[fs.op.is.directory]
31 Input/output library [input.output]
31.12 File systems [filesystems]
31.12.13 Filesystem operation functions [fs.op.funcs]
31.12.13.19 Is directory [fs.op.is.directory]
bool filesystem::is_directory(file_status s) noexcept;
Returns: s.type() == file_type::directory.
bool filesystem::is_directory(const path& p);
bool filesystem::is_directory(const path& p, error_code& ec) noexcept;