_wfopen support

[Nekto89]

On Windows it is important to use UTF-16 file paths for opening files (at least until recent versions that allow using UTF-8 locale for WinAPI).

std::fstream has overload for std::wstring
for std::FILE* there is Windows-specific "_wfopen" method as a replacement for standard std::fopen.

Is it possible to provide a way to use fmt::output_file with UTF-16 encoded filepath?

[vitaut]

Supporting UTF-16 in a portable way is problematic but we could replace sopen_s with _wopen in

https://github.com/fmtlib/fmt/blob/master/src/os.cc#L224

This will allow passing UTF-8 paths. A PR to do this would be welcome.

[nenin-sc]

This will allow passing UTF-8 paths.

Is it possible just transfer a handle to already opened file?

[vitaut]

Is it possible just transfer a handle to already opened file?

It is not possible with the current API.

[Fros1er]

I'd like to work on this.
My current idea is adding a new ctor file(wcstring_view path, int oflag);, meanwhile cast UTF-8 path to UTF-16 in original ctor.
I wonder if it's a proper way to deal with the problem.

[vitaut]

Converting UTF-8 to UTF-16 and invoking _wopen on Windows makes sense but I don't think we should be adding a new ctor.

[Fros1er]

Converting UTF-8 to UTF-16 and invoking _wopen on Windows makes sense but I don't think we should be adding a new ctor.

Maybe sometimes only wchar_t * is available. Converting from UTF-16 to UTF-8 then convert it back seems a little weird.

Not sure if this is a common situation. I'll just start with no extra ctor.

[vitaut]

I think it would be OK to add a static "factory" function with the name that indicates that it is Windows-specific, e.g. open_windows_file, that takes a wide filename and returns file.