Comment by bern4444

Yeah that's the engineering part in software engineer :)

If you have VerifiedUserWithBirthday, any value that fails the parsing function is implicitly UnverifiedUserOrUserWithoutBirthday... No need to define it separately. You get the inverse type for free IE a value that is of type User and not of type VerifiedUserWithBirthday.

A new property doesn't mean a new derived type. Only if that new property impacts what a VerifiedUserWithBirthday should represent should the VerifiedUserWithBirthday type be updated and even then, it's not a new type, just an update to an existing type. Again minimal updates needed.

The compiler handles all the validation and will tell you exactly where there are any issues - the compiler is what makes the maintenance cost quite low.