this is a real life example when i was using semantic-ui framework，
they often have multiple class names tide to one condition.
My instinct tell me to type like the first example, however Vue gave me an error.

So I tried as the second example. and I failed again

The second exemple would work as an expression returning a string, not a list.
The best way to it imo would be to use a list as a property computed from flag

https://jsfiddle.net/x54ju3em/1/

@Axnyff use a string works well.
the first case is still worth consideration (I've edit a little to reflect the use case)