HasToString.cpp

template <typename T> 
class HasToString 
{ 
private: 
    typedef char YesType[1]; 
    typedef char NoType[2]; 
    
    template <typename C> static YesType& test( decltype(&C::ToString) ) ; 
    template <typename C> static NoType& test(...); 

public: 
    enum { value = sizeof(test<T>(0)) == sizeof(YesType) }; 
};

Hi! we don't need bodies of the functions as it's used only at compile time. So we "Call" them without running the code... "0" converts to nullp pointer and that matches with the member function pointer &C::ToString. But if the ToString function is not available then we call default "NoType test(...) that matches all params.
You can also have a look at my updated post, with C++17 techniques: https://www.bfilipek.com/2019/07/detect-overload-from-chars.html