Fixed a bug which caused you to get a compiler error if you tried

to call dot() on two 1x1 matrices which were statically dimensioned.

dlib/matrix/matrix_utilities.h

@@ -596,7 +596,7 @@ namespace dlib
     }
 
     template < typename EXP1, typename EXP2 >
-    typename enable_if_c<EXP1::NR == 1 && EXP2::NR == 1, typename EXP1::type>::type 
+    typename enable_if_c<EXP1::NR == 1 && EXP2::NR == 1 && EXP1::NC != 1 && EXP2::NC != 1, typename EXP1::type>::type 
     dot ( const matrix_exp<EXP1>& m1, const matrix_exp<EXP2>& m2) 
     { 
         DLIB_ASSERT(m1.size() == m2.size(), 
@@ -610,7 +610,7 @@ namespace dlib
     }
 
     template < typename EXP1, typename EXP2 >
-    typename enable_if_c<EXP1::NR == 1 && EXP2::NC == 1, typename EXP1::type>::type 
+    typename enable_if_c<EXP1::NR == 1 && EXP2::NC == 1 && EXP1::NC != 1 && EXP2::NR != 1, typename EXP1::type>::type 
     dot ( const matrix_exp<EXP1>& m1, const matrix_exp<EXP2>& m2) 
     { 
         DLIB_ASSERT(m1.size() == m2.size(), 
@@ -624,7 +624,7 @@ namespace dlib
     }
 
     template < typename EXP1, typename EXP2 >
-    typename enable_if_c<EXP1::NC == 1 && EXP2::NR == 1, typename EXP1::type>::type 
+    typename enable_if_c<EXP1::NC == 1 && EXP2::NR == 1 && EXP1::NR != 1 && EXP2::NC != 1, typename EXP1::type>::type 
     dot ( const matrix_exp<EXP1>& m1, const matrix_exp<EXP2>& m2) 
     { 
         DLIB_ASSERT(m1.size() == m2.size(), 
@@ -638,7 +638,7 @@ namespace dlib
     }
 
     template < typename EXP1, typename EXP2 >
-    typename enable_if_c<EXP1::NC == 1 && EXP2::NC == 1, typename EXP1::type>::type 
+    typename enable_if_c<EXP1::NC == 1 && EXP2::NC == 1 && EXP1::NR != 1 && EXP2::NR != 1, typename EXP1::type>::type 
     dot ( const matrix_exp<EXP1>& m1, const matrix_exp<EXP2>& m2) 
     { 
         DLIB_ASSERT(m1.size() == m2.size(), 
@@ -651,6 +651,20 @@ namespace dlib
         return trans(m1)*m2; 
     }
 
+    template < typename EXP1, typename EXP2 >
+    typename enable_if_c<(EXP1::NC == 1 && EXP1::NR == 1) || (EXP2::NC == 1 && EXP2::NR == 1), typename EXP1::type>::type 
+    dot ( const matrix_exp<EXP1>& m1, const matrix_exp<EXP2>& m2) 
+    { 
+        DLIB_ASSERT(m1.size() == m2.size(), 
+            "\t type dot(const matrix_exp& m1, const matrix_exp& m2)"
+            << "\n\t You can only compute the dot product between vectors of equal length"
+            << "\n\t m1.size():     " << m1.size() 
+            << "\n\t m2.size():     " << m2.size() 
+            );
+        
+        return m1(0)*m2(0);
+    }
+
 // ----------------------------------------------------------------------------------------
 
     template <typename M, long R, long C>

dlib/test/matrix4.cpp

@@ -909,6 +909,29 @@ namespace
             DLIB_TEST(m == m2);
 
         }
+
+        {
+            matrix<double,1,1> a, b;
+            a = 2;
+            b = 3;
+            DLIB_TEST(dot(a,b) == 6);
+        }
+        {
+            matrix<double,1,1> a;
+            matrix<double,0,1> b(1);
+            a = 2;
+            b = 3;
+            DLIB_TEST(dot(a,b) == 6);
+            DLIB_TEST(dot(b,a) == 6);
+        }
+        {
+            matrix<double,1,1> a;
+            matrix<double,1,0> b(1);
+            a = 2;
+            b = 3;
+            DLIB_TEST(dot(a,b) == 6);
+            DLIB_TEST(dot(b,a) == 6);
+        }
     }
 
     class matrix_tester : public tester