How to extract the highest-indexed specialization from a structure?

I'm trying to do some template metaprogramming and I'm finding the need to "extract" the highest index of a specialization of some structure in some type.

For example, if I have some types:

struct A
{
    template<unsigned int> struct D;
    template<> struct D<0> { };
};

struct B
{
    template<unsigned int> struct D;
    template<> struct D<0> { };
    template<> struct D<1> { };
};

struct C
{
    template<unsigned int> struct D;
    template<> struct D<0> { };
    template<> struct D<1> { };
    template<> struct D<2> { };
};

How can I then write a metafunction like this:

template<class T>
struct highest_index
{
    typedef ??? type;
    // could also be:   static size_t const index = ???;
};

to give me the highest-indexed D that has been specialized inside an arbitrary struct like the ones above, without requiring the struct to have declared the count explicitly?