Lean Kernel Arena / corner-cases/nested-nonuniform-param

Checks that a parameter supplied to a nested inductive occurrence really acts as the datatype's parameter, i.e. that it is the parameter itself and does not change between recursive occurrences (as is already enforced for non-nested occurrences).

The inductive E : W → Type has constructor E.mk : (w : W) → L (E ⟨false⟩) → E w, where L (α : Type) is nested. The occurrence E ⟨false⟩ inside the nested L uses the constant ⟨false⟩ in the position of E's parameter, instead of the actual parameter w. That argument is type-correct, so it is not caught by merely type-checking the nested application (leanprover/lean4#14577); a correct checker must also verify that it is the expected parameter.

This particular declaration is not known to yield a proof of False: here L stores no value of type α, so the nested occurrence is phantom and E w is isomorphic to Unit for every w. The variant where L actually stores an α (so recursion would descend into an E ⟨false⟩ while the motive is fixed at E w) is already rejected by the kernel's positivity check ("non valid occurrence"). Since it is not a demonstrated unsoundness, it is not settled whether a checker should accept or reject it, so the expected outcome is either and the test does not count towards completeness or soundness.

Origin: raised by @arthur-adjedj on leanprover/lean4#14577 (https://github.com/leanprover/lean4/pull/14577#issuecomment-5101819377) as a case not covered by that PR's fix; related to leanprover/lean4#14576.