Android Studio suddenly cannot resolve symbols

Question

Android Studio 0.4.2 was working fine and today I opened it and almost everything was red and the auto-completion had stopped working. I look at the imports and AS seems to

Answer 1

Another very subtle cause:

Multi-flavor library should be compiled in specific way than a normal single-flavored. Otherwise it silently produces cannot resolve symbols error.

Multi flavor app based on multi flavor library in Android Gradle