Steps to reproduce:

1. Create new Unity project
2. Add a script component to any game object
3. Add "[RequireComponent (typeof(Rigidbody))]" in the line above the class name in the script
4. Save the script
5. Switch back to editor
6. Let it recompile the script
7. Notice the Rigidbody component will not be added to the object that already had the script

Note: Adding this script to game object after the change, creates the needed components as expected.