Add a NoPreferredStorageOrderBit flag for expression having no preferred storage order.

It is currently only used in Product.

Eigen/src/Core/DiagonalMatrix.h

@@ -30,7 +30,7 @@ class DiagonalBase : public EigenBase<Derived>
       MaxRowsAtCompileTime = DiagonalVectorType::MaxSizeAtCompileTime,
       MaxColsAtCompileTime = DiagonalVectorType::MaxSizeAtCompileTime,
       IsVectorAtCompileTime = 0,
-      Flags = 0
+      Flags = NoPreferredStorageOrderBit
     };
 
     typedef Matrix<Scalar, RowsAtCompileTime, ColsAtCompileTime, 0, MaxRowsAtCompileTime, MaxColsAtCompileTime> DenseMatrixType;
@@ -159,7 +159,7 @@ struct traits<DiagonalMatrix<_Scalar,SizeAtCompileTime,MaxSizeAtCompileTime> >
   typedef Dense StorageKind;
   typedef DenseIndex Index;
   enum {
-    Flags = LvalueBit
+    Flags = LvalueBit | NoPreferredStorageOrderBit
   };
 };
 }
@@ -287,7 +287,7 @@ struct traits<DiagonalWrapper<_DiagonalVectorType> >
     ColsAtCompileTime = DiagonalVectorType::SizeAtCompileTime,
     MaxRowsAtCompileTime = DiagonalVectorType::SizeAtCompileTime,
     MaxColsAtCompileTime = DiagonalVectorType::SizeAtCompileTime,
-    Flags =  traits<DiagonalVectorType>::Flags & LvalueBit
+    Flags =  (traits<DiagonalVectorType>::Flags & LvalueBit) | NoPreferredStorageOrderBit
 #ifndef EIGEN_TEST_EVALUATORS
     ,
     CoeffReadCost = traits<_DiagonalVectorType>::CoeffReadCost

Eigen/src/Core/Product.h

@@ -82,7 +82,10 @@ struct traits<Product<Lhs, Rhs, Option> >
 #endif
     
     // The storage order is somewhat arbitrary here. The correct one will be determined through the evaluator.
-    Flags = (MaxRowsAtCompileTime==1 ? RowMajorBit : 0)
+    Flags = (   MaxRowsAtCompileTime==1
+             || ((LhsCleaned::Flags&NoPreferredStorageOrderBit) && (RhsCleaned::Flags&RowMajorBit))
+             || ((RhsCleaned::Flags&NoPreferredStorageOrderBit) && (LhsCleaned::Flags&RowMajorBit)) )
+          ? RowMajorBit : (MaxColsAtCompileTime==1 ? 0 : NoPreferredStorageOrderBit)
   };
 };
 
@@ -156,6 +159,16 @@ public:
 
 } // namespace internal
 
+#ifdef EIGEN_TEST_EVALUATORS
+// Generic API dispatcher
+template<typename Lhs, typename Rhs, int Option, typename StorageKind>
+class ProductImpl : public internal::generic_xpr_base<Product<Lhs,Rhs,Option>, MatrixXpr, StorageKind>::type
+{
+  public:
+    typedef typename internal::generic_xpr_base<Product<Lhs,Rhs,Option>, MatrixXpr, StorageKind>::type Base;
+};
+#endif
+
 template<typename Lhs, typename Rhs, int Option>
 class ProductImpl<Lhs,Rhs,Option,Dense>
   : public internal::dense_product_base<Lhs,Rhs,Option>

Eigen/src/Core/util/Constants.h

@@ -154,6 +154,16 @@ const unsigned int AlignedBit = 0x80;
 
 const unsigned int NestByRefBit = 0x100;
 
+/** \ingroup flags
+  *
+  * for an expression, this means that the storage order
+  * can be either row-major or column-major.
+  * The precise choice will be decided at evaluation time or when
+  * combined with other expressions.
+  * \sa \ref RowMajorBit, \ref TopicStorageOrders */
+const unsigned int NoPreferredStorageOrderBit = 0x200;
+
+
 // list of flags that are inherited by default
 const unsigned int HereditaryBits = RowMajorBit
                                   | EvalBeforeNestingBit