This was a top of my head example of how you fan catch many invariants, not meant to be exhaustive perfect code. I was also implying the user type is something

This was a top of my head example of how you fan catch many invariants, not meant to be exhaustive perfect code. I was also implying the user type is something you use within your application, not something you expose to other people directly. If your application has to expose internal data it makes often more sense to add a specific interface for it (e.g. an API with versions). If there is code where people rely on your internal types, it is your responsibility to use semantic versioning to communicate breaking changes, while it is the other person's responsibility to assume breaking changes when versions change. If we are talking about someone other relying on your user to have 3 fields that is a collegue: it is a good idea to test your code against the whole codebase. Then you know ahead what will fail and can act accordingly.