template <class T> class has_sub_type
{
  typedef size_1 char[1];
  typedef size_2 char[2];
  template <class wrapped> class wrapper { };
  template <u> static size_1& test(wrapper<typename u::type>*);
  template <u> static size_2& test(...);
public:
  static const bool value = (sizeof(test<T>(0)) == 1);
};