Sfinae detection for all compilers.

Test on all compilers

#include <utility>

class A
{
    friend bool operator<(const A &lhs, const A &rhs)
    {
        return false;
    }
};


class B: A
{};


template <typename T>
class HasLess {
  template <typename U>
    static char (&check (int))[1 + sizeof(decltype(std::declval<U>() < std::declval<U>()))];

    template <typename>
    static char (&check (...))[1];

public:
    static constexpr const bool value = sizeof (check<T> (0)) != 1;
};



int main(void)
{
    B b;
    static_assert(HasLess<A>::value, "");
    static_assert(!HasLess<B>::value, "");
    return 0;
}