How can I convert two C++ template classes into a single variadic template class?

It seems like the challenge you're facing comes from C++ treating template types differently than you might expect. A common workaround is to use a metafunction to handle the container types instead of directly passing them as templates. You can define a helper structure like this:

template
struct array_n_type { template using type = std::array; };
struct vector_type { template using type = std::vector; };

Then you can define your Test class like this:

template
class Test {
typename Fn1::template type c1t1;
typename Fn2::template type c2t2;
};

With that, your instantiations would look like:

Test<int, float, vector_type, array_n_type> t1;
Test t2;

This should give you the flexibility you're looking for without running into compilation problems!

You might also want to consider the reasons behind wanting to consolidate these classes. Templates in C++ can be a bit tricky, especially since this particular style is becoming less common with newer practices. Just make sure that combining them doesn't make your code harder to maintain or understand later on!

If you're looking for an alternative implementation, consider using variadic templates in a slightly different way. Instead of trying to access containers by index like your initial attempt, you could use a helper function that can unpack the parameter pack for you. This might simplify some of the handling you're trying to achieve.