Added asserts to catch a common user error.

dlib/matrix/matrix.h

@@ -1435,6 +1435,9 @@ namespace dlib
             }
             else
             {
+                DLIB_ASSERT(size() == 0, 
+                    "\t const matrix::operator+=(m)"
+                    << "\n\t You are trying to add two matrices that have incompatible dimensions.");
                 *this = m;
             }
             return *this;
@@ -1469,6 +1472,9 @@ namespace dlib
             }
             else
             {
+                DLIB_ASSERT(size() == 0, 
+                    "\t const matrix::operator-=(m)"
+                    << "\n\t You are trying to subtract two matrices that have incompatible dimensions.");
                 *this = -m;
             }
             return *this;
@@ -1495,6 +1501,10 @@ namespace dlib
             }
             else
             {
+                DLIB_ASSERT(this->size() == 0, 
+                    "\t const matrix::operator+=(m)"
+                    << "\n\t You are trying to add two matrices that have incompatible dimensions.");
+
                 set_size(m.nr(), m.nc());
                 for (long i = 0; i < size; ++i)
                     data(i) = m.data(i);
@@ -1514,6 +1524,9 @@ namespace dlib
             }
             else
             {
+                DLIB_ASSERT(this->size() == 0, 
+                    "\t const matrix::operator-=(m)"
+                    << "\n\t You are trying to subtract two matrices that have incompatible dimensions.");
                 set_size(m.nr(), m.nc());
                 for (long i = 0; i < size; ++i)
                     data(i) = -m.data(i);

dlib/matrix/matrix_abstract.h

@@ -492,6 +492,10 @@ namespace dlib
         /*!
             requires
                 - matrix_exp<EXP>::type == T
+                - One of the following is true:
+                    - nr() == m.nr() && nc() == m.nc()
+                    - size() == 0
+                  (i.e. this matrix must have matching dimensions or it must be empty)
             ensures
                 - if (nr() == m.nr() && nc() == m.nc()) then
                     - #(*this) == *this + m
@@ -509,6 +513,10 @@ namespace dlib
         /*!
             requires
                 - matrix_exp<EXP>::type == T
+                - One of the following is true:
+                    - nr() == m.nr() && nc() == m.nc()
+                    - size() == 0
+                  (i.e. this matrix must have matching dimensions or it must be empty)
             ensures
                 - if (nr() == m.nr() && nc() == m.nc()) then
                     - #(*this) == *this - m