If a constraint targets a method's parameters as per validationAppliesTo() but has no cross-parameter validator, a constraint definition exception should be thrown:

If the constraint targets the parameters of an executable either implicitly or by the use of validationAppliesTo in the constraint - see 3.1.1.4, then the cross-parameter ConstraintValidator is used. If none is present, a ConstraintDefinitionException is raised.

The RI instead implicitly applies the constraint to the method's return value (or throws a declaration exception for void methods).