HashMap with std::future as the key

Since we cannot get the arguments from std::future, I am trying to maintain a map between std::future and std::string. The following statement:

std::unordered_map<std::future<void>, std::string> map fails with the following error:

error: no match for call to '(const std::hash<std::future<void> >) (const std::future<void>&)' │ =• noexcept(declval<const _Hash&>()(declval<const _Key&>()))>

How can I solve this?