In examples 18.104.22.168. of the spec (§5.1.2. Methods for validating method and constructor constraints) it says "Do we need more examples?". The answer is yes ;o)
Method validation is new so it would make things clearer to have more examples. I've been playing with this new feature  and I have some doubts on the way I am using validateReturnValue (check ).
So I think a more concrete and complete example in the spec would help. You can have a look at my GitHub  and the experiments I've been playing with.