Deprecate passing nil as model arg instead of raising ArgumentError (r…

…ails#50931)

actionview/CHANGELOG.md

@@ -17,7 +17,7 @@
 
     *Sean Doyle*
 
-*   Raise `ArgumentError` when `nil` is passed as `model:` argument value to the `form_with` method.
+*   Deprecate passing `nil` as value for the `model:` argument to the `form_with` method.
 
     *Collin Jilbert*
 

actionview/lib/action_view/helpers/form_helper.rb

@@ -754,7 +754,7 @@ def apply_form_for_options!(object, options) # :nodoc:
       #     form_with(**options.merge(builder: LabellingFormBuilder), &block)
       #   end
       def form_with(model: false, scope: nil, url: nil, format: nil, **options, &block)
-        raise ArgumentError, "The :model argument cannot be nil" if model.nil?
+        ActionView.deprecator.warn("Passing nil to the :model argument is deprecated and will raise in Rails 7.3") if model.nil?
 
         options = { allow_method_names_outside_object: true, skip_default_ids: !form_with_generates_ids }.merge!(options)
 

actionview/test/template/form_helper/form_with_test.rb

@@ -340,11 +340,10 @@ def url_for(object)
   end
 
   def test_form_with_when_given_nil_model_argument
-    error = assert_raises(ArgumentError) do
+    assert_deprecated(ActionView.deprecator) do
       form_with(model: nil) do
       end
     end
-    assert_equal "The :model argument cannot be nil", error.message
   end
 
   def test_form_with