Fixed #33613 -- Made createsuperuser detect uniqueness of USERNAME_FIELD when using Meta.constraints.

tests/auth_tests/models/with_unique_constraint.py

@@ -0,0 +1,22 @@
+from django.contrib.auth.models import AbstractBaseUser, BaseUserManager
+from django.db import models
+
+
+class CustomUserWithUniqueConstraintManager(BaseUserManager):
+    def create_superuser(self, username, password):
+        user = self.model(username=username)
+        user.set_password(password)
+        user.save(using=self._db)
+        return user
+
+
+class CustomUserWithUniqueConstraint(AbstractBaseUser):
+    username = models.CharField(max_length=150)
+
+    objects = CustomUserWithUniqueConstraintManager()
+    USERNAME_FIELD = "username"
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(fields=["username"], name="unique_custom_username"),
+        ]