Fix https://github.com/opencv/opencv/issues/8693

modules/core/src/matrix.cpp

@@ -4021,6 +4021,11 @@ void cv::reduce(InputArray _src, OutputArray _dst, int dim, int op, int dtype)
     CV_OCL_RUN(_dst.isUMat(),
                ocl_reduce(_src, _dst, dim, op, op0, stype, dtype))
 
+    // Fake reference to source. Resolves issue 8693 in case of src == dst.
+    UMat srcUMat;
+    if (_src.isUMat())
+        srcUMat = _src.getUMat();
+
     Mat src = _src.getMat();
     _dst.create(dim == 0 ? 1 : src.rows, dim == 0 ? src.cols : 1, dtype);
     Mat dst = _dst.getMat(), temp = dst;

modules/core/src/umatrix.cpp

@@ -961,6 +961,8 @@ void UMat::convertTo(OutputArray _dst, int _type, double alpha, double beta) con
         }
     }
 #endif
+    UMat src = *this;  // Fake reference to itself.
+                       // Resolves issue 8693 in case of src == dst.
     Mat m = getMat(ACCESS_READ);
     m.convertTo(_dst, _type, alpha, beta);
 }