User Admin models use `django.contrib.auth.models.User` instead of `User = get_user_model()`

[Ariel Pontes]

I have a custom User model that is exactly the same as Django's default User model except for the fact that we allow longer usernames (I'm on Django 1.9, which has a 30 character limit on usernames). Because this is the only difference, I use all default User models in the Admin. However, when I try to create a user with a long username, I get the 30 character limit error. In the shell everything works fine. When investigating I saw that in the django/contrib/auth/forms.py file, the UserCreationForm referred to django.contrib.auth.models.User. I believe it should refer instead to UserModel (which is declared in the file using get_user_model()).

Relevant lines:

• ​https://github.com/django/django/blob/stable/2.1.x/django/contrib/auth/forms.py#L86
• ​https://github.com/django/django/blob/stable/2.1.x/django/contrib/auth/forms.py#L135

[Tim Graham]

Duplicate of #28608.