A better suggestion: fix the defect. Just delete primary_extension. At best, it does nothing that the extensions array can't do, as it doesn't appear at first glance that any check to primary_extension does not also include a check to extensions.
At worst, it is confusing to implementers and requires chicanery to work around... which is exactly the case we're in. We're in the worst case scenario for this bit of code, and there is no upside to its best-case scenario. Just delete the code.
