Missing functionality in bulk visit

[hlverstoep]

I want to write a concurrent cache where I can lookup multiple items at once:

template <class K, class V>
std::vector<std::optional<V>> lookup(const concurrent_flat_map<K, V>& map, const std::vector<K>& keys){
    std::vector<std::optional<V>> result;
    map.cvisit(keys.begin(), keys.end(), [&](const auto& x){
        // PROBLEM: how to get 'i'??
        result[i] = x->second;
    });
    return result;
}

I can't find a way to write this with the current API. I would like a new function that traverses an additional iterator in the same order:

template <class K, class V>
std::vector<std::optional<V>> lookup(const concurrent_flat_map<K, V>& map, const std::vector<K>& keys){
    std::vector<std::optional<V>> result;
    map.cvisit(keys.begin(), keys.end(), result.begin(), [&](const auto& x, auto& it){
        *it = x->second;
    });
    return result;
}