additional defer() to offset the effect of using the type before decl… (#424)

* additional defer() to offset the effect of using the type before declaring

* linter fix

* linter fix

* linter fix

* test case

Co-authored-by: Kacper Szmigiel <szmigielkacper@gmai.com>

mypy_django_plugin/lib/helpers.py

@@ -343,6 +343,7 @@ def copy_method_to_another_class(ctx: ClassDefContext, self_type: Instance,
     arguments = []
     bound_return_type = semanal_api.anal_type(method_type.ret_type,
                                               allow_placeholder=True)
+
     assert bound_return_type is not None
 
     if isinstance(bound_return_type, PlaceholderNode):
@@ -352,6 +353,10 @@ def copy_method_to_another_class(ctx: ClassDefContext, self_type: Instance,
                                                      method_type.arg_types[1:],
                                                      method_node.arguments[1:]):
         bound_arg_type = semanal_api.anal_type(arg_type, allow_placeholder=True)
+        if bound_arg_type is None and not semanal_api.final_iteration:
+            semanal_api.defer()
+            return
+
         assert bound_arg_type is not None
 
         if isinstance(bound_arg_type, PlaceholderNode):

test-data/typecheck/managers/test_managers.yml

@@ -335,3 +335,25 @@
                     objects = MyManager()
                 class ChildUser(models.Model):
                     objects = MyManager()
+
+-   case: custom_manager_annotate_method_before_type_declaration 
+    main: |
+        from myapp.models import ModelA, ModelB, ManagerA
+        reveal_type(ModelA.objects)  # N: Revealed type is 'myapp.models.ModelA_ManagerA1[myapp.models.ModelA]'
+        reveal_type(ModelA.objects.do_something)  # N: Revealed type is 'def (other_obj: myapp.models.ModelB) -> builtins.str'
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+                from django.db import models
+                class ManagerA(models.Manager):
+                    def do_something(self, other_obj: "ModelB") -> str:
+                        return 'test'
+                class ModelA(models.Model):
+                    title = models.TextField()
+                    objects = ManagerA()
+                class ModelB(models.Model):
+                    movie = models.TextField()
+