How do I create a packaged_task with a member function?

On the program below, I'm trying to create a packaged_task with a member function:

#include <future>
using namespace std;

struct S
{
    int calc(int& a)
    {
        return a*a;
    }
};

int main()
{
    S s;
    auto bnd = std::bind(&S::calc, s);
    std::packaged_task<int(int&)> task( bnd);
    return 0;
}

Unfortunately the attempt results in an error.

How can this be done??