Workaround various "returning reference to temporary" warnings.

Eigen/src/Core/CoreEvaluators.h

@@ -1221,7 +1221,9 @@ struct evaluator<Diagonal<ArgType, DiagIndex> >
  
   typedef typename XprType::Index Index;
   typedef typename XprType::Scalar Scalar;
-  typedef typename XprType::CoeffReturnType CoeffReturnType;
+  // FIXME having to check whether ArgType is sparse here i not very nice.
+  typedef typename internal::conditional<!internal::is_same<typename ArgType::StorageKind,Sparse>::value,
+                                         typename XprType::CoeffReturnType,Scalar>::type CoeffReturnType;
 
   EIGEN_DEVICE_FUNC CoeffReturnType coeff(Index row, Index) const
   {

Eigen/src/Core/util/Constants.h

@@ -449,6 +449,9 @@ enum Action {GetAction, SetAction};
 /** The type used to identify a dense storage. */
 struct Dense {};
 
+/** The type used to identify a general sparse storage. */
+struct Sparse {};
+
 /** The type used to identify a permutation storage. */
 struct PermutationStorage {};