code: public void warning(int pos, Warning warningKey) {
          report(diags.warning(null, source, wrap(pos), warningKey));
      }
docs: Report a warning, unless suppressed by the  -nowarn option or the
  maximum number of warnings has been reached.
  @param pos    The source position at which to report the warning.
  @param warningKey    The key for the localized warning message.