App: Regression registry_cls no longer had any effect. Closes #3613

celery/app/base.py

@@ -261,7 +261,7 @@ class Celery(object):
         self._pending = deque()
         self._tasks = tasks
         if not isinstance(self._tasks, TaskRegistry):
-            self._tasks = TaskRegistry(self._tasks or {})
+            self._tasks = self.registry_cls(self._tasks or {})
 
         # If the class defines a custom __reduce_args__ we need to use
         # the old way of pickling apps: pickling a list of

t/unit/app/test_app.py

@@ -901,6 +901,17 @@ class test_App:
         beat = self.app.Beat()
         assert isinstance(beat, Beat)
 
+    def test_registry_cls(self):
+
+        class TaskRegistry(self.app.registry_cls):
+            pass
+
+        class CustomCelery(type(self.app)):
+            registry_cls = TaskRegistry
+
+        app = CustomCelery(set_as_current=False)
+        assert isinstance(app.tasks, TaskRegistry)
+
 
 class test_defaults: