Emit error and set fallback type for managers that can't be resolved (#999)

* Emit error and set fallback type for managers that can't be resolved

* fixup! Emit error and set fallback type for managers that can't be resolved

tests/typecheck/managers/test_managers.yml

@@ -442,3 +442,82 @@
                 class MyModel(models.Model):
                     site = models.ForeignKey(Site, on_delete=models.CASCADE)
                     on_site = CurrentSiteManager()
+
+-   case: test_emits_error_for_unresolved_managers
+    main: |
+        from myapp import models
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+                from django.db import models
+
+                def LocalManager() -> models.Manager:
+                    """
+                    Returns a manager instance of an inlined manager type that can't
+                    be resolved.
+                    """
+                    class InnerManager(models.Manager):
+                        ...
+
+                    return InnerManager()
+
+                class User(models.Model):
+                    name = models.TextField()
+
+                class Booking(models.Model):
+                    renter = models.ForeignKey(User, on_delete=models.PROTECT)
+                    owner = models.ForeignKey(User, on_delete=models.PROTECT, related_name='bookingowner_set')
+
+                    objects = LocalManager()
+
+                class TwoUnresolvable(models.Model):
+                    objects = LocalManager()
+                    second_objects = LocalManager()
+
+                class AbstractUnresolvable(models.Model):
+                    objects = LocalManager()
+
+                    class Meta:
+                        abstract = True
+
+                class InvisibleUnresolvable(AbstractUnresolvable):
+                    text = models.TextField()
+
+                def process_booking(user: User):
+                    reveal_type(User.objects)
+                    reveal_type(User._default_manager)
+
+                    reveal_type(Booking.objects)
+                    reveal_type(Booking._default_manager)
+
+                    reveal_type(TwoUnresolvable.objects)
+                    reveal_type(TwoUnresolvable.second_objects)
+                    reveal_type(TwoUnresolvable._default_manager)
+
+                    reveal_type(InvisibleUnresolvable.objects)
+                    reveal_type(InvisibleUnresolvable._default_manager)
+
+                    reveal_type(user.bookingowner_set)
+                    reveal_type(user.booking_set)
+    out: |
+        myapp/models:13: error: Couldn't resolve related manager for relation 'booking' (from myapp.models.Booking.myapp.Booking.renter).
+        myapp/models:13: error: Couldn't resolve related manager for relation 'bookingowner_set' (from myapp.models.Booking.myapp.Booking.owner).
+        myapp/models:20: error: Could not resolve manager type for "myapp.models.Booking.objects"
+        myapp/models:23: error: Could not resolve manager type for "myapp.models.TwoUnresolvable.objects"
+        myapp/models:24: error: Could not resolve manager type for "myapp.models.TwoUnresolvable.second_objects"
+        myapp/models:27: error: Could not resolve manager type for "myapp.models.AbstractUnresolvable.objects"
+        myapp/models:32: error: Could not resolve manager type for "myapp.models.InvisibleUnresolvable.objects"
+        myapp/models:36: note: Revealed type is "django.db.models.manager.Manager[myapp.models.User]"
+        myapp/models:37: note: Revealed type is "django.db.models.manager.Manager[myapp.models.User]"
+        myapp/models:39: note: Revealed type is "django.db.models.manager.Manager[myapp.models.Booking]"
+        myapp/models:40: note: Revealed type is "django.db.models.manager.BaseManager[myapp.models.Booking]"
+        myapp/models:42: note: Revealed type is "django.db.models.manager.Manager[myapp.models.TwoUnresolvable]"
+        myapp/models:43: note: Revealed type is "django.db.models.manager.Manager[myapp.models.TwoUnresolvable]"
+        myapp/models:44: note: Revealed type is "django.db.models.manager.BaseManager[myapp.models.TwoUnresolvable]"
+        myapp/models:46: note: Revealed type is "django.db.models.manager.Manager[myapp.models.InvisibleUnresolvable]"
+        myapp/models:47: note: Revealed type is "django.db.models.manager.BaseManager[myapp.models.InvisibleUnresolvable]"
+        myapp/models:49: note: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Booking]"
+        myapp/models:50: note: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Booking]"