bool exists(file_status s) noexcept;
Returns: status_known(s) && s.type() != file_type::not_found.
bool exists(const path& p); bool exists(const path& p, error_code& ec) noexcept;
Let s be a file_status, determined as if by status(p) or status(p, ec), respectively.
Effects: The signature with argument ec calls ec.clear() if status_known(s).
Returns: exists(s).
Throws: As specified in [fs.err.report].