The payload element was added to attach arbitrary data to any constraint.

If I now write a constraint and the related validator, I can access the constraint in the validator and therefore the payload from it.

The Callback validator is something special: it executes custom validation code, but is limited in that it has no access to the constraint object itself – and so no access to the payload.

Quesiton: should we pass the payload (or even the complete Callback constraint to be forward-compatible for future additional fields) into the validation method?