Cannot resolve symbol '?attr/actionBarSize' after updating Android Studio from 2.3 to 3.0 for buildToolsVersion '26.0.2'

Nothing helped, but changing:

?attr/actionBarSize to ?android:attr/actionBarSize did the job.

That's actually a bug in few versions. Even if you won't fix it, Android will automatically fix it at runtime.

Update all your library versions to 26.1.0 (to the most recent version) and also add:

compile 'com.android.support:support-v4:26.1.0'

if you are using Android Studio 3.0.0 and above then use

implementation 'com.android.support:support-v4:26.1.0'

Sync your project and the error will automatically resolve because ?attr/actionBarSize is part of v4 library.

In the project directory hierarchy switch "Android" to "Project". Then delete a folder ".idea/libraries", only "libraries". Select an option from the menu "File -> Invalidate Caches / Restart... -> Invalidate and Restart".