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 T1, typename T2>
class HasLess {
template<typename T> struct Identity;
template<typename U1, typename U2> static std::true_type
test(Identity<decltype(std::declval<U1>() < std::declval<U2>())>*);
template<typename U1, typename U2> static std::false_type
test(...);
public:
static constexpr bool value = decltype(test<T1, T2>(nullptr))::value;
};
template<typename T1, typename T2, bool HasLess>
class LessResultImpl {
public:
using Type = decltype(std::declval<T1>() < std::declval<T2>());
};
template<typename T1, typename T2>
class LessResultImpl<T1, T2, false> {
};
template<typename T1, typename T2>
class LessResultT
: public LessResultImpl<T1, T2, HasLess<T1, T2>::value> {
};
template<typename T1, typename T2>
using LessResult = typename LessResultT<T1, T2>::Type;