C++ metaprogramming assign unique ID to tree structures

I have an odd problem. I have a metaprogramming type defined like so:

template <int N, typename... Args>
struct mytype {
    ...
};

By default, I create them like this:

using type = mytype<-1, mytype<-1, mytype<-1, ...>>>;

Later on, I need to recurse through the type and recursively set each number to a unique ID. The IDs need to be sequential and start at 0 for long technical reasons. For instance, if I had this:

mytype<-1
    mytype<-1, a, b>
>

I want it to become something like this:

mytype<0,
    mytype<1, a, b>
>

It doesn't matter the order for the numbers' assignment.

I don't quite know how to approach this problem and have tried several things that didn't get anywhere. Any help would be greatly appreciated.