Which is exactly what's going on here. A username has a string as a payload, but that payload has restrictions (not every string will do) and methods which expect a username should get a username, not any old string.
Meanwhile the real world usages of this term I've seen in the past have all been things like enums as strings, lists as strings, numbers as strings, etc... Not arbitrary textual inputs from the user.
By having stronger types this won’t come up as a problem. You don’t have to rely on having the best programmers in the world that never make mistakes (tm) to be on your team and instead rely on the computer making guard rails for you so you can’t screw up minor things like that.
I would probably type to the level of Url, Email, Name but not PersonProfileTwitterLink.
In our codebase, we use wrapper classes and the only time we had a defect with this is when one developer got lazy and used 3 primitive Strings in a class instead of wrapper classes. Another developer needed to update the code and populated the wrong wrapper class as they were not as familiar with that part of the codebase. Had the original developer simply used wrapper classes, the person maintaining the code wouldn't have had that confusion.
That can be handled transparently in languages that have good support for strong type systems, like Rust or Haskell, using traits or type classes.
What you're saying is essentially that addressing stringly typing can only be taken so far in weakly typed languages, without becoming inconvenient.
> Meanwhile the real world usages of this term I've seen in the past have all been things like enums as strings, lists as strings, numbers as strings, etc... Not arbitrary textual inputs from the user.
The definitional question is not that interesting. The point is that the concept applies just as much to a username represented as a string as it does to any other kind of value being represented as a string.
The reason is simple, which is just that "string" is a general type that can represent anything, whereas "username" is a subset of all possible strings. If you're trying to use your type system to ensure correct code, you want to be able to type check a function signature like `f(user, company, motto)`, just to take a simple example.