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> // for declval()
#include <type_traits> // for true_type and false_type
template<typename T>
class IsContextualBoolT {
private:
template<typename T> struct Identity;
template<typename U> static std::true_type
test(Identity<decltype(declval<U>()? 0 : 1)>*);
template<typename U> static std::false_type
test(...);
public:
static constexpr bool value = decltype(test<T>(nullptr))::value;
};
template<typename T>
constexpr bool IsContextualBool = IsContextualBoolT<T>::value;