The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
#include "hasplus.hpp"
template<typename T1, typename T2, bool = HasPlusT<T1, T2>::value>
struct PlusResultT { // primary template, used when HasPlusT yields true
using Type = decltype(std::declval<T1>() + std::declval<T2>());
};
template<typename T1, typename T2>
struct PlusResultT<T1, T2, false> { // partial specialization, used otherwise
};