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/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