In #24722 there is a massive change that turns docblocks into type hints. I think it would be a good idea to make an automated test in coding standard for it.

It would make it simpler to make sure that all new features and refactorings use return type hints.

Also as more and more code gets converted to type hints it will be harder and harder to find the places where type hints are missing.

Even if it were too hard to do it all at once, it would be possible to keep in branch, rebase and use it every now and then in newly created PR adding more type hints.

What do you think? Is it feasible? I wouldn't mind making a PR for the CS check, if it makes sense.