE_IF(has_member_data_tm_zone<T>::value)>
-  void format_tz_name_impl(const T& tm) {
+  template <typename T, FMT_ENABLE_IF(has_tm_zone<T>::value)>
+  void format_tz_name(const T& tm) {
     if (is_classic_)
       out_ = write_tm_str<Char>(out_, tm.tm_zone, loc_);
     else
       format_localized('Z');
   }
-  template <typename T, FMT_ENABLE_IF(!has_member_data_tm_zone<T>::value)>
-  void format_tz_name_impl(const T&) {
+  template <typename T, FMT_ENABLE_IF(!has_tm_zone<T>::value)>
+  void format_tz_name(const T&) {
     format_localized('Z');
   }
 
@@ -1389,8 +1401,8 @@ class tm_writer {
     out_ = copy<Char>(std::begin(buf) + offset, std::end(buf), out_);
   }
 
-  void on_utc_offset(numeric_system ns) { format_utc_offset_impl(tm_, ns); }
-  void on_tz_name() { format_tz_name_impl(tm_); }
+  void on_utc_offset(numeric_system ns) { format_utc_offset(tm_, ns); }
+  void on_tz_name() { format_tz_name(tm_); }
 
   void on_year(numeric_system ns, pad_type pad) {
     if (is_classic_ || ns == numeric_system::standard)
@@ -2236,7 +2248,7 @@ template <typename Char> struct formatter<std::tm, Char> {
 
  public:
   FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) -> const Char* {
-    return do_parse(ctx, detail::has_member_data_tm_gmtoff<std::tm>::value);
+    return do_parse(ctx, detail::has_tm_gmtoff<std::tm>::value);
   }
 
   template <typename FormatContext>
@@ -2261,17 +2273,20 @@ struct formatter<sys_time<Duration>, Char> : private formatter<std::tm, Char> {
     if (detail::const_check(
             period::num == 1 && period::den == 1 &&
             !std::is_floating_point<typename Duration::rep>::value)) {
+      detail::set_tm_zone(tm, detail::utc());
       return formatter<std::tm, Char>::format(tm, ctx);
     }
     Duration epoch = val.time_since_epoch();
     Duration subsecs = detail::duration_cast<Duration>(
         epoch - detail::duration_cast<std::chrono::seconds>(epoch));
     if (subsecs.count() < 0) {
       auto second = detail::duration_cast<Duration>(std::chrono::seconds(1));
-      if (tm.tm_sec != 0)
+      if (tm.tm_sec != 0) {
         --tm.tm_sec;
-      else
+      } else {
         tm = gmtime(val - second);
+        detail::set_tm_zone(tm, detail::utc());
+      }
       subsecs += second;
     }
     return formatter<std::tm, Char>::do_format(tm, ctx, &subsecs);

test/chrono-test.cc

@@ -335,30 +335,16 @@ TEST(chrono_test, local_time) {
                    fmt::format_error, "no timezone");
 }
 
-template <typename T,
-          FMT_ENABLE_IF(fmt::detail::has_member_data_tm_gmtoff<T>::value)>
+template <typename T, FMT_ENABLE_IF(fmt::detail::has_tm_gmtoff<T>::value)>
 bool set_tm_gmtoff(T& time, long offset) {
   time.tm_gmtoff = offset;
   return true;
 }
-template <typename T,
-          FMT_ENABLE_IF(!fmt::detail::has_member_data_tm_gmtoff<T>::value)>
+template <typename T, FMT_ENABLE_IF(!fmt::detail::has_tm_gmtoff<T>::value)>
 bool set_tm_gmtoff(T&, long) {
   return false;
 }
 
-template <typename T,
-          FMT_ENABLE_IF(fmt::detail::has_member_data_tm_zone<T>::value)>
-bool set_tm_zone(T& time, char* tz) {
-  time.tm_zone = tz;
-  return true;
-}
-template <typename T,
-          FMT_ENABLE_IF(!fmt::detail::has_member_data_tm_zone<T>::value)>
-bool set_tm_zone(T&, char*) {
-  return false;
-}
-
 TEST(chrono_test, tm) {
   auto time = fmt::gmtime(290088000);
   test_time(time);
@@ -371,7 +357,7 @@ TEST(chrono_test, tm) {
                      fmt::format_error, "no timezone");
   }
   char tz[] = "EET";
-  if (set_tm_zone(time, tz)) {
+  if (fmt::detail::set_tm_zone(time, tz)) {
     EXPECT_EQ(fmt::format(fmt::runtime("{:%Z}"), time), "EET");
   } else {
     EXPECT_THROW_MSG((void)fmt::format(fmt::runtime("{:%Z}"), time),