In a previous post there's a discussion on an intriguing example of a passive construction of a transitive (allegedly) deponent verb: Ab amīcīs hortārētur (Did Latin have any ergative verbs? ). The cautious parentheses are added because of the existence of the non-deponent forms horto, -are: e.g., cf. http://micmap.org/dicfro/search/gaffiot/hortor
As a follow-up question from this discussion, it is worth pointing out that impersonal passives of intransitive deponent verbs like *Mortuum est beate ‘{One/people} died happily’ are ill-formed, compared to ok Pugnatum est acriter. The explanation of this contrast has been argued to be related to the fact that the former is an unaccusative verb, whereas the latter is an unergative one (vid. https://en.wikipedia.org/wiki/Impersonal_passive_voice ; for the well-known distinction between unaccusative vs. unergative verbs, vid. https://en.wikipedia.org/wiki/Unaccusative_verb ).
This distinction notwithstanding, I was wondering why the previous impersonal passive construction with the deponent verb mori (*Mortuum est beate) is ill-formed whereas Moriendum est omnibus is well-formed. Any thoughts on this contrast?
Here are some additional remarks drawn from my comments on Cerberus's interesting answer/contribution below: intransitive agentive verbs (e.g., pugnare) can appear in impersonal passive constructions but intransitive non-agentive (i.e. "lexically passive") verbs (e.g., mori) cannot. In short, it seems that intransitive verbs with an already "lexically passive meaning" cannot appear in a syntactic passive construction (e.g., into an impersonal passive construction). NB: the ill-formed example *Mortuum est beate is an impersonal passive construction but the well-formed example Moriendum est nobis is also an impersonal construction (but, crucially, it is not passive).