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#include <utility>
#include <memory>
#include <iostream>
 
template<class T>
T&& smart_wrap_unwrap(T&& v, long)
{
  return std::forward<T>(v);
}
 
template<class T>
auto smart_wrap_unwrap(T&& v, int)
  -> decltype(smart_wrap_unwrap(std::forward<T>(v).getPtr(), 0))
{
  return smart_wrap_unwrap(std::forward<T>(v).getPtr(), 0);
}
 
template<class T>
struct smart{
  template<class U>
  smart(U* v) : _ptr(v){}
  T& getPtr(){ return _ptr; }
  T _ptr;
};
 
struct X{
  void foo(){ std::cout << "!\n"; }
};
 
int main(){
  smart<smart<smart<std::shared_ptr<X>>>> smartception(new X());
  smart_wrap_unwrap(smartception, 0)->foo();
}

prog.cpp: In function 'decltype (smart_wrap_unwrap(forward<T>(v).getPtr(), 0)) smart_wrap_unwrap(T&&, int) [with T = smart<smart<smart<std::shared_ptr<X> > > >&, decltype (smart_wrap_unwrap(forward<T>(v).getPtr(), 0)) = smart<smart<std::shared_ptr<X> > >&]':
prog.cpp:32:36:   instantiated from here
prog.cpp:15:58: error: invalid initialization of reference of type 'std::shared_ptr<X>&' from expression of type 'smart<smart<std::shared_ptr<X> > >'

Standard output is empty