Workaround various "returning reference to temporary" warnings.

Eigen/SparseCore

@@ -26,13 +26,6 @@
   * This module depends on: Core.
   */
 
-namespace Eigen {
-
-/** The type used to identify a general sparse storage. */
-struct Sparse {};
-
-}
-
 #include "src/SparseCore/SparseUtil.h"
 #include "src/SparseCore/SparseMatrixBase.h"
 #include "src/SparseCore/SparseAssign.h"

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 {};
 

Eigen/src/SparseCore/CompressedStorage.h

@@ -143,7 +143,7 @@ class CompressedStorage
     }
 
     /** Like at(), but the search is performed in the range [start,end) */
-    inline const Scalar& atInRange(size_t start, size_t end, Index key, const Scalar& defaultValue = Scalar(0)) const
+    inline Scalar atInRange(size_t start, size_t end, Index key, const Scalar &defaultValue = Scalar(0)) const
     {
       if (start>=end)
         return defaultValue;

Eigen/src/SparseCore/SparseMatrix.h

@@ -179,7 +179,7 @@ class SparseMatrix
 
     /** \returns the value of the matrix at position \a i, \a j
       * This function returns Scalar(0) if the element is an explicit \em zero */
-    inline const Scalar& coeff(Index row, Index col) const
+    inline Scalar coeff(Index row, Index col) const
     {
       eigen_assert(row>=0 && row<rows() && col>=0 && col<cols());
       
@@ -1297,7 +1297,7 @@ struct evaluator<SparseMatrix<_Scalar,_Options,_Index> >
   operator const SparseMatrixType&() const { return *m_matrix; }
   
   typedef typename DenseCoeffsBase<SparseMatrixType,ReadOnlyAccessors>::CoeffReturnType CoeffReturnType;
-  CoeffReturnType coeff(Index row, Index col) const
+  Scalar coeff(Index row, Index col) const
   { return m_matrix->coeff(row,col); }
   
   Scalar& coeffRef(Index row, Index col)