SBT + Idea 13 => Cannot resolve symbol

To fix this, remove the folder .idea and reimport the project.

The root cause remains a mystery...

In case none of the other solutions work and you are in the same boat as me, where you see a Cannot resolve symbol ... in the build.sbt and all files that have sbt dependencies, and where there are no sbt dependencies in External Libraries, running File | New | Module from Existing Sources... and selecting the build.sbt may fix it for you as it fixed it for me.

I had a similar problem when I updated to SBT 0.13.7. IntelliJ started marking operators such as := and ++ in red with message "Cannot resolve symbol"

My steps to fix this solution were as follows:

1. In IntelliJ under Files | Settings, I switched my SBT Runner to use a Custom Launcher (and gave my path to sbt-launch.jar which for me was /opt/sbt/bin/sbt-launch.jar).

2. After testing that I can still compile after #1 above, I then had to invalidate the cache and restart; under Files | Invalidate Caches / Restart...

That's it! :-)