code: private static <T> T 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);
          }
          @SuppressWarnings("unchecked") T t = (T) r;
          return t;
      }
docs: Decodes outcome to return result or throw unchecked exception.