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Thank you @nicolas-grekas.

What is the exact reason for this? Right now, I cannot overwrite/overload/whatever the newly introduced services with 2.6. I'd want to overwrite the security.authorization_checker service with a custom service, but that is impossible right now. This commit doesn't seem to add anything or fix any bugs, why is it merged? @fabpot @nicolas-grekas @stof

@tobias-93 Replacing the class is not a right extension point for services, because it means your own class must have the exact same arguments and method calls than the core one, and this will break if we add more arguments or method calls in a newer versions of Symfony (calls done in a service definition are not covered by our BC policy at all btw, so you have strictly no guarantee about it to work in a newer version, even patch ones if it is necessary to fix a bug).
There are several reliable ways to replace a service:

• redefining a service with the same name (which erases the previous definition entirely)
• decorating the existing service with the decorates feature of Symfony 2.5 in case you want to apply composition on your service.

This is why we decided to stop providing a broken extension point into our services (3.0 is removing all these parameters. this PR just avoided to add new ones in 2.6)

Btw, if you really want to replace the class of an existing service definition (taking into account the limitations listed above by acknowledging it goes outside the BC policy), a compiler pass can still alter the definition itself.