Fix timezone handling in tm

Author: vitaut

include/fmt/chrono.h

@@ -420,14 +420,11 @@ auto write(OutputIt out, const std::tm& time, const std::locale& loc,
   return write_encoded_tm_str(out, string_view(buf.data(), buf.size()), loc);
 }
 
-template <typename Rep1, typename Rep2>
-struct is_same_arithmetic_type
-    : public std::integral_constant<bool,
-                                    (std::is_integral<Rep1>::value &&
-                                     std::is_integral<Rep2>::value) ||
-                                        (std::is_floating_point<Rep1>::value &&
-                                         std::is_floating_point<Rep2>::value)> {
-};
+template <typename T, typename U>
+using is_similar_arithmetic_type =
+    bool_constant<(std::is_integral<T>::value && std::is_integral<U>::value) ||
+                  (std::is_floating_point<T>::value &&
+                   std::is_floating_point<U>::value)>;
 
 FMT_NORETURN inline void throw_duration_error() {
   FMT_THROW(format_error("cannot format duration"));
@@ -486,9 +483,9 @@ auto duration_cast(std::chrono::duration<FromRep, FromPeriod> from) -> To {
 #endif
 }
 
-template <
-    typename To, typename FromRep, typename FromPeriod,
-    FMT_ENABLE_IF(!is_same_arithmetic_type<FromRep, typename To::rep>::value)>
+template <typename To, typename FromRep, typename FromPeriod,
+          FMT_ENABLE_IF(
+              !is_similar_arithmetic_type<FromRep, typename To::rep>::value)>
 auto duration_cast(std::chrono::duration<FromRep, FromPeriod> from) -> To {
   // Mixed integer <-> float cast is not supported by safe_duration_cast.
   return std::chrono::duration_cast<To>(from);
@@ -520,6 +517,7 @@ template <typename... T> auto current_zone(T...) -> time_zone* {
 template <typename... T> void _tzset(T...) {}
 }  // namespace tz
 
+// DEPRECATED!
 inline void tzset_once() {
   static bool init = []() {
     using namespace tz;
@@ -2227,8 +2225,10 @@ template <typename Char> struct formatter<std::tm, Char> {
         ctx.out(), basic_string_view<Char>(buf.data(), buf.size()), specs);
   }
 
-  FMT_CONSTEXPR auto do_parse(parse_context<Char>& ctx,
-                              bool no_timezone = false) -> const Char* {
+  FMT_CONSTEXPR auto do_parse(
+      parse_context<Char>& ctx,
+      bool no_timezone = !detail::has_member_data_tm_gmtoff<std::tm>::value)
+      -> const Char* {
     auto it = ctx.begin(), end = ctx.end();
     if (it == end || *it == '}') return it;
 

test/chrono-test.cc

@@ -16,7 +16,6 @@
 
 using fmt::runtime;
 using fmt::sys_time;
-using fmt::sys_time;
 using testing::Contains;
 
 #if defined(__MINGW32__) && !defined(_UCRT)
@@ -336,9 +335,31 @@ 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)>
+bool set_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)>
+bool set_gmtoff(T&, long) {
+  return false;
+}
+
 TEST(chrono_test, tm) {
   auto time = fmt::gmtime(290088000);
   test_time(time);
+  if (set_gmtoff(time, -28800)) {
+    EXPECT_EQ(fmt::format(fmt::runtime("{:%z}"), time), "-0800");
+    EXPECT_EQ(fmt::format(fmt::runtime("{:%Ez}"), time), "-08:00");
+    EXPECT_EQ(fmt::format(fmt::runtime("{:%Oz}"), time), "-08:00");
+  } else {
+    EXPECT_THROW_MSG((void)fmt::format(fmt::runtime("{:%z}"), time),
+                     fmt::format_error, "no timezone");
+    EXPECT_THROW_MSG((void)fmt::format(fmt::runtime("{:%Z}"), time),
+                     fmt::format_error, "no timezone");
+  }
 }
 
 TEST(chrono_test, daylight_savings_time_end) {