IntelliJ won't import modules from gradle composite builds - gradle

Question

UPDATE: Not only is IntelliJ having trouble, but the reason seems to be because my setup fails to build in Gradle. Still unsure why.
I'm trying to set intellij up to recognize composite gradle builds. I have Project A which depends on Project B, like this:
/p/projecta
/p/projectb
I've tried MANY things, but IntelliJ can't find the declarations of anything in project-b that I try to access from project-a. For example, in ProjectA.java below, IntelliJ marks the import for Project B as Cannot resolve symbol ProjectB
// projecta/src/main/java/projecta/ProjectA.java
import projectb.ProjectB; // 'Cannot resolve symbol ProjectB'
// projectb/src/main/java/projectb/ProjectB.java
public class ProjectB {
}
// projecta/settings.gradle
rootProject.name = 'projecta'
includeBuild '../projectb'
// projectb/settings.gradle
rootProject.name = 'projectb'
Despite the above configuration, it does not work. IntelliJ continues to mark all symbols from Project B as not resolvable.
Other things I've tried
removing the includeBuild from settings.gradle, and right clicking in the gradle window on Project A, going to 'Composite Build Configuration' and adding Project B from there. Same issue.
Adding this code to build.gradle in Project A
tasks.register('run') {
dependsOn gradle.includedBuild('projectb').task(':run')
}
Putting Project B inside of Project A on the disk: /p/projecta/projectb and then changing includeBuild in projecta/settings.gradle to includeBuild 'projectb'
Many variations of #2 to try to get IntelliJ to recognize the gradle dependency.

Answer 1

I recently ran into this problem and figured it out so I wanted to respond here so others can benefit from my foraging through the jungle of parameters and trial and error.
I have the exact same project set up as described in the question. Using Gradle 5.0, Intellij was able to correctly import and link my sources and I was able to build correctly on the command line. When I bumped the Gradle version to 5.6 and higher, Intellij was no longer able to link the sources correctly. Re-importing my project would actually work but the dependency that represents the included build would be highlighted red and the console would say that it couldn't resolve it. This happened for every Gradle version after 5.6.
It seems the root cause was Intellij was using Java 11 for Gradle while I had sourceCompatibility set to 10. It looks like this breaks things in more recent versions of Gradle (understandably).
I was also able to fix this by adding the following to my project's build.gradle
java {
disableAutoTargetJvm()
}
However, I just updated my JDK for Gradle in Intellij to be Java 10 and everything worked....

Answer 2

My problem was just like for #Troy, had to make all projects work under the same JDK by adding this to the build.gradle.kts:
java {
toolchain {
languageVersion.set(JavaLanguageVersion.of(8))
}
}