From 5f2e61fdd5eb10695e4535cede6e2631f174d6f8 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Sun, 6 Apr 2025 09:54:47 -0700
Subject: [PATCH] Cleanup chrono detail

---
 include/fmt/chrono.h | 40 ++++++++++++++++------------------------
 1 file changed, 16 insertions(+), 24 deletions(-)

diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index c3a697d6..8024ddf5 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -505,26 +505,28 @@ auto to_time_t(sys_time<Duration> time_point) -> std::time_t {
 }
 
 namespace tz {
+
+// DEPRECATED!
 struct time_zone {
-  template <typename Duration, typename T>
-  auto to_sys(T)
-      -> std::chrono::time_point<std::chrono::system_clock, Duration> {
+  template <typename Duration, typename LocalTime>
+  auto to_sys(LocalTime) -> sys_time<Duration> {
     return {};
   }
 };
-template <typename... T> inline auto current_zone(T...) -> time_zone* {
+template <typename... T> auto current_zone(T...) -> time_zone* {
   return nullptr;
 }
 
-template <typename... T> inline void _tzset(T...) {}
+template <typename... T> void _tzset(T...) {}
 }  // namespace tz
 
-// Workaround a bug in libstdc++ which sets __cpp_lib_chrono to 201907 without
-// providing current_zone(): https://github.com/fmtlib/fmt/issues/4160.
-template <typename T> FMT_CONSTEXPR auto has_current_zone() -> bool {
-  using namespace std::chrono;
-  using namespace tz;
-  return !std::is_same<decltype(current_zone()), tz::time_zone*>::value;
+inline void tzset_once() {
+  static bool init = []() {
+    using namespace tz;
+    _tzset();
+    return false;
+  }();
+  ignore_unused(init);
 }
 }  // namespace detail
 
@@ -535,7 +537,7 @@ FMT_BEGIN_EXPORT
  * expressed in local time. Unlike `std::localtime`, this function is
  * thread-safe on most platforms.
  */
-inline auto localtime(std::time_t time) -> std::tm {
+FMT_DEPRECATED inline auto localtime(std::time_t time) -> std::tm {
   struct dispatcher {
     std::time_t time_;
     std::tm tm_;
@@ -572,9 +574,8 @@ inline auto localtime(std::time_t time) -> std::tm {
 }
 
 #if FMT_USE_LOCAL_TIME
-template <typename Duration,
-          FMT_ENABLE_IF(detail::has_current_zone<Duration>())>
-FMT_DEPRECATED inline auto localtime(std::chrono::local_time<Duration> time)
+template <typename Duration>
+FMT_DEPRECATED auto localtime(std::chrono::local_time<Duration> time)
     -> std::tm {
   using namespace std::chrono;
   using namespace detail::tz;
@@ -1003,15 +1004,6 @@ template <typename T>
 struct has_member_data_tm_zone<T, void_t<decltype(T::tm_zone)>>
     : std::true_type {};
 
-inline void tzset_once() {
-  static bool init = []() {
-    using namespace tz;
-    _tzset();
-    return false;
-  }();
-  ignore_unused(init);
-}
-
 // 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)>
 inline auto to_nonnegative_int(T value, Int upper) -> Int {