Tests for various emptyqs tickets

The tickets are either about different signature between qs.none() and
qs or problems with subclass types (either EmptyQS overrided the custom
qs class, or EmptyQS was overridden by another class - values() did
this).

Fixed #15959, fixed #17271, fixed #17712, fixed #19426

tests/modeltests/basic/tests.py

@@ -4,7 +4,7 @@ from datetime import datetime
 
 from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
 from django.db.models.fields import Field, FieldDoesNotExist
-from django.db.models.query import EmptyQuerySet
+from django.db.models.query import QuerySet, EmptyQuerySet, ValuesListQuerySet
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
 from django.utils.translation import ugettext_lazy
@@ -646,3 +646,43 @@ class ModelTest(TestCase):
         with self.assertRaises(TypeError):
             EmptyQuerySet()
         self.assertTrue(isinstance(Article.objects.none(), EmptyQuerySet))
+
+    def test_emptyqs_values(self):
+        # test for #15959
+        Article.objects.create(headline='foo', pub_date=datetime.now())
+        with self.assertNumQueries(0):
+            qs = Article.objects.none().values_list('pk')
+            self.assertTrue(isinstance(qs, EmptyQuerySet))
+            self.assertTrue(isinstance(qs, ValuesListQuerySet))
+            self.assertEqual(len(qs), 0)
+
+    def test_emptyqs_customqs(self):
+        # A hacky test for custom QuerySet subclass - refs #17271
+        Article.objects.create(headline='foo', pub_date=datetime.now())
+        class CustomQuerySet(QuerySet):
+            def do_something(self):
+                return 'did something'
+
+        qs = Article.objects.all()
+        qs.__class__ = CustomQuerySet
+        qs = qs.none()
+        with self.assertNumQueries(0):
+            self.assertEqual(len(qs), 0)
+            self.assertTrue(isinstance(qs, EmptyQuerySet))
+            self.assertEqual(qs.do_something(), 'did something')
+
+    def test_emptyqs_values_order(self):
+        # Tests for ticket #17712
+        Article.objects.create(headline='foo', pub_date=datetime.now())
+        with self.assertNumQueries(0):
+            self.assertEqual(len(Article.objects.none().values_list('id').order_by('id')), 0)
+        with self.assertNumQueries(0):
+            self.assertEqual(len(Article.objects.none().filter(
+                id__in=Article.objects.values_list('id', flat=True))), 0)
+
+    @skipUnlessDBFeature('can_distinct_on_fields')
+    def test_emptyqs_distinct(self):
+        # Tests for #19426
+        Article.objects.create(headline='foo', pub_date=datetime.now())
+        with self.assertNumQueries(0):
+            self.assertEqual(len(Article.objects.none().distinct('headline', 'pub_date')), 0)