When a CompletionFailure is thrown when user code is running, it shouldn't be thrown out to the client code, but rather skipped, and then rethrown later if javac itself will complete the Symbol. On all places where javac invokes client code (e.g. TaskListeners, annotation Processors), the userCodeHandler should be set using DeferredCompletionFailureHandler#setHandler, and the original handler should be restored when the control returns back to javac. Implementations of API methods should use Symbol#apiComplete() instead of Symbol#complete, as the apiComplete method will invoke DeferredCompletionFailureHandler#handleAPICompletionFailure, which will catch the CompletionFailure and will either rethrow it or skip it, depending on the context.