Function register not found in ServiceProvider

I created a new Laravel 5.6 project, and I made an abstract Service Provider, where all my service providers will extend from.

My abstract ServiceProvider should extend the original one from Laravel Illuminate\Support\ServiceProvider.

In Laravel 5.1, I always work in this way without any problem, until today when I decided to try the 5.6 version, and now I have a problem, that register() function is not found in Illuminate\Support\ServiceProvider.
If I take a look in the file in vendor directory, then that is right, the function is not there, but if I go to official documentation, I can see it...
Also for the version 5.1, the function is there if I look in the life in vendor directory

Any solution?

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