The problem with this for me is that most of the time "verifying that our ccode does what it did yesterday" is not a useful condition : if you make no change to code, its going to do what it did yesterday. If you do make a change to the code, then you are probably intending for it to do something different, so now you have to change the test accordingly. It usually just means you have to make the same change in 2 different spots for every piece of unit-tested code you want to change.
> If you do make a change to the code, then you are probably intending for it to do something different, so now you have to change the test accordingly. It usually just means you have to make the same change in 2 different spots for every piece of unit-tested code you want to change.
Sure, but that's how unit-tested code works in general.
> then you are probably intending for it to do something different
If you have decided that your software is going to do something different, you probably want to deprecate the legacy functionality to give the users some time to adapt, not change how things work from beneath them. If you eventually remove what is deprecated, the tests can be deleted along with it. There should be no need for them to change except maybe in extreme circumstances (e.g. a feature under test has a security vulnerability that necessitates a breaking change).
If you are testing internal implementation details, where things are likely to change often... Don't do that. It's not particularly useful. Test as if you are the user. That is what you want to be consistent and well documented.
