Unable to build with clang-cl

[Neumann-A]

/// <returns>0 if the read request is still outstanding, -1 if the request failed, otherwise the size of the data read
/// into the buffer</returns>
size_t _read_file_async(.....

auto res =_read_file_async();
switch(res) {
case(-1): <- clang-cl will rightfully reject this since -1 is never returned 
}