code: private static Object reportJoin(Object r) {
      if (r instanceof AltResult) {
        Throwable x;
        if ((x = ((AltResult) r).ex) == null)
          return null;
        if (x instanceof CancellationException)
          throw (CancellationException) x;
        if (x instanceof CompletionException)
          throw (CompletionException) x;
        throw new CompletionException(x);
      }
      return r;
    }
docs: Decodes outcome to return result or throw unchecked exception.