Specializing std::hash for private member class

I have a class (call it Outer) which has a private member class (Inner). I want to store instances of Outer::Inner in unordered standard containers, so I want to specialize std::hash<Outer::Inner>.

However, when writing this:

namespace std {
    template<>
    struct hash<Outer::Inner> {
        std::size_t operator()(const Outer::Inner &arg) const
        {
            return std::hash<int>()(arg.someSpecialProperty);
        }
    };
}

the compiler complains:

error: 'Inner' is a private member of 'Outer'
            std::size_t operator()(const Outer::Inner &p) const
                                                ^

I have tried to make std::hash a friend struct by following this answer, but that didn't work either: the forward declaration of Outer::Inner failed with:

error: use of undeclared identifier 'Outer'

So how should I proceed (if what I intend to do is possible at all)?

Since it's a private inner type, I assume that you have a private or protected std::unordered_map member in the enclosing class. If that's the case, just write a private inner hash functor and pass it as the third argument of the std::unordered_map. It's the easiest solution to your problem, I think.