Add failing test for abstract page managers

Custom page managers set on abstract pages would cause an error on start
up, as accessing the manager of an abstract page is invalid.

wagtail/tests/testapp/migrations/0023_mycustompage.py

@@ -0,0 +1,27 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9 on 2015-12-23 22:49
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailcore', '0023_alter_page_revision_on_delete_behaviour'),
+        ('tests', '0022_uploaded_by_user_on_delete_set_null'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='MyCustomPage',
+            fields=[
+                ('page_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='wagtailcore.Page')),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=('wagtailcore.page',),
+        ),
+    ]

wagtail/tests/testapp/models.py

@@ -588,3 +588,17 @@ class CustomManager(PageManager):
 
 class CustomManagerPage(Page):
     objects = CustomManager()
+
+
+class MyBasePage(Page):
+    """
+    A base Page model, used to set site-wide defaults and overrides.
+    """
+    objects = CustomManager()
+
+    class Meta:
+        abstract = True
+
+
+class MyCustomPage(MyBasePage):
+    pass

wagtail/wagtailcore/tests/test_page_model.py

@@ -17,7 +17,7 @@ from wagtail.tests.testapp.models import (
     MTIBasePage, MTIChildPage, AbstractPage, TaggedPage,
     BlogCategory, BlogCategoryBlogPage, Advert, ManyToManyBlogPage,
     GenericSnippetPage, BusinessNowherePage, SingletonPage,
-    CustomManager, CustomManagerPage)
+    CustomManager, CustomManagerPage, MyCustomPage)
 from wagtail.tests.utils import WagtailTestUtils
 
 
@@ -1028,3 +1028,11 @@ class TestPageManager(TestCase):
         custom Manager inherits from PageManager.
         """
         self.assertIs(type(CustomManagerPage.objects), CustomManager)
+
+    def test_abstract_base_page_manager(self):
+        """
+        Abstract base classes should be able to override their default Manager,
+        and Wagtail should respect this. It is up to the developer to ensure
+        their custom Manager inherits from PageManager.
+        """
+        self.assertIs(type(MyCustomPage.objects), CustomManager)