Adding a ROW_NUMBER() with no column to ORDER BY?
The canonical way to do this is the following: ROW_NUMBER() OVER(ORDER BY (SELECT NULL)). If you're golfing, you might try something like this:

SELECT value, n = ROW_NUMBER() OVER(ORDER BY (SELECT 1))
FROM STRING_SPLIT('one,two,three,four,five',',')


It works for the simple case you posted in the question:



I should say that there is not a documented guarantee that the ROW_NUMBER() value will be in the precise order that you expect. But it's code golf, so this seems good enough.