message: core: emit warning ONCE on ambiguous MatExpr processing
diff: // */
  #include "precomp.hpp"
  +#include <opencv2/core/utils/logger.hpp>
  namespace cv
  {
  @@ -1319,10 +1320,18 @@ void MatOp_AddEx::assign(const MatExpr& e, Mat& m, int _type) const
  cv::add(dst, e.s, dst);
  }
  else
  + {
  + if (e.a.channels() > 1)
  + CV_LOG_ONCE_WARNING(NULL, "OpenCV/MatExpr: processing of multi-channel arrays might be changed in the future: "
  + "https://github.com/opencv/opencv/issues/16739");
  cv::addWeighted(e.a, e.alpha, e.b, e.beta, e.s[0], dst);
  }
  + }
  else if( e.s.isReal() && (dst.data != m.data || fabs(e.alpha) != 1))
  {
  + if (e.a.channels() > 1)
  + CV_LOG_ONCE_WARNING(NULL, "OpenCV/MatExpr: processing of multi-channel arrays might be changed in the future: "
  + "https://github.com/opencv/opencv/issues/16739");
  e.a.convertTo(m, _type, e.alpha, e.s[0]);
  return;
  }
  