More tests to validate the const-correctness of the tensor code.

Eigen/src/Core/GenericPacketMath.h

@@ -95,6 +95,8 @@ template<typename T> struct packet_traits : default_packet_traits
   };
 };
 
+template<typename T> struct packet_traits<const T> : packet_traits<T> { };
+
 /** \internal \returns a + b (coeff-wise) */
 template<typename Packet> EIGEN_DEVICE_FUNC inline Packet
 padd(const Packet& a,

Eigen/src/Core/util/XprHelper.h

@@ -415,6 +415,14 @@ template <typename A> struct promote_storage_type<A,A>
 {
   typedef A ret;
 };
+template <typename A> struct promote_storage_type<A, const A>
+{
+  typedef A ret;
+};
+template <typename A> struct promote_storage_type<const A, A>
+{
+  typedef A ret;
+};
 
 /** \internal gives the plain matrix or array type to store a row/column/diagonal of a matrix type.
   * \param Scalar optional parameter allowing to pass a different scalar type than the one of the MatrixType.