So I got an IntelliJ out of memory error, and now all my Kotlin projects which were building fine in IntelliJ, are peppered with red all over the place. Example errors are listOf, it and lazy being an unresolved reference. The projects are gradle based and still build perfectly in gradle, Any ideas ? IntelliJ 2019.1.3 ultimate, Kotlin plugin v1.3.41-release-IJ2019.1-1

Try

Build > Rebuild Project

and if it does not work go for

File > Invalidate Caches and Restart

Ta, rebuild doesn't work, invalidate caches and restart did. Thanks ever so much, you saved my day !

Maybe closing/reopening the project or IDE would have been enough BTW

No, that wasn't enough.