Closed
Description
Along the lines of #19823 in the 4.3.x line, we should revise ReactiveAdapterRegistry towards CompletionStage support, considering all implementations of that interface as adaptable (instead of just CompletableFuture). This makes reactive return types for WebFlux consistent with MVC/messaging handler methods, most importantly allowing return type declarations to use the CompletionStage interface even if they effectively return CompletableFuture instances at runtime.