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#include <iostream>
 
    template<class T>struct identity{using type=T;};
    template<class T>using identity_t=typename identity<T>::type;
 
    template <typename T, template<class>class Z=identity_t>
    struct Yin {
      Z<T> *m_ptr;
    };
 
    template <typename T, template<class>class Z=identity_t>
    struct Yang {
      Z<T> *m_ptr;
    };
 
    template<
      template<class, template<class>class>class Yin,
      template<class, template<class>class>class Yang
    >
    struct flopper {
      template<class T> struct flip {
      	template<class U> using flop_t=typename flopper<Yang, Yin>::template flip_t<U>;
        using type = Yang<T, flop_t>;
      };
      template<class T> using flip_t=typename flip<T>::type;
    };
 
    using yin = Yin< void, flopper<Yin, Yang>::template flip_t >;
    using yang = Yang< void, flopper<Yang, Yin>::template flip_t >;
 
 
int main() {
	yin a={nullptr};
	yang b={&a};
	// your code goes here
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:34:12: error: cannot convert ‘yin* {aka Yin<void, flopper<Yin, Yang>::flip_t>*}’ to ‘flopper<Yang, Yin>::flip_t<void>* {aka Yin<void, flopper<Yang, Yin>::flip<void>::flop_t>*}’ in initialization
  yang b={&a};
            ^
prog.cpp:34:7: warning: unused variable ‘b’ [-Wunused-variable]
  yang b={&a};
       ^

Standard output is empty