Decltype get function from function call

I have this code:

template<typename T, bool> class Ensure
{
    typedef T type;
};

template<typename T> class Ensure<T, false> {};

template<typename T, typename U>
T mix(const T& x, const T& y, const U& a)
{
    return x * (static_cast<U>(1.0) - a) + y * a;
}

template<typename T, typename U>
T mix(typename Ensure<T, std::is_scalar<T>::value>::type x,
      typename Ensure<T, std::is_scalar<T>::value>::type y,
      typename Ensure<T, std::is_scalar<T>::value>::type a)
{
    return x * (static_cast<U>(1.0) - a) + y * a;
}

I think that I have understood that mix take const T& with object types and T with scalar types. However, I want to check it.

I want to test if the right function is called by this code:

float a = 0, b = 0, c = 0;
mix(a, b, c); // const float& or float ?

COncretly how to get a function pointer or the type of function with a function call ?

Something like: decltype((&mix)(a, b, c)) ptr; std::cout << typeid(ptr).name() << std::endl