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 IsEqualityComparable
{
private:
// test convertibility of == and ! == to bool:
static void* conv(bool); // to check convertibility to bool
template<typename U>
static std::true_type test(decltype(conv(std::declval<U const&>() ==
std::declval<U const&>())),
decltype(conv(!(std::declval<U const&>() ==
std::declval<U const&>())))
);
// fallback:
template<typename U>
static std::false_type test(...);
public:
static constexpr bool value = decltype(test<T>(nullptr,
nullptr))::value;
};