Challenge 2: A Variadic sum_all Using a Fold Expression — Possible Solution
====================================================================

#include <iostream>

template<typename... Args>
auto sum_all(Args... args) {
    return (args + ...);  // C++17 fold expression
}

int main() {
    std::cout << sum_all(1, 2) << std::endl;
    std::cout << sum_all(1, 2, 3) << std::endl;
    std::cout << sum_all(1, 2, 3, 4, 5) << std::endl;
    return 0;
}

Output:

    3
    6
    15

WHY THIS WORKS AS AN ANSWER
------------------------------
The single variadic template definition, template<typename... Args>,
accepts genuinely different numbers of arguments across the three
calls (2, 3, and 5), and the fold expression (args + ...) expands, at
compile time, into the equivalent of args1 + args2 + ... for however
many arguments were actually passed -- exactly the "arbitrary number of
arguments" capability the chapter describes as powering functions like
make_unique throughout the STL.