Currently methods returning CompletionStage (from the JDK) or Publisher (from ReactiveStreams) are handled as asynchronous.

It would be nice to be able to return PublisherBuilder (from MP RSO) as well, since at present the following does not work:

PublisherBuilder test() {
   return ReactiveStreams.of("x");
}

Instead you have to

Publisher test() {
   return ReactiveStreams.of("x")
        .buildRs()
}

So it is presently the responsibility of the user to remember to call PublisherBuilder.buildRs()