I just updated the Android studio de 2.3 and it marks me as error Cannot resolve symbol AppCompatActivity - Support v7 libraries aren't recognized? but compile and run the app, without any problem, that is, it seems that something was deconfigured and it marks me error in many lines.
I have tried the typical clean/build clean the project and go back to build, but it keeps showing me some lines with errors.
Many details that have arisen when updating both support libraries and Android Studio, in this case invalidates the cache :
Another solution can be in " Gradle Scripts/build.gradle(Module:app) ". In the last line (highlighted in bold) you only need to delete a number and rewrite it as you had before deleting.
androidTestImplementation 'com.android.support.test.espresso:espresso core:3.0.1' delete the directory .idea of the application, then in the file menu give it to invalidate cache and restart:
It is very important that you delete the .idea directory before doing this.
Check if compileSdkVersion and targetSdkVersion have the same value
The other thing that can work is adding buildtoolsversion 26.0.2