Emulate range-for begin/end lookup rules about range-v3 HOT 5 CLOSED

constexpr int MaxPrioLevel = 2;
template<int Level> struct Prio : public Prio<Level-1>
{
  constexpr Prio(){}
  static_assert(Level<=MaxPrioLevel,"Level must not be greater than MaxPrioLevel, to make sure MaxPrio means maximum priority. It is safe to increase MaxPrioLevel if needed.");
};
template<> struct Prio<0>{constexpr Prio(){}};
constexpr Prio<MaxPrioLevel> maxPrio;

struct begin_fn
{
private:
  //Array. Case 6.5.4.1.3 of N4296
  //TODO with Prio<2>

  //Member function. Case 6.5.4.1.2 of N4296
  template<typename T, typename... Args>
  auto f(Prio<1>, T&& x, Args&&... args) const ->
  decltype(std::forward<T>(x).begin(std::forward<Args>(args)...))
  {
    return std::forward<T>(x).begin(std::forward<Args>(args)...);
  }

  //Free function (e.g. from argument dependent lookup (ADL)). Case 6.5.4.1.3 of N4296
  template<typename... Args>
  auto f(Prio<0>, Args&&... args) const ->
  decltype(begin(std::forward<Args>(args)...))
  {
    return begin(std::forward<Args>(args)...);
  }
public:
  template<typename... Args>
  decltype(auto) operator()(Args&&... args) const
  {
    return this->f(maxPrio, std::forward<Args>(args)...);
  }
};