Heh, recently I had to fix a bug in some code that had one of these comments. Feels like a sign of bad code or laziness. Why make a path that should not happen? I can get it when it's on some while loop that should find something to return, but on a if else sequence it feels really wrong.
Strong disagree about laziness. If the dev is lazy they will not make a path for it. When they are not lazy they actually make a path and write a comment explaining why they think this is unreachable. Taking the time to write a comment is not a sign of laziness. It’s the complete opposite. You can debate whether the comment is detailed enough to convey why the dev thinks it’s unreachable, but it’s infinitely better than no comment and leaving the unreachability in their head.
Before sealed classes and ultra-robust type checking, sometimes private functions would have, say, 3 states that should be possible, but 3 years later, a new state is added but wasn’t checked because the compiler didn’t stop it because the language didn’t support it at that time.
It's much better to have a `panic!("this should never happen")` statement than to let your program get into an inconsistent state and then keep going. Ideally, you can use your type system to make inconsistent states impossible, but type systems can only express so much. Even Haskell can't enforce typeclass laws at the compiler level.
A program that never asserts its invariants is much more likely to be a program that breaks those invariants than a program that probably doesn't.
