extends.cpp

#include <iostream>
#include <type_traits>

class Base{
  public:
    int get(){ return 1234; } 
};
class A{};
class B : public Base{};

template<typename T, std::enable_if_t<std::is_base_of<Base, T>::value, std::nullptr_t> = nullptr>
void f(T c){
    std::cout <<"OK:"<< c.get() << std::endl;
}

int main(){
    A a;
    B b;
    // f(1); compile error!
    // f(a); compile error!
    f(b);
    return 0;
}