Detect operator support with decltype/SFINAE

Question

A (somewhat) outdated article explores ways to use decltype along with SFINAE to detect if a type supports certain operators, such as == or &

Answer 1

You need to make your less_than_test function a template, since SFINAE stands for Substitution Failure Is Not An Error and there's no template function that can fail selection in your code.

template 
struct supports_less_than
{
    template 
    static auto less_than_test(const U* u) -> decltype(*u < *u, char(0))
    { }

    static std::array less_than_test(...) { }

    static const bool value = (sizeof(less_than_test((T*)0)) == 1);
};

int main()
{
    std::cout << std::boolalpha << supports_less_than::value << endl;
}