thenCombineAsync
abstract override fun <U, V> thenCombineAsync(other: CompletionStage<out U>, fn: BiFunction<in T, in U, out V>): DelayCompletionStage<V>
abstract override fun <U, V> thenCombineAsync(other: CompletionStage<out U>, fn: BiFunction<in T, in U, out V>, executor: Executor): DelayCompletionStage<V>