future::unwrap hangs for launch::deferred

[eschnett]

Unwrapping a future that has been created with async(launch::deferred) hangs:

#include <hpx/hpx.hpp>
#include <iostream>
using namespace hpx;
using namespace std;

int hpx_main(int argc, char **argv) {
  cout << "Starting...\n";
  future<future<void> > f1 =
      async(launch::deferred, []() { return async([]() {}); });
  future<void> f2 = f1.unwrap();
  f2.wait();
  cout << "Done.\n";
  return 0;
}