<chrono>: Time zone Africa/Juba causes runtime_error in std::chrono::tzdb::current_zone

[lat9nq]

Describe the bug

A user can choose the time zone Africa/Juba in Windows 10 by selecting (UTC+02:00) Juba in the Date & Time options. This time zone has a path and name, however causes a runtime_error if used here.

Command-line test case

cmake_minimum_required(VERSION 3.12)

project(tzdb_test VERSION 1.0)

add_executable(tzdb_test
    main.cpp)

if (MSVC)
    target_compile_options(tzdb_test PRIVATE
        /std:c++20)
endif()

// main.cpp
#include <chrono>
#include <cstdio>
#include <stdexcept>
#include <string_view>

int main() {
  std::string location_name;
  try {
    const struct std::chrono::tzdb &time_zone_data = std::chrono::get_tzdb();
    const std::chrono::time_zone *current_zone = time_zone_data.current_zone();
    std::string_view current_zone_name = current_zone->name();
    location_name = current_zone_name;
  } catch (std::runtime_error &runtime_error) {
    location_name = std::string{"FAILURE: "} + runtime_error.what();
  }

  std::printf("%s\n", location_name.c_str());

  return 0;
}

PS C:\Temp> mkdir build
PS C:\Temp> cd build
PS C:\Temp\build> cmake ..
-- Building for: Visual Studio 17 2022
-- Selecting Windows SDK version 10.0.20348.0 to target Windows 10.0.19045.
-- The C compiler identification is MSVC 19.36.32532.0
-- The CXX compiler identification is MSVC 19.36.32532.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.36.32532/bin/Hostx64/x64/cl.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.36.32532/bin/Hostx64/x64/cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (6.9s)
-- Generating done (0.0s)
-- Build files have been written to: C:/Temp/build
PS C:\Temp\build> cmake --build . --config Release
MSBuild version 17.6.3+07e294721 for .NET Framework

  Checking Build System
  Building Custom Rule C:/Temp/CMakeLists.txt
  main.cpp
  tzdb_test.vcxproj -> C:\Temp\build\Release\tzdb_test.exe
  Building Custom Rule C:/Temp/CMakeLists.txt
PS C:\Temp\build> .\Release\tzdb_test.exe
FAILURE: unable to locate time_zone with given name

Expected behavior

It is expected to not receive a std::runtime_error, thus crash if it's not caught, when using these functions. Instead, this program should output the string Africa/Juba.

STL version

Visual Studio version on compile VM:

Microsoft Visual Studio Community 2022 (64-bit) - Current
Version 17.6.2

[achabense]

I try to set my timezone to (UTC+02:00) Juba and get the same error.

current_zone calls _Tzdb_generate_current_zone, which calls __std_tzdb_get_current_zone inside.

The following example shows that _Tzdb_generate_current_zone is generating "Etc/Unknown":

#include <chrono>
#include <iostream>
int main() {
    std::cout << std::chrono::_Tzdb_generate_current_zone();//"Etc/Unknown"
}

An emulation of __std_tzdb_get_current_zone shows that ucal_getDefaultTimeZone is generating "Etc/Unknown" for this time zone.

#include<iostream>
#include<icu.h>
#include<Windows.h>

int main() {
    const HMODULE _Icu_module = LoadLibraryExW(L"icu.dll", nullptr, LOAD_LIBRARY_SEARCH_SYSTEM32);
    if (_Icu_module != nullptr) {
        auto getter = (decltype(+ucal_getDefaultTimeZone))GetProcAddress(_Icu_module, "ucal_getDefaultTimeZone");
        UChar result[3000]{};
        UErrorCode ec;
        getter(result, 2500, &ec);
        std::wcout << reinterpret_cast<wchar_t*>(result);//"Etc/Unknown"
    }
}

Additional context:
zoned_time can produce correct local time for Africa/Juba:

#include <chrono>
#include <iostream>

using namespace std::chrono;
int main() {
    zoned_time zt("Africa/Juba", system_clock::now());
    std::cout << zt.get_time_zone()->name() << ":" << zt; //Africa/Juba:2023-07-09 19:43:35.7872691 GMT+2 (correct when testing)
}

[cpplearner]

MSVC STL uses ICU to get time zone information. ICU uses CLDR to map Windows time zones to IANA time zones. CLDR added mapping for "Africa/Juba" in 2021 (unicode-org/cldr@85885ed), but it seems that Windows doesn't contain this update 😕

[StephanTLavavej]

Thanks for reporting this issue. As @cpplearner noted, Windows needs to update its copy of ICU before this will start working. MSVC's STL absolutely relies on ICU (via Windows) for time zone information as this is the only way for us to obtain the info and have it updated (in situations exactly like this one!) without requiring users to rebuild and reship their apps that are using the STL. As a result, we're closing this issue as external.