I have the following code snipped, which produces the following output on mingw(gcc) and msvc:
std::cout << std::filesystem::relative("D:/home/freddy", "C:/home") << std::endl;
mingw(gcc):
"\\home\\freddy"
msvc:
""
gcc and also clang (both executed on linux):
"../../D:/home/freddy"
In my opinion it should be "" or "../../D:/home/freddy", since there is no relative path over root_paths, but not the relative path of an erased root path.
Additionally MSVC will fail, when the drive does not exist, because the drive is "not ready yet":
weakly_canonical: Das Gerõt ist nicht bereit.: "D:/home/freddy"
Which also seems like a MSVC bug in std::filesystem::weakly_canonical