@fabpot  Is my fault I didn't think that would be BC (just updated the table to BC). But you don't like the idea of a method isEmpty() at all?

If you like, there is a way of doing this without BC, a new interface ConstraintViolationListExtendedInterface could be added that would extend ConstraintViolationListInterface and ConstraintViolationList  class would implement it instead of ConstraintViolationListInterface. The method isEmpty() would be added at the extended interface. Is this acceptable? It would take more lines of code, but I can do it.