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#include<iostream>
#include<type_traits>
 
struct A{};
struct B{};
struct C{};
 
 
 
template <typename ... Args>
struct Convert;
 
template <typename T>
struct Convert<T,T> {
 
    static int param() { return 0; }
};
 
template <typename T, typename U>
struct Convert<T,U> {
 
    static decltype(-Convert<U,T>::param()) param() { return -Convert<U,T>::param(); }
};
 
template <>
struct Convert<A,B> {
    static int param() { return 42; }
};
 
template <>
struct Convert<A,C> {
    static int param() { return 43; }
};
 
template <>
struct Convert<B,C> {
    static int param() { return 44; }
};
 
int main()
{
    std::cout<<Convert<A,B>::param()<<std::endl;
    std::cout<<Convert<B,A>::param()<<std::endl;
    std::cout<<Convert<A,C>::param()<<std::endl;
    std::cout<<Convert<C,A>::param()<<std::endl;
    std::cout<<Convert<B,C>::param()<<std::endl;
    std::cout<<Convert<C,B>::param()<<std::endl;
 
    Convert<int,double>::param();
}

prog.cpp: In instantiation of ‘struct Convert<double, int>’:
prog.cpp:24:42:   required from ‘struct Convert<int, double>’
prog.cpp:51:24:   required from here
prog.cpp:24:42: error: incomplete type ‘Convert<int, double>’ used in nested name specifier
     static decltype(-Convert<U,T>::param()) param() { return -Convert<U,T>::param(); }
                                          ^
prog.cpp: In instantiation of ‘struct Convert<int, double>’:
prog.cpp:51:24:   required from here
prog.cpp:24:42: error: ‘param’ is not a member of ‘Convert<double, int>’
prog.cpp: In function ‘int main()’:
prog.cpp:51:5: error: ‘param’ is not a member of ‘Convert<int, double>’
     Convert<int,double>::param();
     ^

Standard output is empty