C++: Specify default value of class template argument based on previous argument?

Question

I have the following forward declaration:

template
    < typename Vector
    , typename Scalar = Vector::Scalar >
class RayPlaneHitTest;

This works fine with MSVC++ 2010, but G++ rejects it. I think the purpose is self-explanatory. How do I need to modify it s.t. it works with G++?

I've been trying it with G++ 4.6.3.