[eigen] const references (bug 218)

[Robert Carnecky <crobi@xxxxxxxxxxx>]

I have a question/comment on bug 218 
(http://eigen.tuxfamily.org/bz/show_bug.cgi?id=218)
The bug is closed as wontfix, but I suspect that it might be legitimate, 
hence my post here.

What happens:
Visual Studio users get the following warning: C4181: qualifier applied 
to reference type; ignored
This happens for example inside MatrixBase<Derived>::cross 
(OrthoMethods.h, line 46)
The offending code is: const typename internal::nested<Derived,2>::type 
lhs(derived());

Now my question:
Why do we have a const keyword there?
If internal::nested<Derived,2>::type turns out to be a reference type, 
the const is applied to the reference and not to the referenced type and 
the warning is legitimate (references are always constant). Also, 
internal::nested uses ref_selector, and ref_selector<T>::type is already 
a reference to const T, so no need for an additional const.
If internal::nested<Derived,2>::type turns out to be a plain type, the 
const keyword might help the compiler optimize code, which would however 
surprise me here. If the const is really helpful, a new member 
internal::nested::const_type might be more appropriate.

Regards,
Robert