Lean Kernel Arena / other/orphan-ctor

Proof of False from a constructor of an inductive type that does not exist.

The export contains False exactly as the prelude has it — an empty Prop-valued inductive with no constructors, its ctors field is the empty list — together with its ordinary False.rec. Smuggled into the same inductive block is a constructor named rogue, of type False, with no parameters and no fields, whose induct field names Orphan, a name for which the export has no declaration at all. The theorem inconsistent : False is then simply rogue.

A checker must derive the constructors of an inductive group from the inductive declarations and reject any exported constructor that is not one of them; here that fails because False has no constructors, and the inductive type rogue claims to come from does not exist. A checker that instead registers exported constructors as given — or that only checks constructors whose induct field points at a declaration it knows — ends up with an inhabitant of the genuine empty type.

This is the constructor-side counterpart of bugs/orphan-rec: in both cases the bogus declaration escapes by not being attached to any inductive declaration that the checker verifies, rather than by disagreeing with one.