Ticket #51469: consterr.cpp

template <class T, int N>
struct static_poly {
   T m_data[N]; //constexpr std::array isn't modifiable in C++14 (P0107R0)

   // construct:
   constexpr static_poly() : m_data{} {}

   constexpr T& operator[] (int i) {
      return m_data[i];
   }

   constexpr const T& operator[] (int i) const {
      return m_data[i];
   }
};

template <class T, int N1, int N2>
constexpr static_poly<T, N1 + N2> operator * (const static_poly<T, N1>& a, const static_poly<T, N2>& b) {
   static_poly<T, N1 + N2> prod;
   for (int i = 0; i < N1; ++i)
      for (int j = 0; j < N2; ++j)
         prod[i+j] += a[i] * b[j];
   return prod;
}

int main() {
    constexpr static_poly<int, 3> x2p1; // zeros
    constexpr static_poly<int, 2> xm1; // zeros
    constexpr auto prod = x2p1 * xm1;
    return 0;
}