rails4-rc1-routes.rb

# config/routes.rb

  get '/sign_in' => 'user_sessions#new', :as => :sign_in
  post '/sign_in' => 'user_sessions#create', :as => :sign_in

# Output

#=> Invalid route name, already in use: 'sign_in' 
#=> You may have defined two routes with the same name using the `:as` option, or you may be overriding a route already defined by a resource with the same naming. For the latter, you can restrict the routes created with `resources` as explained here: 
#=> http://guides.rubyonrails.org/routing.html#restricting-the-routes-created