when can we use “auto” in function prototype?

#include <iostream>
#include <deque>
#include <list>
#include <vector>
#include <functional>

static auto consumer(auto& container) {
    return [&](auto val) {container.push_back(val); };
}

static void print(const auto& container) {
    for (auto i : container) {
        std::cout << i << ",";
    }
    std::cout << std::endl;
}

int main()
{
    std::list<int> l;
    std::vector<int> v;
    std::deque<int> d;

    const std::vector<std::function<void(int)>>consumers{ consumer(l),consumer(v),consumer(d) };
    for (int i = 0; i < 10; ++i) {
        for (auto &&consumer : consumers) {
            consumer(i);
        }
    }
    print(l);
    print(v);
    print(d);
    return 0;
}