I read about French passport renewals (Google Translate):
The old passport must be returned. If it included a visa, it can be kept for the period of validity of this visa.
Assume that the old passport did include a valid visa, thereby allowing the passport owner to keep it when retrieving the new passport. What happens if one doesn't return the old passport once the visa on the old passport expired?
Original quote in French:
L'ancien passeport doit être restitué. S'il comportait un visa, il peut être conservé pendant la durée de validité de ce visa.