Problems when running Eclipse with JDK9

[Brett Randall]

I'm seeing an issue which only occurs when Eclipse is launched using a JDK9 VM (macOS, Java(TM) SE Runtime Environment (build 9.0.4+11), the default on my machine) as the JVM.  When I switch to jdk1.8.0_151 by adding to eclipse.ini:

-vm
/Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/bin/java

... and restarting Eclipse and rebuilding the workspace, the problem goes away.

The symptom with Eclipse running under JDK9 is that the Scala project (in my multi-project Gradle build) does not produce .class files under bin/main (Gradle Eclipse Buildship customizes the output directories).  When Eclipse runs under JDK8 I do see bin/...../MyScalaClass.class output generated.

Important to note that this relates to the Eclipse IDE _launch_ JDK - it is not impacted by the project/workspace JDK.  When running Eclipse launching with JDK9, if I set the default workspace JDK to JDK8 and check that all the projects are using that, the output is still not generated.  The only fix I have found is to add the JDK -vm arg to eclipse.ini.

Could this be a JDT weaving issue, or would that occur after the compiler output?  Are there any (platform?) known-issues with Eclipse and JDK9?

macOS High Sierra version 10.13.5 (17F77)
Eclipse Oxygen EE Version: Oxygen.3a Release (4.7.3a) Build id: 20180405-1200
Added TestNG, Scala IDE (initially 4.7.0, now 4.7.1.RC3 - Scala IDE for Eclipse 4.7.1.v-2_12-201801021323-2dfe808)