#include <iostream>

auto dummy_lambda = []{};
using lambda_type = decltype(dummy_lambda);

template<typename F1, typename F2 = lambda_type const&>
void foo(F1&& f1, F2&& f2 = dummy_lambda)
{
  std::forward<F1>(f1)();
  std::forward<F2>(f2)();
}


void bar()
{
  std::cout << "hello!\n";
}

int main(int, const char*[])
{
  foo(&bar, &bar);
  foo(&bar);
}