> returning `bool` to `optional<bool>`.

In that case you need to pay attention to what you're doing. Semantically you'd be altering your code to convert a function that always returns a value into a function that maybe won't return it. How do you expect to replace a type with a wrapper that maybe won't wrap that type and still assume you won't need to check if a return value exists?

That would be like replacing a reference with a pointer and then forgetting to check if the pointer is null. Making this mistake has nothing to do with luck and has everything to do with incompetence, and no language specs saves you from that.