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 <utility>
template<typename T1, typename T2>
struct PlusResultT {
using Type = decltype(std::declval<T1>() + std::declval<T2>());
};
template<typename T1, typename T2>
using PlusResult = typename PlusResultT<T1, T2>::Type;