hoge.cpp

template<typename F>
class FixPoint : F
{
public:
  explicit constexpr FixPoint(F&& f) noexcept
    : F(std::forward<F>(f))
  {}

  template<typename... Args>
  constexpr decltype(auto)
  operator()(Args&&... args) const
  {
    return F::operator()(*this, std::forward<Args>(args)...);
  }
};  // class FixPoint


int
main()
{
  auto result = FixPoint{[](auto&& f, int n) -> int {
    return n <= 1 ? n : (f(n - 1) + f(n - 2));
  }}(10);
  std::cout << result << std::endl;
}