title: std::wait_until always required to block?
body: <p>Is the 2-parameter version (i.e. the one without the predicate) of <code>std::wait_until</code> (C++11) required to block when the provided timeout <code>time_point</code> parameter is earlier than the "current time"?</p>
  
  <p>If it does not block, does it still return <code>std::cv_status::timeout</code>?</p>