the behavior of std::async with std::launch::async policy

I have some question about behavior of std::async function with std::launch::async policy & std::future object returned from async.

In following code, main thread waits for the completion of foo() on the thread created by async call.

#include <thread>
#include <future>
#include <iostream>

void foo()
{
  std::cout << "foo:begin" << std::endl;
  std::this_thread::sleep_for(std::chrono::seconds(10));
  std::cout << "foo:done" << std::endl;
}

int main()
{
  std::cout << "main:begin" << std::endl;
  {
    auto f = std::async(std::launch::async, foo);
    // dtor f::~f blocks until completion of foo()... why??
  }
  std::this_thread::sleep_for(std::chrono::seconds(2));
  std::cout << "main:done" << std::endl;
}

And I know http://www.stdthread.co.uk/doc/headers/future/async.html says

The destructor of the last future object associated with the asynchronous state of the returned std::future shall block until the future is ready.

My question is:

• Q1. Does this behavior conform to the current C++ standard?
• Q2. If Q1's answer is yes, which statements say that?