diff --git a/Eigen/src/Core/ProductBase.h b/Eigen/src/Core/ProductBase.h
index 287ea554f..3bd3487d6 100644
--- a/Eigen/src/Core/ProductBase.h
+++ b/Eigen/src/Core/ProductBase.h
@@ -134,7 +134,7 @@ class ProductBase : public MatrixBase<Derived>
       return m_result;
     }
 
-    const Diagonal<FullyLazyCoeffBaseProductType,0> diagonal() const
+    const Diagonal<const FullyLazyCoeffBaseProductType,0> diagonal() const
     { return FullyLazyCoeffBaseProductType(m_lhs, m_rhs); }
 
     template<int Index>
diff --git a/Eigen/src/Core/products/CoeffBasedProduct.h b/Eigen/src/Core/products/CoeffBasedProduct.h
index 878956a33..dc20f7e1e 100644
--- a/Eigen/src/Core/products/CoeffBasedProduct.h
+++ b/Eigen/src/Core/products/CoeffBasedProduct.h
@@ -213,14 +213,14 @@ class CoeffBasedProduct
     const _LhsNested& lhs() const { return m_lhs; }
     const _RhsNested& rhs() const { return m_rhs; }
 
-    const Diagonal<LazyCoeffBasedProductType,0> diagonal() const
+    const Diagonal<const LazyCoeffBasedProductType,0> diagonal() const
     { return reinterpret_cast<const LazyCoeffBasedProductType&>(*this); }
 
     template<int DiagonalIndex>
-    const Diagonal<LazyCoeffBasedProductType,DiagonalIndex> diagonal() const
+    const Diagonal<const LazyCoeffBasedProductType,DiagonalIndex> diagonal() const
     { return reinterpret_cast<const LazyCoeffBasedProductType&>(*this); }
 
-    const Diagonal<LazyCoeffBasedProductType,Dynamic> diagonal(Index index) const
+    const Diagonal<const LazyCoeffBasedProductType,Dynamic> diagonal(Index index) const
     { return reinterpret_cast<const LazyCoeffBasedProductType&>(*this).diagonal(index); }
 
   protected: