Fixed #19150 -- Added validation for USERNAME_FIELD being included in REQUIRED_FIELDS.

import sys

from django.conf import settings

from django.core.management.color import color_style

from django.utils.encoding import force_str

from django.utils.itercompat import is_iterable

            # No need to perform any other validation checks on a swapped model.

            continue

        # This is the current User model. Check known validation problems with User models

        if settings.AUTH_USER_MODEL == '%s.%s' % (opts.app_label, opts.object_name):

            # Check that the USERNAME FIELD isn't included in REQUIRED_FIELDS.

            if cls.USERNAME_FIELD in cls.REQUIRED_FIELDS:

                e.add(opts, 'The field named as the USERNAME_FIELD should not be included in REQUIRED_FIELDS on a swappable User model.')

        # Model isn't swapped; do field-specific validation.

        for f in opts.local_fields:

            if f.name == 'id' and not f.primary_key and opts.pk.name == 'id':