CompletableFuture.minimalCompletionStage().toCompletableFuture() returns "this", i.e. another minimal CompletableFuture that throws UnsupportedOperationException for most operations, which is inconvenient without providing any additional safety. Users can always write their own external toCompletableFuture as follows:

    static <T> CompletableFuture<T> toCompletableFuture(CompletionStage<T> stage) {
        CompletableFuture<T> f = new CompletableFuture<>();
        stage.handle((T t, Throwable ex) -> {
                         if (ex != null) f.completeExceptionally(ex);
                         else f.complete(t);
                         return null;
                     });
        return f;
    }

but this is not entirely obvious.