#include <iostream>
using namespace std;

class B{};
template<class T1,class T2,class T3=B>class E{};   //<- "class T3=B"
//^ library

//v user
class C{};
template<class T1,class T3=B> using F=E<T1,C,T3>;  //<- "class T3=B" is duplicated
int main() {
    F<B> f1;
    F<B,C> f2;
    return 0;
}