Fixed #30097 -- Made 'obj' arg of InlineModelAdmin.has_add_permission() optional.

Restored backwards compatibility after refs #27991.
Regression in be6ca89396.

tests/modeladmin/tests.py

@@ -734,6 +734,10 @@ class ModelAdminPermissionTests(SimpleTestCase):
         def has_perm(self, perm):
             return perm == 'modeladmin.add_band'
 
+    class MockAddUserWithInline(MockUser):
+        def has_perm(self, perm):
+            return perm == 'modeladmin.add_concert'
+
     class MockChangeUser(MockUser):
         def has_perm(self, perm):
             return perm == 'modeladmin.change_band'
@@ -793,6 +797,26 @@ class ModelAdminPermissionTests(SimpleTestCase):
         self.assertEqual(len(inline_instances), 1)
         self.assertIsInstance(inline_instances[0], ConcertInline)
 
+    def test_inline_has_add_permission_without_obj(self):
+        # This test will be removed in Django 3.1 when `obj` becomes a required
+        # argument of has_add_permission() (#27991).
+        class ConcertInline(TabularInline):
+            model = Concert
+
+            def has_add_permission(self, request):
+                return super().has_add_permission(request)
+
+        class BandAdmin(ModelAdmin):
+            inlines = [ConcertInline]
+
+        ma = BandAdmin(Band, AdminSite())
+        request = MockRequest()
+        request.user = self.MockAddUserWithInline()
+        band = Band(name='The Doors', bio='', sign_date=date(1965, 1, 1))
+        inline_instances = ma.get_inline_instances(request, band)
+        self.assertEqual(len(inline_instances), 1)
+        self.assertIsInstance(inline_instances[0], ConcertInline)
+
     def test_has_change_permission(self):
         """
         has_change_permission returns True for users who can edit objects and