beast: Can not read file name with umlauts in boost::beast::http::file_body

Thanks again for the very cool library, first and foremost! We have a minor issue with umlauts (low priority).

Version of Beast

#define BOOST_BEAST_VERSION 290

Steps necessary to reproduce the problem

My code is based on the advanced_server_flex.cpp example, and my problem appears specifically around line https://github.com/boostorg/beast/blob/develop/example/advanced/server-flex/advanced_server_flex.cpp#L179:

    // Attempt to open the file
    beast::error_code ec;
    http::file_body::value_type body;
    body.open(path.c_str(), beast::file_mode::scan, ec);

    // Handle the case where the file doesn't exist
    if(ec == beast::errc::no_such_file_or_directory)  //< This is true for us when the file name has umlauts.

We used this code successfully on multiple platforms (MSVC, Ubuntu and MacOSX) for about a year now. However as soon as we started adding clang-cl to the list of compilers, the webserver can no longer read files that have Umlauts in the file name.

We tested with a file named

üöälaut.txt

that is not found on Windows when using clang-cl, whereas all other platforms/compilers work fine.

All relevant compiler information

The error comes with Visual Studio 2019 x64 16.7.1 with the Microsoft-supplied clang-cl version 10.0.0. Using the Microsoft-Compiler and not clang-cl, everything works.