Properly app_template_dirs when INSTALLED_APPS change.

django/template/loaders/app_directories.py

@@ -13,7 +13,8 @@ from django.template.loader import BaseLoader
 from django.utils._os import safe_join
 from django.utils import six
 
-# At compile time, cache the directories to search.
+
+def calculate_app_template_dirs():
     if six.PY2:
         fs_encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
     app_template_dirs = []
@@ -23,9 +24,11 @@ for app_config in apps.get_app_configs():
             if six.PY2:
                 template_dir = template_dir.decode(fs_encoding)
             app_template_dirs.append(template_dir)
+    return tuple(app_template_dirs)
 
-# It won't change, so convert it to a tuple to save memory.
-app_template_dirs = tuple(app_template_dirs)
+
+# At compile time, cache the directories to search.
+app_template_dirs = calculate_app_template_dirs()
 
 
 class Loader(BaseLoader):

django/test/signals.py

@@ -33,6 +33,9 @@ def update_installed_apps(**kwargs):
         # Rebuild any AppDirectoriesFinder instance.
         from django.contrib.staticfiles.finders import get_finder
         get_finder.cache_clear()
+        # Rebuild app_template_dirs cache.
+        from django.template.loaders import app_directories as mod
+        mod.app_template_dirs = mod.calculate_app_template_dirs()
 
 
 @receiver(setting_changed)