std::hash specialization using sfinae?

Question

As an exercise I was trying to see if I could use SFINAE to create a std::hash specialization for std::pair and std::tuple when all of

Answer 1

You are not supposed to specialize std::hash for types that doesn't depend on a type you defined yourself.

That said, this hack might work:

template<class T, class E>
using first = T;

template <typename T>
struct hash<first<std::pair<T, T>, std::enable_if_t<std::is_unsigned<T>::value>>>
{
    size_t operator()(const std::pair<T, T>& x) const
    {
        return x;
    }
};

Really, though, don't do this. Write your own hasher.