Использован тернарный оператор.

Код:

• #include <iostream>
• template <typename T>
• constexpr T even_sum(T seq[], int n, int i, T sum) {
•    return i == n || seq[i] % 2 != 0 ? sum : even_sum(seq, n, i+1, sum + seq[i]);
• }
• int main() {
•    constexpr int sequence[] = {4, 6, 8, 10};
•    constexpr int n = sizeof(sequence) / sizeof(*sequence);
•    constexpr int k = even_sum(const_cast<int *>(sequence), n, 0, 0);
•    std::cout << k << std::endl;
•    return 0;
• }