Howto enable hashing for std::chrono::year_month and std::chrono::year_month_day?

[maximiliank]

Is it ok to add a specialization of AbslHashValue in the namespace absl::hash_internal for std::chrono::year_month/std::chrono::year_month_day? Doing something in an *_internal namespace seems dangerous.

If I add the following, I can use those types in std::unordered_map with absl::Hash<KeyType>:

namespace absl::hash_internal {
    // Support std::chrono::year_month and std::chrono_year_month_day
    template<typename T, typename H,
            typename = std::enable_if_t<std::is_same_v<T, std::chrono::year_month> ||
                                        std::is_same_v<T, std::chrono::year_month_day>>>
    H AbslHashValue(H state, const T ymd)
    {
        state = H::combine(std::move(state), static_cast<int>(ymd.year()), unsigned{ymd.month()});
        if constexpr (std::is_same_v<T, std::chrono::year_month_day>)
        {
            state = H::combine(std::move(state), unsigned{ymd.day()});
        }
        return state;
    }
}

Is there a way to enable hashing for those types without adding a specialization in an internal namespace?