<filesystem>: prevent filesystem::path dangerous conversions to/from default code page encoding

Hello.

I would like to ask for a way to disable implicit conversions of const char* to std::filesystem::path using the default 8bit code page encoding and vice versa. If we had to switch from boost::filesystem to std::filesystem, these innocently looking conversions would be a major mine field to us. Indeed, we have had the same problem for years in our product using the C++ multi platform library wxWidgets, where the wxString class behaves the same way on Windows as now the std::filesystem::path in regard to default conversion to/from const char*, and the team members have to be mindful to not trigger these harmful conversions.

I understand the decision to use 16bit characters on Windows for performance reasons. What I do not understand is the decision to convert to / from a local code page by default in the year 2020. It would be great if one had a way to switch the default conversion to UTF-8, either in runtime or by a preprocessor macro. AFAIK the only way to convince std::filesystem to convert to/from UTF-8 by default is to switch the whole Windows 10 to an experimental UTF-8 code page, which is not an option for a production code really, which shall run on Windows 7 and older Windows 10.

I also understand that it is difficult to change the std::filesystem API now as it has already been standardized. I would therefore like to ask for some way to trigger compiler warnings or errors when these default from / to local code page conversions are triggered. Without that we would most likely not switch from boost::filesystem to std::filesystem, as such move would be too risky.

Thank you for consideration,
Vojtech Bubnik, Prusa Research

Start by reviewing the std::filesystem::path conversion APIs on Windows and the issue discussion about default code-page conversions. The issue does not name files or tests; completion would require a maintainer-approved way to diagnose, disable, or change these implicit conversions.