There are some cases where @@app is not defined

actionpack/lib/action_dispatch/testing/integration.rb

@@ -717,7 +717,11 @@ module Behavior
 
       module ClassMethods
         def app
-          @@app || ActionDispatch.test_app
+          if defined?(@@app) && @@app
+            @@app
+          else
+            ActionDispatch.test_app
+          end
         end
 
         def app=(app)

actionpack/test/controller/integration_test.rb

@@ -397,25 +397,6 @@ def test_integration_methods_called
   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 'fake_app', self.class.app
-  end
-end
-
 class IntegrationProcessTest < ActionDispatch::IntegrationTest
   class IntegrationController < ActionController::Base
     def get

railties/test/application/integration_test_case_test.rb

@@ -42,4 +42,32 @@ class MailerIntegrationTest < ActionDispatch::IntegrationTest
       assert_match(/0 failures, 0 errors/, output)
     end
   end
+
+  class IntegrationTestDefaultApp < ActiveSupport::TestCase
+    include ActiveSupport::Testing::Isolation
+
+    setup do
+      build_app
+    end
+
+    teardown do
+      teardown_app
+    end
+
+    test "app method of integration tests returns test_app by default" do
+      app_file 'test/integration/default_app_test.rb', <<-RUBY
+        require 'test_helper'
+
+        class DefaultAppIntegrationTest < ActionDispatch::IntegrationTest
+          def test_app_returns_action_dispatch_test_app_by_default
+            assert_equal ActionDispatch.test_app, app
+          end
+        end
+      RUBY
+
+      output = Dir.chdir(app_path) { `bin/rails test 2>&1` }
+      assert_equal 0, $?.to_i, output
+      assert_match(/0 failures, 0 errors/, output)
+    end
+  end
 end