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
using std::declval;
template<typename T, typename U>
class CommonTypeT
{
public:
using Type = decltype(true? declval<T>() : declval<U>());
};
template<typename T, typename U>
using CommonType = typename CommonTypeT<T, U>::Type;