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template <typename T>
struct event_data;
 
template<typename T>
struct tovoid {
	typedef void type;
};
 
template <typename T, typename enable = void>
struct get_data{
  typedef event_data<T> type;
};
 
template <typename T>
struct get_data<T, typename tovoid<typename T::data_t>::type >{
  typedef typename T::data_t type;
};
 
template <typename T>
struct base{
	typedef typename get_data<T>::type data_type;
 
	base(){
		data_type();
	}
};
 
struct non_default_impl: public base<non_default_impl>{
	struct data{};
 
	typedef data data_t;
};
struct default_impl{
 
};
 
int main(){
	non_default_impl e1;
//	base<default_impl> e2;
	return 0;
}

prog.cpp: In instantiation of 'base<T>::base() [with T = non_default_impl]':
prog.cpp:28:8:   required from here
prog.cpp:24:3: error: invalid use of incomplete type 'base<non_default_impl>::data_type {aka struct event_data<non_default_impl>}'
   data_type();
   ^
prog.cpp:2:8: note: declaration of 'base<non_default_impl>::data_type {aka struct event_data<non_default_impl>}'
 struct event_data;
        ^

Standard output is empty