Return ActionDispatch.test_app when no app is set on IntegrationTest.app method

Fixes #25926

2 files changed, 20 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/testing/integration.rb b/actionpack/lib/action_dispatch/testing/integration.rb
index 4897f44268..c6577d3d15 100644
--- a/actionpack/lib/action_dispatch/testing/integration.rb
+++ b/actionpack/lib/action_dispatch/testing/integration.rb
@@ -717,7 +717,7 @@ module ActionDispatch
 
       module ClassMethods
         def app
-          defined?(@@app) ? @@app : ActionDispatch.test_app
+          @@app || ActionDispatch.test_app
         end
 
         def app=(app)
diff --git a/actionpack/test/controller/integration_test.rb b/actionpack/test/controller/integration_test.rb
index 3b89531e90..fc24857570 100644
--- a/actionpack/test/controller/integration_test.rb
+++ b/actionpack/test/controller/integration_test.rb
@@ -397,6 +397,25 @@ class IntegrationTestUsesCorrectClass < ActionDispatch::IntegrationTest
   end
 end
 
+class IntegrationTestDefaultApp < ActionDispatch::IntegrationTest
+  def setup
+    @app = self.class.app
+    self.class.app = nil
+
+    @test_app = ActionDispatch.test_app
+    ActionDispatch.test_app = 'fake_app'
+  end
+
+  def teardown
+    self.class.app = @app
+    ActionDispatch.test_app = @test_app
+  end
+
+  def test_class_app_returns_ad_test_app_by_default
+    assert_equal ActionDispatch.test_app, self.class.app
+  end
+end
+
 class IntegrationProcessTest < ActionDispatch::IntegrationTest
   class IntegrationController < ActionController::Base
     def get