using SFINAE for template class specialisation

Question

suppose I have these declarations

template class User;
template class Data;

and want to implement

Answer 1

IF the original declaration of User<> can be adapted to

template class User;

then we can find a solution (following Luc Danton's comment, instead of using std::enable_if)

template
struct is_Data : std::false_type {};
template
struct is_Data> : std::true_type {};

template
class User::type >
{ /* ... */ };

However, this doesn't answer the original question, since it requires to change the original definition of User. I'm still waiting for a better answer. This could be one that conclusively demonstrates that no other solution is possible.