Android Studio 3.1 Cannot Resolve Symbol (Themes, Widget, attr, etc.)

Question

I upgraded Android Studio today to 3.1, and now Android Studio says it cannot resolve symbols for most of the resources (for example ThemeOverlay in style

Answer 1

Close and reopen project as existing Android Studio project

Answer 2

After upgrading Android Studio, you can invalidate cache and restart.

File > Invalidate Caches / Restart…

Answer 3

I closed and reopened project as existing Android Studio project (did not import it), and it worked! Thanks to Andrew Glukhoff 's comment.

Answer 4

Closing a project and then opening as existing one does wonders!

Answer 5

Close the project and import it again. Worked for me today.

Answer 6

You can try to close the project and exit Android Studio (only closing and reimporting it didn't do it for me). Launch Android Studio and open the project again. That worked for me!