Lean Kernel Arena / bugs/proj-of-imax-prop

A closed proof of False, with no axioms, via a data projection out of a proposition whose sort is Prop only up to universe level normalization.

ImaxProp : Sort (imax 1 0) is a proposition, since imax 1 0 normalizes to 0. The exploit uses two definitionally equal spellings of that type. Proof irrelevance is stated through ImaxAsProp : Prop := ImaxProp, whose type is the literal Sort 0, so it is accepted; the data projection imaxProjBool is stated on ImaxProp, whose type is the literal Sort (imax 1 0). A kernel that tests sorts for Prop syntactically does not recognize the latter as a proposition and wrongly allows projecting its Bool field out of a proof. Congruence on the proof-irrelevance equation then equates false and true, giving False.

This is https://github.com/leanprover/lean4/pull/14613, a bug in the official kernel.