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/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