Lean Kernel Arena / perf/irrelevance-before-evaluation

The declaration to check is

slowTriv (count #n) = True.intro

proved by Eq.refl, where slowTriv m : True recurses over m, so forcing it to a constructor evaluates the numeral in Θ(n²) reductions.

Checking compares the two proofs as arguments of Eq, whose head is rigid: nothing can be unfolded instead. Proof irrelevance settles the proofs by their type for Θ(1); evaluating the left one to a constructor costs Θ(n²) and yields the answer irrelevance already gave. The test asks whether a checker consults proof irrelevance before it reduces.

N=200 in the Lean source.