struct IntMember {
   const int value = 1;
};

struct ArrayMember {
   const std::vector<int> array;
};

template <bool IsArray>
struct Poly final : public std::conditional_t<IsArray, ArrayMember, IntMember>
{
   static constexpr bool isArray() noexcept { return IsArray; };
};

template<bool A, bool B>
int process(Poly<A> const& a, Poly<B> const& i)
{
   if constexpr (a.isArray() && i.isArray())
       return a.array == i.array;
   else if constexpr (!a.isArray() && !i.isArray())
       return a.i == i.i;
   else
       return -1;
}

int main()
{
   Poly<true> a;
   Poly<false> i;

   return process(a, i);
}