#include <iostream>
#include <type_traits>
 
template <int I, class T>
using enableIf = typename std::enable_if<I, T>::type;
 
template <class T>
struct optional{
  template <class U>
  enableIf<std::is_convertible<U, T>::value, T> value_or(U&& v) const{
  	std::cout << "value_or(U&& v)\n";
  	return std::move(v);
  }
 
  template <class F>
  auto value_or(F&& action) const -> decltype(action()) {
  	return action();
  }
};
 
int main() {
	optional<int> o1;
	o1.value_or(1);
	int i;
	o1.value_or(i);
	o1.value_or([](){ std::cout<< "lambda\n"; return 0; });
	return 0;
}

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