#include <iostream>
 
template <typename Func>
void apply(Func&& f, ...) { std::cout << "default\n"; }
 
template <typename Func, typename T>
auto apply(Func&& f, T&& t) -> decltype(f(std::forward<T>(t))) {
    return f(std::forward<T>(t));
}
 
 
struct Foo {};
struct Bar {};
 
struct F {
    void operator()(Foo) { std::cout << "Foo\n"; }
    void operator()(Bar) { std::cout << "Bar\n"; }
};
 
int main() {
    F f;
    Foo foo;
    Bar bar;
    int i;
    apply(f, foo);
    apply(f, bar);
    apply(f, i);
}

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