argument error is done for us

actionpack/Rakefile

@@ -18,7 +18,8 @@ Rake::TestTask.new(:test_action_pack) do |t|
   # this will not happen automatically and the tests (as a whole) will error
   t.test_files = Dir.glob('test/{abstract,controller,dispatch,template}/**/*_test.rb').sort
 
-  # t.warning = true
+  #t.warning = true
+  t.verbose = true
 end
 
 namespace :test do

actionpack/lib/action_view/helpers/form_helper.rb

@@ -840,8 +840,6 @@ module ActionView
       private
 
         def instantiate_builder(record, record_object = nil, options = nil, &block)
-          raise ArgumentError, "Missing block" unless block_given?
-
           options, record_object = record_object, nil if record_object.is_a?(Hash)
           options ||= {}
 

actionpack/test/template/form_helper_test.rb

@@ -640,6 +640,12 @@ class FormHelperTest < ActionView::TestCase
     )
   end
 
+  def test_form_for_requires_block
+    assert_raises(ArgumentError) do
+      form_for(:post, @post, :html => { :id => 'create-post' })
+    end
+  end
+
   def test_form_for
     assert_deprecated do
       form_for(:post, @post, :html => { :id => 'create-post' }) do |f|