Many thanks to all that have participated to this thread.
I took the time to summarize the discussions and decisions in
http://beanvalidation.org/proposals/BVAL-232/
You can find the diff at
https://github.com/beanvalidation/beanvalidation.org/commit/647260172be59...
Here is what I propose:
1. start with the generic approach and put that in the spec
2. explore the idea of returning the parameter indexes involved
3. add the type-safe approach as a proposal in the spec
In particular, I think 2. is more important than 3.
Of course if you have ideas on the open questions feel free to continue
on this thread.