core: fix mat matx multiplication

modules/core/include/opencv2/core/mat.hpp

@@ -3627,9 +3627,9 @@ CV_EXPORTS MatExpr operator * (const MatExpr& e, double s);
 CV_EXPORTS MatExpr operator * (double s, const MatExpr& e);
 CV_EXPORTS MatExpr operator * (const MatExpr& e1, const MatExpr& e2);
 template<typename _Tp, int m, int n> static inline
-MatExpr operator * (const Mat& a, const Matx<_Tp, m, n>& b) { return a + Mat(b); }
+MatExpr operator * (const Mat& a, const Matx<_Tp, m, n>& b) { return a * Mat(b); }
 template<typename _Tp, int m, int n> static inline
-MatExpr operator * (const Matx<_Tp, m, n>& a, const Mat& b) { return Mat(a) + b; }
+MatExpr operator * (const Matx<_Tp, m, n>& a, const Mat& b) { return Mat(a) * b; }
 
 CV_EXPORTS MatExpr operator / (const Mat& a, const Mat& b);
 CV_EXPORTS MatExpr operator / (const Mat& a, double s);

modules/core/test/test_mat.cpp

@@ -2001,6 +2001,22 @@ TEST(Core_Vectors, issue_13078_workaround)
     ASSERT_EQ(7, ints[3]);
 }
 
+TEST(Core_MatExpr, issue_13926)
+{
+    Mat M1 = (Mat_<double>(4,4,CV_64FC1) << 1, 2, 3, 4,
+                                           5, 6, 7, 8,
+                                           9, 10, 11, 12,
+                                           13, 14, 15, 16);
+
+    Matx44d M2(1, 2, 3, 4,
+               5, 6, 7, 8,
+               9, 10, 11, 12,
+               13, 14, 15, 16);
+
+    EXPECT_GE(1e-6, cvtest::norm(M1*M2, M1*M1, NORM_INF)) << Mat(M1*M2) << std::endl << Mat(M1*M1);
+    EXPECT_GE(1e-6, cvtest::norm(M2*M1, M2*M2, NORM_INF)) << Mat(M2*M1) << std::endl << Mat(M2*M2);
+}
+
 
 #ifdef HAVE_EIGEN
 TEST(Core_Eigen, eigen2cv_check_Mat_type)