hehe, the complicated nesting scheme in Flagged in the previous commit

was a sign that we were doing something wrong. In fact, having
NestByValue as a special case of Flagged was wrong, and the previous
commit, while not buggy, was inefficient because then when the resulting
NestByValue xpr was nested -- hence copied -- the original xpr which was
already nested by value was copied again; hence instead of 1 copy we got
3 copies.
The solution was to ressuscitate the old Temporary.h (renamed
NestByValue.h) as it was the right approach.

Eigen/Core

@@ -30,6 +30,7 @@ namespace Eigen {
 #include "src/Core/Coeffs.h"
 #include "src/Core/Assign.h"
 #include "src/Core/MatrixStorage.h"
+#include "src/Core/NestByValue.h"
 #include "src/Core/Flagged.h"
 #include "src/Core/Matrix.h"
 #include "src/Core/CwiseBinaryOp.h"

Eigen/src/Core/Flagged.h

@@ -94,11 +94,7 @@ template<typename ExpressionType, unsigned int Added, unsigned int Removed> clas
     }
 
   protected:
-    const typename ei_meta_if<
-      Added & ~Removed & NestByValueBit,
-      ExpressionType,
-      typename ExpressionType::Nested
-    >::ret m_matrix;
+    typename ExpressionType::Nested m_matrix;
 };
 
 /** \returns an expression of *this with added flags
@@ -121,13 +117,4 @@ MatrixBase<Derived>::lazy() const
   return derived();
 }
 
-/** \returns an expression of *this with the NestByValueBit flag added.
-  */
-template<typename Derived>
-inline const Flagged<Derived, NestByValueBit, 0>
-MatrixBase<Derived>::nestByValue() const
-{
-  return derived();
-}
-
 #endif // EIGEN_FLAGGED_H

Eigen/src/Core/MatrixBase.h

@@ -305,8 +305,8 @@ template<typename Derived> class MatrixBase
     Transpose<Derived> transpose();
     const Transpose<Derived> transpose() const;
     const Transpose<
-            Flagged<CwiseUnaryOp<ei_scalar_conjugate_op<typename ei_traits<Derived>::Scalar>, Derived>
-            , NestByValueBit, 0> >
+      NestByValue<CwiseUnaryOp<ei_scalar_conjugate_op<typename ei_traits<Derived>::Scalar>, Derived> >
+    >
     adjoint() const;
     //@}
 
@@ -452,7 +452,6 @@ template<typename Derived> class MatrixBase
     template<unsigned int Added>
     const Flagged<Derived, Added, 0> marked() const;
     const Flagged<Derived, 0, EvalBeforeNestingBit | EvalBeforeAssigningBit> lazy() const;
-    const Flagged<Derived, NestByValueBit, 0> nestByValue() const;
 
     /** \returns number of elements to skip to pass from one row (resp. column) to another
       * for a row-major (resp. column-major) matrix.
@@ -460,6 +459,8 @@ template<typename Derived> class MatrixBase
       * of the underlying matrix.
       */
     inline int stride(void) const { return derived()._stride(); }
+
+    inline const NestByValue<Derived> nestByValue() const;
     //@}
 
     /// \name Coefficient-wise operations

Eigen/src/Core/NestByValue.h

@@ -0,0 +1,93 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra. Eigen itself is part of the KDE project.
+//
+// Copyright (C) 2008 Gael Guennebaud <g.gael@free.fr>
+// Copyright (C) 2006-2008 Benoit Jacob <jacob@math.jussieu.fr>
+//
+// Eigen is free software; you can redistribute it and/or
+// modify it under the terms of the GNU Lesser General Public
+// License as published by the Free Software Foundation; either
+// version 3 of the License, or (at your option) any later version.
+//
+// Alternatively, you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation; either version 2 of
+// the License, or (at your option) any later version.
+//
+// Eigen is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License or the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License and a copy of the GNU General Public License along with
+// Eigen. If not, see <http://www.gnu.org/licenses/>.
+
+#ifndef EIGEN_NESTBYVALUE_H
+#define EIGEN_NESTBYVALUE_H
+
+/** \class NestByValue
+  *
+  * \brief Expression which must be nested by value
+  *
+  * \param ExpressionType the type of the object of which we are requiring nesting-by-value
+  *
+  * This class is the return type of MatrixBase::nestByValue()
+  * and most of the time this is the only way it is used.
+  *
+  * \sa MatrixBase::nestByValue()
+  */
+template<typename ExpressionType>
+struct ei_traits<NestByValue<ExpressionType> >
+{
+  typedef typename ExpressionType::Scalar Scalar;
+  enum {
+    RowsAtCompileTime = ExpressionType::RowsAtCompileTime,
+    ColsAtCompileTime = ExpressionType::ColsAtCompileTime,
+    MaxRowsAtCompileTime = ExpressionType::MaxRowsAtCompileTime,
+    MaxColsAtCompileTime = ExpressionType::MaxColsAtCompileTime,
+    Flags = ExpressionType::Flags,
+    CoeffReadCost = ExpressionType::CoeffReadCost
+  };
+};
+
+template<typename ExpressionType> class NestByValue
+  : public MatrixBase<NestByValue<ExpressionType> >
+{
+  public:
+
+    EIGEN_GENERIC_PUBLIC_INTERFACE(NestByValue)
+
+    inline NestByValue(const ExpressionType& matrix) : m_expression(matrix) {}
+
+  private:
+
+    inline int _rows() const { return m_expression.rows(); }
+    inline int _cols() const { return m_expression.cols(); }
+    inline int _stride() const { return m_expression.stride(); }
+
+    inline const Scalar _coeff(int row, int col) const
+    {
+      return m_expression.coeff(row, col);
+    }
+
+    template<int LoadMode>
+    inline const PacketScalar _packetCoeff(int row, int col) const
+    {
+      return m_expression.template packetCoeff<LoadMode>(row, col);
+    }
+
+  protected:
+    const ExpressionType m_expression;
+};
+
+/** \returns an expression of the temporary version of *this.
+  */
+template<typename Derived>
+inline const NestByValue<Derived>
+MatrixBase<Derived>::nestByValue() const
+{
+  return NestByValue<Derived>(derived());
+}
+
+#endif // EIGEN_NESTBYVALUE_H

Eigen/src/Core/Product.h

@@ -165,7 +165,7 @@ template<typename T> class ei_product_eval_to_column_major
 template<typename T, int n=1> struct ei_product_nested_rhs
 {
   typedef typename ei_meta_if<
-    (ei_traits<T>::Flags & NestByValueBit) && (!(ei_traits<T>::Flags & RowMajorBit)) && (int(ei_traits<T>::Flags) & DirectAccessBit),
+    ei_must_nest_by_value<T>::ret && (!(ei_traits<T>::Flags & RowMajorBit)) && (int(ei_traits<T>::Flags) & DirectAccessBit),
     T,
     typename ei_meta_if<
         ((ei_traits<T>::Flags & EvalBeforeNestingBit)
@@ -181,7 +181,7 @@ template<typename T, int n=1> struct ei_product_nested_rhs
 template<typename T, int n=1> struct ei_product_nested_lhs
 {
   typedef typename ei_meta_if<
-    ei_traits<T>::Flags & NestByValueBit && (int(ei_traits<T>::Flags) & DirectAccessBit),
+    ei_must_nest_by_value<T>::ret && (int(ei_traits<T>::Flags) & DirectAccessBit),
     T,
     typename ei_meta_if<
          int(ei_traits<T>::Flags) & EvalBeforeNestingBit

Eigen/src/Core/Transpose.h

@@ -126,8 +126,8 @@ MatrixBase<Derived>::transpose() const
   * \sa transpose(), conjugate(), class Transpose, class ei_scalar_conjugate_op */
 template<typename Derived>
 inline const Transpose<
-               Flagged<CwiseUnaryOp<ei_scalar_conjugate_op<typename ei_traits<Derived>::Scalar>, Derived >
-               , NestByValueBit, 0> >
+               NestByValue<CwiseUnaryOp<ei_scalar_conjugate_op<typename ei_traits<Derived>::Scalar>, Derived > >
+             >
 MatrixBase<Derived>::adjoint() const
 {
   return conjugate().nestByValue();

Eigen/src/Core/util/Constants.h

@@ -45,7 +45,6 @@ const unsigned int SelfAdjointBit = 0x100;  ///< means the matrix is selfadjoint
 const unsigned int UpperTriangularBit = 0x200;    ///< means the strictly triangular lower part is 0
 const unsigned int LowerTriangularBit = 0x400;    ///< means the strictly triangular upper part is 0
 const unsigned int DirectAccessBit = 0x800; ///< means the underlying matrix data can be direclty accessed
-const unsigned int NestByValueBit = 0x1000;   ///< means the expression should be copied by value when nested
 
 // list of flags that are inherited by default
 const unsigned int HereditaryBits = RowMajorBit

Eigen/src/Core/util/ForwardDeclarations.h

@@ -35,6 +35,7 @@ template<typename _Scalar, int _Rows, int _Cols,
 class Matrix;
 
 template<typename ExpressionType, unsigned int Added, unsigned int Removed> class Flagged;
+template<typename ExpressionType> class NestByValue;
 template<typename MatrixType> class Minor;
 template<typename MatrixType, int BlockRows=Dynamic, int BlockCols=Dynamic> class Block;
 template<typename MatrixType> class Transpose;

Eigen/src/Core/util/Macros.h

@@ -135,7 +135,6 @@ typedef typename Eigen::NumTraits<Scalar>::Real RealScalar; \
 typedef typename Base::PacketScalar PacketScalar; \
 typedef typename Eigen::ei_nested<Derived>::type Nested; \
 typedef typename Eigen::ei_eval<Derived>::type Eval; \
-typedef Eigen::Flagged<Derived, NestByValueBit, 0> Temporary; \
 enum { RowsAtCompileTime = Base::RowsAtCompileTime, \
        ColsAtCompileTime = Base::ColsAtCompileTime, \
        MaxRowsAtCompileTime = Base::MaxRowsAtCompileTime, \

Eigen/src/Core/util/Meta.h

@@ -192,11 +192,14 @@ template<typename T> struct ei_unref<T&> { typedef T type; };
 template<typename T> struct ei_unconst { typedef T type; };
 template<typename T> struct ei_unconst<const T> { typedef T type; };
 
+template<typename T> struct ei_must_nest_by_value { enum { ret = false }; };
+template<typename T> struct ei_must_nest_by_value<NestByValue<T> > { enum { ret = true }; };
+
 
 template<typename T, int n=1> struct ei_nested
 {
   typedef typename ei_meta_if<
-    ei_traits<T>::Flags & NestByValueBit,
+    ei_must_nest_by_value<T>::ret,
     T,
     typename ei_meta_if<
       int(ei_traits<T>::Flags) & EvalBeforeNestingBit