§1. The provision of an office which by law is not vacant is by that fact invalid and is not validated by subsequent vacancy.
§2. Nevertheless, if it concerns an office which by law is conferred for a determined period of time, provision can be made within six months before the expiration of this time and takes effect from the day of the vacancy of the office.
§3. A promise of some office, no matter by whom it is made, produces no juridic effect.
Source: Code of Canon Law (Vatican.va)