[jb1991]

You are saying that the standard specifies that the standard file system features themselves do not work?

[skitter]

If another program (or thread) is using the same filesystem, calling std::filesystem functions can be UB.

> Behavior is undefined if calls to functions provided by subclause [filesystems] introduce a file system race.

http://eel.is/c++draft/fs.race.behavior#1.sentence-2