Made ModelForms raise ImproperlyConfigured if the list of fields is not specified.

Also applies to modelform(set)_factory and generic model views.

refs #19733.

tests/model_formsets/tests.py

@@ -6,6 +6,7 @@ from datetime import date
 from decimal import Decimal
 
 from django import forms
+from django.core.exceptions import ImproperlyConfigured
 from django.db import models
 from django.forms.models import (_get_foreign_key, inlineformset_factory,
     modelformset_factory, BaseModelFormSet)
@@ -131,6 +132,15 @@ class DeletionTests(TestCase):
 
 
 class ModelFormsetTest(TestCase):
+    def test_modelformset_factory_without_fields(self):
+        """ Regression for #19733 """
+        message = (
+            "Calling modelformset_factory without defining 'fields' or 'exclude' "
+            "explicitly is prohibited."
+        )
+        with self.assertRaisesMessage(ImproperlyConfigured, message):
+            modelformset_factory(Author)
+
     def test_simple_save(self):
         qs = Author.objects.all()
         AuthorFormSet = modelformset_factory(Author, fields="__all__", extra=3)