Unresolved reference: R

Unlike other posts from here (R is unresolved - #18 by cairomenso for example), my project compiles fine.

There is no Gradle error, I can compile and run on my phone without any warnings, however, IntelliJ Ultimate shows any usage of the R class in red with a “Unresolved reference: R” error. Even code completion works.

I don’t really know where this could come from, so here’s some info about the system:
Debian 9
IntelliJ Ultimate 2018.3.4 with bundled Android Support plugin (the latest version I believe)
The project was created through the wizard. The only modifications I made to the config is to change the Kotlin dependancy from the JRE7 to the JDK7 version:
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

About the Gradle wrapper:

Gradle 4.10.1

Build time:   2018-09-12 11:33:27 UTC
Revision:     76c9179ea9bddc32810f9125ad97c3315c544919

Kotlin DSL:   1.0-rc-6
Kotlin:       1.2.61
Groovy:       2.4.15
Ant:          Apache Ant(TM) version 1.9.11 compiled on March 23 2018
JVM:          1.8.0_181 (Oracle Corporation 25.181-b13)
OS:           Linux 4.9.0-8-amd64 amd64

(this project is using a regular Groovy build.gradle).

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