Don't modify return values of manager methods in place, modify a copy…

… instead.

mypy_django_plugin/transformers/managers.py

@@ -1,6 +1,7 @@
 from typing import Final, Optional, Union
 
 from mypy.checker import TypeChecker
+from mypy.copytype import copy_type
 from mypy.nodes import (
     GDEF,
     CallExpr,
@@ -121,7 +122,7 @@ def _process_dynamic_method(
     is_fallback_queryset: bool,
 ) -> CallableType:
     variables = method_type.variables
-    ret_type = method_type.ret_type
+    ret_type = copy_type(method_type.ret_type)
 
     manager_model = get_proper_type(find_member("model", manager_instance, manager_instance))
     assert isinstance(manager_model, TypeType), manager_model

tests/typecheck/managers/querysets/test_as_manager.yml

@@ -236,6 +236,7 @@
         reveal_type(MyModel.objects.dummy_override())  # N: Revealed type is "myapp.models.MyModel"
         reveal_type(MyModel.objects.example_mixin(MyModel()))  # N: Revealed type is "myapp.models.MyModel"
         reveal_type(MyModel.objects.example_other_mixin())  # N: Revealed type is "myapp.models.MyModel"
+        reveal_type(MyModel.objects.example_union())  # N: Revealed type is "Union[myapp.models.MyModel, None]"
         reveal_type(MyOtherModel.objects.example())  # N: Revealed type is "myapp.models.MyOtherModel"
         reveal_type(MyOtherModel.objects.example_2())  # N: Revealed type is "myapp.models.MyOtherModel"
         reveal_type(MyOtherModel.objects.override())  # N: Revealed type is "myapp.models.MyOtherModel"
@@ -245,13 +246,14 @@
         reveal_type(MyOtherModel.objects.example_other_mixin())  # N: Revealed type is "myapp.models.MyOtherModel"
         reveal_type(MyOtherModel.objects.test_self())  # N: Revealed type is "myapp.models._MyModelQuerySet2[myapp.models.MyOtherModel]"
         reveal_type(MyOtherModel.objects.test_sub_self())  # N: Revealed type is "myapp.models._MyModelQuerySet2[myapp.models.MyOtherModel]"
+        reveal_type(MyOtherModel.objects.example_union())  # N: Revealed type is "Union[myapp.models.MyOtherModel, None]"
     installed_apps:
         - myapp
     files:
         -   path: myapp/__init__.py
         -   path: myapp/models.py
             content: |
-              from typing import TypeVar, Generic
+              from typing import TypeVar, Generic, Optional
               from django.db import models
               from typing_extensions import Self
 
@@ -263,6 +265,7 @@
 
               class OtherMixin(models.QuerySet[T]):
                   def example_other_mixin(self) -> T: ...
+                  def example_union(self) -> Optional[T]: ...
 
               class _MyModelQuerySet(OtherMixin[T], models.QuerySet[T], Generic[T]):
                   def example(self) -> T: ...