Variants can be declared without a block to signify their presence in the controller

actionpack/lib/action_controller/metal/mime_responds.rb

@@ -466,9 +466,11 @@ module ActionController #:nodoc:
         end
 
         def method_missing(name)
+          if block_given?
             yield if name == @variant || (name == :none && @variant.nil?)
           end
         end
       end
     end
+  end
 end

actionpack/test/controller/mime/respond_to_test.rb

@@ -170,7 +170,7 @@ class RespondToController < ActionController::Base
     respond_to do |format|
       format.html do |variant|
         variant.phone { render text: "phone" }
-        variant.none  { render text: "none" }
+        variant.none
       end
     end
   end

actionpack/test/fixtures/respond_to/variant_plus_none_for_format.html.erb

@@ -0,0 +1 @@
+none