Fixed #34438 -- Reallowed extending UserCreationForm.

Regression in 298d02a77a.

django/contrib/auth/forms.py

@@ -147,16 +147,22 @@ class BaseUserCreationForm(forms.ModelForm):
 
 
 class UserCreationForm(BaseUserCreationForm):
-    error_messages = {
-        **BaseUserCreationForm.error_messages,
-        "unique": _("A user with that username already exists."),
-    }
-
     def clean_username(self):
         """Reject usernames that differ only in case."""
         username = self.cleaned_data.get("username")
-        if username and User.objects.filter(username__iexact=username).exists():
-            raise forms.ValidationError(self.error_messages["unique"], code="unique")
+        if (
+            username
+            and self._meta.model.objects.filter(username__iexact=username).exists()
+        ):
+            self._update_errors(
+                ValidationError(
+                    {
+                        "username": self.instance.unique_error_message(
+                            self._meta.model, ["username"]
+                        )
+                    }
+                )
+            )
         else:
             return username
 

django/contrib/auth/locale/en/LC_MESSAGES/django.po

@@ -77,10 +77,6 @@ msgstr ""
 msgid "Enter the same password as before, for verification."
 msgstr ""
 
-#: contrib/auth/forms.py:152 contrib/auth/models.py:353
-msgid "A user with that username already exists."
-msgstr ""
-
 #: contrib/auth/forms.py:168
 msgid ""
 "Raw passwords are not stored, so there is no way to see this user’s "
@@ -240,6 +236,10 @@ msgstr ""
 msgid "Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only."
 msgstr ""
 
+#: contrib/auth/models.py:353
+msgid "A user with that username already exists."
+msgstr ""
+
 #: contrib/auth/models.py:356
 msgid "first name"
 msgstr ""