Fixed #27673 -- Made admin's checks run at check time instead of during registration.

Thanks Morgan Aubert for the test.

tests/admin_checks/tests.py

@@ -7,7 +7,9 @@ from django.contrib.contenttypes.admin import GenericStackedInline
 from django.core import checks
 from django.test import SimpleTestCase, override_settings
 
-from .models import Album, Book, City, Influence, Song, State, TwoAlbumFKAndAnE
+from .models import (
+    Album, Author, Book, City, Influence, Song, State, TwoAlbumFKAndAnE,
+)
 
 
 class SongForm(forms.ModelForm):
@@ -52,7 +54,6 @@ class SystemChecksTestCase(SimpleTestCase):
             self.assertEqual(errors, expected)
         finally:
             admin.site.unregister(Song)
-            admin.sites.system_check_errors = []
 
     @override_settings(INSTALLED_APPS=['django.contrib.admin'])
     def test_contenttypes_dependency(self):
@@ -105,7 +106,27 @@ class SystemChecksTestCase(SimpleTestCase):
             self.assertEqual(errors, expected)
         finally:
             custom_site.unregister(Song)
-            admin.sites.system_check_errors = []
+
+    @override_settings(DEBUG=True)
+    def test_allows_checks_relying_on_other_modeladmins(self):
+        class MyBookAdmin(admin.ModelAdmin):
+            def check(self, **kwargs):
+                errors = super(MyBookAdmin, self).check(**kwargs)
+                author_admin = self.admin_site._registry.get(Author)
+                if author_admin is None:
+                    errors.append('AuthorAdmin missing!')
+                return errors
+
+        class MyAuthorAdmin(admin.ModelAdmin):
+            pass
+
+        admin.site.register(Book, MyBookAdmin)
+        admin.site.register(Author, MyAuthorAdmin)
+        try:
+            self.assertEqual(admin.site.check(None), [])
+        finally:
+            admin.site.unregister(Book)
+            admin.site.unregister(Author)
 
     def test_field_name_not_in_list_display(self):
         class SongAdmin(admin.ModelAdmin):