Universal aggregate initialization by variadic templates

This is an interesting problem I'm thinking about a time ago. Given a struct with an underlying aggregate:

#include <array>

template <typename T, size_t N>
struct A
{
   constexpr A() = default;

   template <typename ... Ts>
   constexpr A(const T& value, const Ts& ... values); // magic

   std::array<T, N> arr; // aggregate
};

How would you implement variadic template constructor A(const T& value, const Ts& ... values) to

• accept both values of type T and another A<T, N>
• properly initialize the underlying aggregate based on the values represented by the passed arguments
• respect the capacity of the aggregate
• support C++14 constexpr rules and do not introduce any runtime overhead

---

Satisfying the above requirements, it is possible to do the following:

int main()
{
   A<int, 3> x(1, 2, 3);
   A<int, 3> y(1, 2);

   A<int, 6> a(x, 1, 2, 3);
   A<int, 6> b(1, x, 2, 3);
   A<int, 6> c(1, 2, x, 3);
   A<int, 6> d(1, 2, 3, x);
   A<int, 6> e(x, x);
   A<int, 6> f(y, y, y);

   return 0;
}