Complained on override_settings(INSTALLED_APPS=...).

Currently such overrides aren't reflected in the app cache.

It would  be possible to handle them. But that doesn't look like a very
good API. It makes it complicated to express "add this app" and "remove
this app", which are the most common operations on INSTALLED_APPS.

django/test/signals.py

@@ -17,7 +17,7 @@ setting_changed = Signal(providing_args=["setting", "value", "enter"])
 # except for cases where the receiver is related to a contrib app.
 
 # Settings that may not work well when using 'override_settings' (#19031)
-COMPLEX_OVERRIDE_SETTINGS = set(['DATABASES'])
+COMPLEX_OVERRIDE_SETTINGS = set(['DATABASES', 'INSTALLED_APPS'])
 
 
 @receiver(setting_changed)

tests/app_loading/tests.py

@@ -3,6 +3,7 @@ from __future__ import unicode_literals
 import os
 import sys
 from unittest import TestCase
+import warnings
 
 from django.core.apps import app_cache
 from django.core.apps.cache import AppCache
@@ -77,11 +78,13 @@ class EggLoadingTest(TestCase):
         # Pretend we're the master app cache to test the population process.
         app_cache._apps_loaded = False
         app_cache._models_loaded = False
-        with override_settings(INSTALLED_APPS=('notexists',)):
-            with self.assertRaises(ImportError):
-                app_cache.get_model('notexists', 'nomodel')
-            with self.assertRaises(ImportError):
-                app_cache.get_model('notexists', 'nomodel')
+        with warnings.catch_warnings():
+            warnings.filterwarnings("ignore", "Overriding setting INSTALLED_APPS")
+            with override_settings(INSTALLED_APPS=['notexists']):
+                with self.assertRaises(ImportError):
+                    app_cache.get_model('notexists', 'nomodel')
+                with self.assertRaises(ImportError):
+                    app_cache.get_model('notexists', 'nomodel')
 
 
 class GetModelsTest(TestCase):