In #4342, part of the functionalities of form_for was delegated to form_with, this could potentially cause some regress, as the condition to send polymorphic_path(record, {}) instead of polymorphic_path(record, format: options.delete(:format)) is removed and when format is not passed, this causes issues if the caller is not expecting an extra arg format:nil. This PR adds back the condition inside for with and add a test to make sure it doesn't break for_with

actionview/lib/action_view/helpers/form_helper.rb

@@ -754,7 +754,11 @@ module ActionView
 
         if model
           if url != false
-            url ||= polymorphic_path(model, format: format)
+            url ||= if format.nil?
+              polymorphic_path(model, {})
+            else
+              polymorphic_path(model, format: format)
+            end
           end
 
           model   = _object_for_form_builder(model)

actionview/test/template/form_helper_test.rb

@@ -2057,6 +2057,18 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_new_form_for_without_format
+    form_for(@post, html: { id: "edit_post_123", class: "edit_post" }) do |f|
+      concat f.label(:title)
+    end
+
+    expected = whole_form("/posts/123", "edit_post_123", "edit_post", method: "patch") do
+      "<label for='post_title'>Title</label>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_form_for_with_model_using_relative_model_naming
     blog_post = Blog::Post.new("And his name will be forty and four.", 44)