void resize_file(const path& p, uintmax_t new_size); void resize_file(const path& p, uintmax_t new_size, error_code& ec) noexcept;
Postconditions: file_size(p) == new_size.
Throws: As specified in [fs.err.report].
Remarks: Achieves its postconditions as if by POSIX truncate().