2 files changed, 19 insertions, 1 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index 0dc1d70e37..18a2922fa7 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -55,7 +55,11 @@ module ActionDispatch
         when Class
           klass == middleware
         else
-          klass == ActiveSupport::Inflector.constantize(middleware.to_s)
+          if lazy_compare?(@klass) && lazy_compare?(middleware)
+            normalize(@klass) == normalize(middleware)
+          else
+            klass == ActiveSupport::Inflector.constantize(middleware.to_s)
+          end
         end
       end
 
@@ -72,6 +76,14 @@ module ActionDispatch
       end
 
       private
+        def lazy_compare?(object)
+          object.is_a?(String) || object.is_a?(Symbol)
+        end
+
+        def normalize(object)
+          object.to_s.strip.sub(/^::/, '')
+        end
+
         def build_args
           Array(args).map { |arg| arg.respond_to?(:call) ? arg.call : arg }
         end
diff --git a/actionpack/test/dispatch/middleware_stack_test.rb b/actionpack/test/dispatch/middleware_stack_test.rb
index f4e18308ae..7cf6365af3 100644
--- a/actionpack/test/dispatch/middleware_stack_test.rb
+++ b/actionpack/test/dispatch/middleware_stack_test.rb
@@ -87,4 +87,10 @@ class MiddlewareStackTest < ActiveSupport::TestCase
     end
     assert_equal [:foo], @stack.last.send(:build_args)
   end
+
+  test "lazy compares so unloaded constants can be loaded" do
+    @stack.use "UnknownMiddleware"
+    @stack.use :"MiddlewareStackTest::BazMiddleware"
+    assert @stack.include?("::MiddlewareStackTest::BazMiddleware")
+  end
 end