FutureReturnValueIgnored analogue for CompletionStages

[ninepints]

Hi! I have a proposal for a new check based on a situation that came up at work. Here is (essentially) our code:

CompletionStage<Void> c = CompletableFuture.completedFuture(null);
c.thenRun(() -> {...});

Error Prone compiles this with no complaints. However, if the type of c is changed to CompletableFuture<Void>, we get a warning about the return value of thenRun being ignored. FutureReturnValueIgnored kicks in because the return value is known to be a CompletableFuture rather than just a CompletionStage.

Since it seems equally dangerous to ignore a returned CompletionStage - exceptions thrown by the lambda above are suppressed - maybe Error Prone should warn about that too?

[ninepints]

I guess the real question is whether a (reasonable) CompletionStage implementation could let you retrieve the result of chained computations by some alternate means. It seems unlikely: if you have a CompletionStage<T> and use thenApply to map the T to a U, how are you going to retrieve the U except through the returned stage? I'm having trouble imagining a CompletionStage implementation that's not also a Future.