Detecting constexpr with SFINAE

I'm working on upgrading some C++ code to take advantage of the new functionality in C++11. I have a trait class with a few functions returning fundamental types which would most of the time, but not always, return a constant expression. I would like to do different things based on whether the function is constexpr or not. I came up with the following approach:

template<typename Trait>
struct test
{
    template<int Value = Trait::f()>
    static std::true_type do_call(int){ return std::true_type(); }

    static std::false_type do_call(...){ return std::false_type(); }

    static bool call(){ return do_call(0); }
};

struct trait
{
    static int f(){ return 15; }
};

struct ctrait
{
    static constexpr int f(){ return 20; }
};

int main()
{
   std::cout << "regular: " << test<trait>::call() << std::endl;
   std::cout << "constexpr: " << test<ctrait>::call() << std::endl;
}

The extra int/... parameter is there so that if both functions are available after SFINAE, the first one gets chosen by overloading resolution.

Compiling and running this with Clang 3.2 shows:

regular: 0
constexpr: 1

So this appears to work, but I would like to know if the code is legal C++11. Specially since it's my understanding that the rules for SFINAE have changed.