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);
 }

modules/core/test/test_umat.cpp

@@ -1366,4 +1366,21 @@ TEST(UMat, DISABLED_regression_5991)
     EXPECT_EQ(0, cvtest::norm(mat.getMat(ACCESS_READ), Mat(3, sz, CV_8U, Scalar(1)), NORM_INF));
 }
 
+TEST(UMat, testTempObjects_Mat_issue_8693)
+{
+    UMat srcUMat(3, 4, CV_32FC1);
+    Mat srcMat;
+
+    randu(srcUMat, -1.f, 1.f);
+    srcUMat.copyTo(srcMat);
+
+    reduce(srcUMat, srcUMat, 0, CV_REDUCE_SUM);
+    reduce(srcMat, srcMat, 0, CV_REDUCE_SUM);
+
+    srcUMat.convertTo(srcUMat, CV_64FC1);
+    srcMat.convertTo(srcMat, CV_64FC1);
+
+    EXPECT_EQ(0, cvtest::norm(srcUMat.getMat(ACCESS_READ), srcMat, NORM_INF));
+}
+
 } } // namespace cvtest::ocl