Call to undefined method Illuminate\Foundation\Providers\ArtisanServiceProvider::when()

This error showed when trying to upgrade to Laravel 4.1, Also  when trying to  update composer , the first Solution to this problem is to change all your composer.json file (assumed that you have some in workbench folder), to be compatible with new versions of laravel :

Change this line in Composer.json :

"illuminate/support": "4.1.*" 

to be Like this

 "illuminate/support": "4.*" 

Then Run :

 Composer update --no-scripts  

In the main folder that contain composer.json And also on all the main folders of your packages .