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.

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), [])