Username validation allows '@' character, while auth backend explicitly prevents this.

[sam@…]

When editing users, the help_text for username states: "Required. 30 characters or fewer. Letters, digits and @/./+/-/_ only."
However, when trying to log in with a username that (thus validly) contains "@", you get the error "Usernames cannot contain the '@' character.".

Of course, the login backend can be replaced with a custom one, but if using a non-existent username containing "@", you still get the contradictory error message.

I think the (useless) error message should be removed if the model allows "@".

[Karen Tracey]

This is #8342.