Fixed a bug in the matrix class. Expressions of the form mat *= mat(0) would

evaluate incorrectly because the *= operator took the right hand side by reference
and thus experienced an aliasing problem.  The other op= operators had similar
problems and have also been fixed.

dlib/matrix/matrix.h

@@ -1475,7 +1475,7 @@ namespace dlib
         }
 
         matrix& operator += (
-            const T& val
+            const T val
         )
         {
             const long size = nr()*nc();
@@ -1486,7 +1486,7 @@ namespace dlib
         }
 
         matrix& operator -= (
-            const T& val
+            const T val
         )
         {
             const long size = nr()*nc();
@@ -1497,7 +1497,7 @@ namespace dlib
         }
 
         matrix& operator *= (
-            const T& a
+            const T a
         )
         {
             const long size = data.nr()*data.nc();
@@ -1507,7 +1507,7 @@ namespace dlib
         }
 
         matrix& operator /= (
-            const T& a
+            const T a
         )
         {
             const long size = data.nr()*data.nc();

dlib/test/matrix.cpp

@@ -1212,6 +1212,43 @@ namespace
             m2 = 1;
             m1 = subm(m2,0,0,3,3)*m1;
         }
+
+        {
+            matrix<int> m(2,1);
+
+            m = 3,3;
+            m /= m(0);
+
+            DLIB_TEST(m(0) == 1);
+            DLIB_TEST(m(1) == 1);
+        }
+        {
+            matrix<int> m(2,1);
+
+            m = 3,3;
+            m *= m(0);
+
+            DLIB_TEST(m(0) == 9);
+            DLIB_TEST(m(1) == 9);
+        }
+        {
+            matrix<int> m(2,1);
+
+            m = 3,3;
+            m -= m(0);
+
+            DLIB_TEST(m(0) == 0);
+            DLIB_TEST(m(1) == 0);
+        }
+        {
+            matrix<int> m(2,1);
+
+            m = 3,3;
+            m += m(0);
+
+            DLIB_TEST(m(0) == 6);
+            DLIB_TEST(m(1) == 6);
+        }
     }