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
// an explicitly-provided visitor result type:
template<typename R, typename Visitor, typename... ElementTypes>
class VisitResultT
{
public:
using Type = R;
};
template<typename R, typename Visitor, typename... ElementTypes>
using VisitResult =
typename VisitResultT<R, Visitor, ElementTypes...>::Type;