#include <type_traits>
 
template <typename> struct IsBase : std::false_type {};
 
template <typename Self>
struct Base {
    Base& operator()() {
        return *this;
    };
 
    template <typename Other,
              typename = typename std::enable_if<std::is_same<Other, Self>::value ||
                                                 IsBase<Other>::value>::type>
    Self operator+(const Other& other) const {
        return static_cast<const Self&>(*this);
    }
};
 
template <typename X> struct IsBase<Base<X>> : std::true_type {};
 
 
struct D1 : Base<D1> {};
struct D2 : Base<D2> {};
 
 
int main() {
    D1 d1;
    D2 d2;
    d1 + d1;
    d2 + d2;
    d1() + d2();
    d1 + d2;
}
 
 

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