Fixed #24922 -- Added system check for templates setting

If `'loaders'` is present in the `TEMPLATES` options together with
`APP_DIRS` set to `True`, the template engine raises an exception. This
conflict is now detected by the system check templates.E001.

django/core/checks/__init__.py

@@ -11,6 +11,7 @@ import django.core.checks.model_checks  # NOQA
 import django.core.checks.security.base  # NOQA
 import django.core.checks.security.csrf  # NOQA
 import django.core.checks.security.sessions  # NOQA
+import django.core.checks.templates  # NOQA
 
 __all__ = [
     'CheckMessage',

django/core/checks/registry.py

@@ -15,6 +15,7 @@ class Tags(object):
     models = 'models'
     security = 'security'
     signals = 'signals'
+    templates = 'templates'
 
 
 class CheckRegistry(object):

django/core/checks/templates.py

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.conf import settings
+
+from . import Error, Tags, register
+
+E001 = Error(
+    "You have 'APP_DIRS': True in your TEMPLATES but also specify 'loaders' "
+    "in OPTIONS. Either remove APP_DIRS or remove the 'loaders' option.",
+    id='templates.E001',
+)
+
+
+@register(Tags.templates)
+def check_setting_app_dirs_loaders(app_configs, **kwargs):
+    passed_check = True
+    for conf in settings.TEMPLATES:
+        if not conf.get('APP_DIRS'):
+            continue
+        if 'loaders' in conf.get('OPTIONS', {}):
+            passed_check = False
+    return [] if passed_check else [E001]

docs/ref/checks.txt

@@ -491,3 +491,13 @@ If you're using MySQL, the following checks will be performed:
 
 * **mysql.E001**: MySQL does not allow unique ``CharField``\s to have a
   ``max_length`` > 255.
+
+Templates
+---------
+
+The following checks verify that your :setting:`TEMPLATES` setting is correctly
+configured:
+
+* **templates.E001**: You have ``'APP_DIRS': True`` in your
+  :setting:`TEMPLATES` but also specify ``'loaders'`` in ``OPTIONS``. Either
+  remove ``APP_DIRS`` or remove the ``'loaders'`` option.

tests/check_framework/test_templates.py

@@ -0,0 +1,39 @@
+from django.core.checks.templates import E001
+from django.test import SimpleTestCase
+from django.test.utils import override_settings
+
+
+class CheckTemplateSettingsAppDirsTest(SimpleTestCase):
+    TEMPLATES_APP_DIRS_AND_LOADERS = [
+        {
+            'BACKEND': 'django.template.backends.django.DjangoTemplates',
+            'APP_DIRS': True,
+            'OPTIONS': {
+                'loaders': ['django.template.loaders.filesystem.Loader'],
+            },
+        },
+    ]
+
+    @property
+    def func(self):
+        from django.core.checks.templates import check_setting_app_dirs_loaders
+        return check_setting_app_dirs_loaders
+
+    @override_settings(TEMPLATES=TEMPLATES_APP_DIRS_AND_LOADERS)
+    def test_app_dirs_and_loaders(self):
+        """
+        Error if template loaders are specified and APP_DIRS is True.
+        """
+        self.assertEqual(self.func(None), [E001])
+
+    def test_app_dirs_removed(self):
+        TEMPLATES = self.TEMPLATES_APP_DIRS_AND_LOADERS[:]
+        del TEMPLATES[0]['APP_DIRS']
+        with self.settings(TEMPLATES=TEMPLATES):
+            self.assertEqual(self.func(None), [])
+
+    def test_loaders_removed(self):
+        TEMPLATES = self.TEMPLATES_APP_DIRS_AND_LOADERS[:]
+        del TEMPLATES[0]['OPTIONS']['loaders']
+        with self.settings(TEMPLATES=TEMPLATES):
+            self.assertEqual(self.func(None), [])