I don't really understand how this is relevant. Sure, memoization is more efficient for recursive fibonacci, but in both cases the code is correct. The author

I don't really understand how this is relevant. Sure, memoization is more efficient for recursive fibonacci, but in both cases the code is correct. The author is presenting a case in which code is made demonstrably incorrect by seemingly unrelated changes, because those changes altered invariants about the objects the author was using - making his assumptions about which actions were safe incorrect. That's a good time for tooling to throw warnings/errors. Where you seem to be proposing... nothing?