title: can rethrow_exception really throw the same exception object, rather than a copy?
body: <p>While examining what <code>exception_ptr</code> does, the C++11 standard says (18.8.5/7) that:</p>
  
  <blockquote>
    <p>Use of rethrow_exception on exception_ptr objects that refer to the same exception object shall not introduce a data race. [ Note: if rethrow_exception rethrows the same exception object (rather than a copy), concurrent access to that rethrown exception object may introduce a data race...</p>
  </blockquote>
  
  <p>I don't find the case where this weird "Note" applies, since the described effect of <code>rethrow_exception</code> is "Throws: the exception object to which p refers" but 15.1/3, describing the general exception throwing process mandates that "throwing an exception copy-initializes a temporary object, called the exception object."</p>
  
  <p>The weird note would imply that rethrow_exception skips this copy-initialization. But is this really possible?</p>