Should `std::error_code::message` respect the locale set by the user?

In our program, we use std::setlocale(LC_ALL, ".UTF8"); to support UTF-8 encoding, as noted in the Microsoft documentation. However, when certain C++ STL functions fail, their message() outputs are not in UTF-8.

After investigating the source code, I found that the error messages are always formatted according to the system's default locale, as shown in this STL implementation:
https://github.com/microsoft/STL/blob/e36ee6c2b9bc6f5b1f70776c18cf5d3a93a69798/stl/src/syserror_import_lib.cpp#L38-L54

While it would be acceptable if the system locale used UTF-8 as the codepage, allowing the messages to be correctly printed to log files, this is not the case. There is a "Beta: Use Unicode UTF-8 for worldwide language support" setting in the Region settings, which does the job correctly, but toggling that setting requires a reboot, which is not acceptable for our requirements.

Question

Should the std::error_code::message function respect the locale set by the user (e.g., via std::setlocale), or should it continue to use the system's default locale? If the former, is there a plan to implement this behavior in the Microsoft STL implementation?

Related Information

• I learned that the locale is by default set across the whole program, unless specified by configthreadlocale, from the Microsoft documentation.
• The issue of system_error not honoring the current thread's locale is mentioned in this issue.

Please let me know if you need any additional information or clarification.

Read stl/src/syserror_import_lib.cpp at the linked lines and review related issue #2882 first. Done means reaching a maintainer-backed decision on whether std::error_code::message should use the user or thread locale, with the implementation plan or rationale recorded.