Lots of C API returns a gboolean on success or failure along with an associated GError.

You might have an API like this:
  gboolean badger_async_finish (Badger *self, GAsyncResult *res, GStrv *return1, GStrv *return2, GError **error)

The gboolean is useful for the C API, but in a JS API you need to catch the exception in order to know @error is, so the gboolean is redundant information.

It would be nice to be able to annotate this

  Returns: (ignore): %TRUE on success, %FALSE otherwise (@error will be set)

Which resulted in a binded API similar to:

try {
  let [return1, return2] = b.async_finish(res);
} catch (e) {
  ... stuff
}