For two class template partial specializations, the first is at least as specialized as the second if, given the following rewrite to two function templates, the first function template is at least as specialized as the second according to the ordering rules for function templates ([temp.func.order]):
the first function template has the same template parameters as the first partial specialization and has a single function parameter whose type is a class template specialization with the template arguments of the first partial specialization, and
the second function template has the same template parameters as the second partial specialization and has a single function parameter whose type is a class template specialization with the template arguments of the second partial specialization.
[ Example:
template<int I, int J, class T> class X { };
template<int I, int J> class X<I, J, int> { }; // #1
template<int I> class X<I, I, int> { }; // #2
template<int I, int J> void f(X<I, J, int>); // A
template<int I> void f(X<I, I, int>); // B
The partial specialization #2 is more specialized than the partial specialization #1 because the function template B is more specialized than the function template A according to the ordering rules for function templates. — end example ]