If we're going that route, just forbid making User objects which are not valid. Hide the fields, force the instances to be immutable, validate in the constructo

If we're going that route, just forbid making User objects which are not valid. Hide the fields, force the instances to be immutable, validate in the constructor, problem solved. Does not work in things with duck typing or anything remotely resembling the two most used programming languages in new projects in the industry. But for traditional statically typed languages, it's a good solution. Now a function taking in a user is literally incapable of ever taking in an invalid user, assuming your tests for the class are good and pass.