feat(clock): support chrono Time Zone extensions.

Use chrono Calendars and Time Zones (P0355R7, P1466R3) when available
instead of the `date` library.
Verified with a patched build of a recent GCC 13 snapshot.

include/factory.hpp

@@ -1,7 +1,7 @@
 #pragma once
 
 #include <json/json.h>
-#ifdef HAVE_LIBDATE
+#if defined(HAVE_CHRONO_TIMEZONES) || defined(HAVE_LIBDATE)
 #include "modules/clock.hpp"
 #else
 #include "modules/simpleclock.hpp"

include/modules/clock.hpp

@@ -1,8 +1,7 @@
 #pragma once
 
-#include <date/tz.h>
-
 #include "ALabel.hpp"
+#include "util/date.hpp"
 #include "util/sleeper_thread.hpp"
 
 namespace waybar::modules {

include/util/date.hpp

@@ -1,8 +1,34 @@
 #pragma once
 
-#include <date/tz.h>
 #include <fmt/format.h>
 
+#if HAVE_CHRONO_TIMEZONES
+#include <chrono>
+#include <format>
+
+/* Compatibility layer for <date/tz.h> on top of C++20 <chrono> */
+namespace date {
+
+using namespace std::chrono;
+
+namespace literals {
+using std::chrono::last;
+}
+
+inline auto format(const std::string& spec, const auto& ztime) {
+  return spec.empty() ? "" : std::vformat("{:L" + spec + "}", std::make_format_args(ztime));
+}
+
+inline auto format(const std::locale& loc, const std::string& spec, const auto& ztime) {
+  return spec.empty() ? "" : std::vformat(loc, "{:L" + spec + "}", std::make_format_args(ztime));
+}
+
+}  // namespace date
+
+#else
+#include <date/tz.h>
+#endif
+
 template <typename Duration, typename TimeZonePtr>
 struct fmt::formatter<date::zoned_time<Duration, TimeZonePtr>> {
   std::string_view specs;

meson.build

@@ -123,11 +123,18 @@ gtk_layer_shell = dependency('gtk-layer-shell-0',
         required: get_option('gtk-layer-shell'),
         fallback : ['gtk-layer-shell', 'gtk_layer_shell_dep'])
 systemd = dependency('systemd', required: get_option('systemd'))
-tz_dep = dependency('date',
-    required: false,
-    default_options : [ 'use_system_tzdb=true' ],
-    modules : [ 'date::date', 'date::date-tz' ],
-    fallback: [ 'date', 'tz_dep' ])
+
+cpp_lib_chrono = compiler.compute_int('__cpp_lib_chrono', prefix : '#include <chrono>')
+have_chrono_timezones = cpp_lib_chrono >= 201907
+if have_chrono_timezones
+  tz_dep = declare_dependency()
+else
+  tz_dep = dependency('date',
+      required: false,
+      default_options : [ 'use_system_tzdb=true' ],
+      modules : [ 'date::date', 'date::date-tz' ],
+      fallback: [ 'date', 'tz_dep' ])
+endif
 
 prefix = get_option('prefix')
 sysconfdir = get_option('sysconfdir')
@@ -312,7 +319,10 @@ if get_option('rfkill').enabled() and is_linux
     )
 endif
 
-if tz_dep.found()
+if have_chrono_timezones
+    add_project_arguments('-DHAVE_CHRONO_TIMEZONES', language: 'cpp')
+    src_files += 'src/modules/clock.cpp'
+elif tz_dep.found()
     add_project_arguments('-DHAVE_LIBDATE', language: 'cpp')
     src_files += 'src/modules/clock.cpp'
 else

src/modules/clock.cpp

@@ -9,7 +9,6 @@
 #include <sstream>
 #include <type_traits>
 
-#include "util/date.hpp"
 #include "util/ustring_clen.hpp"
 #ifdef HAVE_LANGINFO_1STDAY
 #include <langinfo.h>