How to use decltype in std::future?

In this code, how can I use decltype in std::future to deduce the return type of bar() ? Although directly using std::future<int> works, I would like to know how can decltype be used in such a situation.

#include <iostream>
#include <future>


int bar(int a)
{
    return 50;
}
int main()
{

    std::packaged_task<decltype(bar)> task(bar);

    //std::future<decltype(bar(int))> f = task.get_future(); //doesn't work need to do something like this

    std::future<int> f = task.get_future();   //works

    std::thread t1(std::move(task), 10);
    t1.detach();
    int val = f.get();
    std::cout << val << "\n";
    return 0;
}

Also, is the use of decltype in std::packaged_task correct ?