For the record, here is a solution for (a*b).diagonal, at the cost of extra copies

if a and/or b as to be evaluated. So in the next commit I'll remove it. A nice
solution would be to evaluate the lhs/rhs into member of the initial product,
but that would be overkill.

2 files changed, 21 insertions, 13 deletions

diff --git a/Eigen/src/Core/ProductBase.h b/Eigen/src/Core/ProductBase.h
index 9fdb74f1c..4c95684f0 100644
--- a/Eigen/src/Core/ProductBase.h
+++ b/Eigen/src/Core/ProductBase.h
@@ -83,7 +83,11 @@ class ProductBase : public MatrixBase<Derived>
     typedef typename RhsBlasTraits::DirectLinearAccessType ActualRhsType;
     typedef typename ei_cleantype<ActualRhsType>::type _ActualRhsType;
 
-    typedef typename ProductReturnType<Lhs,Rhs,LazyCoeffBasedProductMode>::Type LazyCoeffBaseProductType;
+    // here we don't use ProductReturnType<LazyCoeffBasedProductMode>::Type because we must nest it by value.
+    typedef typename ei_nested<_LhsNested, _RhsNested::ColsAtCompileTime, typename ei_plain_matrix_type<_LhsNested>::type >::type LazyLhsNested;
+    typedef typename ei_nested<_RhsNested, _LhsNested::RowsAtCompileTime, typename ei_plain_matrix_type<_RhsNested>::type >::type LazyRhsNested;
+    typedef CoeffBasedProduct<LazyLhsNested, LazyRhsNested, 0> NestedByValueLazyCoeffBaseProductType;
+
   public:
 
     typedef typename Base::PlainMatrixType PlainMatrixType;
@@ -114,25 +118,25 @@ class ProductBase : public MatrixBase<Derived>
     const _LhsNested& lhs() const { return m_lhs; }
     const _RhsNested& rhs() const { return m_rhs; }
 
-    const Diagonal<LazyCoeffBaseProductType,0> diagonal() const
-    { return LazyCoeffBaseProductType(m_lhs, m_rhs); }
+    const Diagonal<NestedByValueLazyCoeffBaseProductType,0> diagonal() const
+    { return NestedByValueLazyCoeffBaseProductType(m_lhs, m_rhs); }
 
-    Diagonal<LazyCoeffBaseProductType,0> diagonal()
-    { return LazyCoeffBaseProductType(m_lhs, m_rhs); }
+    Diagonal<NestedByValueLazyCoeffBaseProductType,0> diagonal()
+    { return NestedByValueLazyCoeffBaseProductType(m_lhs, m_rhs); }
 
     template<int Index>
-    const Diagonal<LazyCoeffBaseProductType,Index> diagonal() const
-    { return LazyCoeffBaseProductType(m_lhs, m_rhs); }
+    const Diagonal<NestedByValueLazyCoeffBaseProductType,Index> diagonal() const
+    { return NestedByValueLazyCoeffBaseProductType(m_lhs, m_rhs); }
 
     template<int Index>
-    Diagonal<LazyCoeffBaseProductType,Index> diagonal()
-    { return LazyCoeffBaseProductType(m_lhs, m_rhs); }
+    Diagonal<NestedByValueLazyCoeffBaseProductType,Index> diagonal()
+    { return NestedByValueLazyCoeffBaseProductType(m_lhs, m_rhs); }
 
-    const Diagonal<LazyCoeffBaseProductType,Dynamic> diagonal(int index) const
-    { return LazyCoeffBaseProductType(m_lhs, m_rhs).diagonal(index); }
+    const Diagonal<NestedByValueLazyCoeffBaseProductType,Dynamic> diagonal(int index) const
+    { return NestedByValueLazyCoeffBaseProductType(m_lhs, m_rhs).diagonal(index); }
 
-    Diagonal<LazyCoeffBaseProductType,Dynamic> diagonal(int index)
-    { return LazyCoeffBaseProductType(m_lhs, m_rhs).diagonal(index); }
+    Diagonal<NestedByValueLazyCoeffBaseProductType,Dynamic> diagonal(int index)
+    { return NestedByValueLazyCoeffBaseProductType(m_lhs, m_rhs).diagonal(index); }
 
     // Implicit convertion to the nested type (trigger the evaluation of the product)
     operator const PlainMatrixType& () const
diff --git a/Eigen/src/Core/products/CoeffBasedProduct.h b/Eigen/src/Core/products/CoeffBasedProduct.h
index c9d333072..af0d2e2a2 100644
--- a/Eigen/src/Core/products/CoeffBasedProduct.h
+++ b/Eigen/src/Core/products/CoeffBasedProduct.h
@@ -129,6 +129,10 @@ class CoeffBasedProduct
 
   public:
 
+    inline CoeffBasedProduct(const CoeffBasedProduct& other)
+      : Base(), m_lhs(other.m_lhs), m_rhs(other.m_rhs)
+    {}
+
     template<typename Lhs, typename Rhs>
     inline CoeffBasedProduct(const Lhs& lhs, const Rhs& rhs)
       : m_lhs(lhs), m_rhs(rhs)