fix the multiple temporary issue for nested products

Eigen/src/Core/ProductBase.h

@@ -42,7 +42,8 @@ struct ei_traits<ProductBase<Derived,_Lhs,_Rhs> > //: ei_traits<typename ei_clea
     ColsAtCompileTime = ei_traits<Rhs>::ColsAtCompileTime,
     MaxRowsAtCompileTime = ei_traits<Lhs>::MaxRowsAtCompileTime,
     MaxColsAtCompileTime = ei_traits<Rhs>::MaxColsAtCompileTime,
-    Flags = EvalBeforeNestingBit | EvalBeforeAssigningBit,
+    Flags = EvalBeforeNestingBit | EvalBeforeAssigningBit | NestByRefBit, // Note that EvalBeforeNestingBit and NestByRefBit
+                                                                          // are not used in practice because ei_nested is overloaded for products
     CoeffReadCost = 0 // FIXME why is it needed ?
   };
 };
@@ -137,11 +138,21 @@ class ProductBase : public MatrixBase<Derived>
     Diagonal<LazyCoeffBaseProductType,Dynamic> diagonal(int index)
     { return Diagonal<LazyCoeffBaseProductType,Dynamic>(LazyCoeffBaseProductType(CoeffBaseProductType(m_lhs, m_rhs))).diagonal(index); }
 
+    // Implicit convertion to the nested type (trigger the evaluation of the product)
+    operator const PlainMatrixType& () const
+    {
+      m_result.resize(m_lhs.rows(), m_rhs.cols());
+      this->evalTo(m_result);
+      return m_result;
+    }
+
   protected:
 
     const LhsNested m_lhs;
     const RhsNested m_rhs;
 
+    mutable PlainMatrixType m_result;
+
   private:
 
     // discard coeff methods
@@ -151,6 +162,14 @@ class ProductBase : public MatrixBase<Derived>
     void coeffRef(int);
 };
 
+// here we need to overload the nested rule for products
+// such that the nested type is a const reference to a plain matrix
+template<typename Lhs, typename Rhs, int Mode, int N, typename PlainMatrixType>
+struct ei_nested<GeneralProduct<Lhs,Rhs,Mode>, N, PlainMatrixType>
+{
+  typedef PlainMatrixType const& type;
+};
+
 template<typename NestedProduct>
 class ScaledProduct;