It makes no sense to have a case-default clause inside a unique-case. The unique keyword is to inform that the case is full and that all case items are mutually exclusive. Any case with a case-default clause is, per definition, full. And, also per definition, a default-case-item will never match any other case-item. So, from a synthesis perspective, adding a case-default to a unique-case it is the same as having a regular-case. The only difference between regular-case and unique-case with case-default will be from simulation tool perspective: for the unique-case, it will generate a warning if two case-items match for the same case expression.