fix: respect transform output type for __in lookups (e.g. __year__in)

mypy_django_plugin/django/context.py

@@ -571,7 +571,29 @@ def resolve_lookup_expected_type(
             return self.get_field_lookup_exact_type(helpers.get_typechecker_api(ctx), field)
 
         if issubclass(lookup_cls, In):
-            exact_type = self.get_field_lookup_exact_type(helpers.get_typechecker_api(ctx), field)
+            api = helpers.get_typechecker_api(ctx)
+            exact_type = self.get_field_lookup_exact_type(api, field)
+            transform_parts = lookup_parts[:-1]
+            transformed_field = field
+            for transform_name in transform_parts:
+                if not isinstance(transformed_field, Field):
+                    break
+
+                transform_cls = transformed_field.get_transform(transform_name)
+                output_field = getattr(transform_cls, "output_field", None)
+                if not isinstance(output_field, Field):
+                    break
+
+                transformed_field = output_field
+            else:
+                if transform_parts:
+                    field_info = helpers.lookup_class_typeinfo(api, transformed_field.__class__)
+                    if field_info is None:
+                        exact_type = AnyType(TypeOfAny.explicit)
+                    else:
+                        exact_type = helpers.get_private_descriptor_type(
+                            field_info, "_pyi_private_get_type", is_nullable=transformed_field.null
+                        )
             return ctx.api.named_generic_type("typing.Iterable", [exact_type])
 
         resolved_type = self._resolve_lookup_type_from_lookup_class(ctx, lookup_cls, field)

tests/typecheck/managers/querysets/test_filter.yml

@@ -87,6 +87,7 @@
 
 -   case: valid_filter_with_lookup
     main: |
+        from datetime import date
         from myapp.models import User
 
         # Simple lookups which expect the same type.
@@ -105,6 +106,16 @@
         User.objects.filter(age__in=[User()])  # Doesn't error because mypy sees the provided type as `list[Any]`.
         users: list[User] = [User()]
         User.objects.filter(age__in=users)  # E: Incompatible type for lookup 'age__in': (got "list[User]", expected "Iterable[str | int]")  [misc]
+
+        year: int = 2026
+        years: list[int] = [2025, 2026]
+        invalid_years: list[str] = ["a"]
+        dates: list[date] = [date(2026, 1, 1)]
+        User.objects.filter(start_date__year=year)
+        User.objects.filter(start_date__in=dates)
+        User.objects.filter(start_date__year__in=years)
+        User.objects.filter(start_date__year__in=123)  # E: Incompatible type for lookup 'start_date__year__in': (got "int", expected "Iterable[int]")  [misc]
+        User.objects.filter(start_date__year__in=invalid_years)  # E: Incompatible type for lookup 'start_date__year__in': (got "list[str]", expected "Iterable[int]")  [misc]
     installed_apps:
         - myapp
     files:
@@ -115,6 +126,7 @@
                 class User(models.Model):
                     username = models.CharField(max_length=100)
                     age = models.IntegerField()
+                    start_date = models.DateField()
 
 
 -   case: invalid_filter_with_lookup