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 <type_traits>
template<typename T, typename = std::void_t<>>
struct IsClassT : std::false_type { // primary template: by default no class
};
template<typename T>
struct IsClassT<T, std::void_t<int T::*>> // classes can have pointer-to-member
: std::true_type {
};