Today, I opened a very old project for a quick update. After upgrading the support libraries, gradle plugins and other tools, I saw this strange issue for the first time. The app was building and running successfully, but when I opened any code file, I was getting huge list of compile errors mostly about Unresolved Reference etc.
Initially, I cleaned project, rebuild it, restarted Android studio few times, but couldn't fix it. Heck, I even resumed my coding by ignoring these silly errors, but due to these strange errors, the autocomplete and other IDE functionalities were not working. So, I had to fix this problem. Then I googled it and found some answers on StackOverflow at here and here.
Most, suggestions were already same what I was doing. What fixed my issue was to delete the .idea and .gradle project from the app directory and Invalidate and Cache Restart the Android Studio and you will be back in the game.