I don't know why IntelliJ and/or Gradle keep doing this.  I have a fresh new install of latest version 21 IntelliJ.  I followed the instructions on how to setup the Forge project with IntelliJ 21.  It all worked great.

I run any of the generated configurations like "Clean Clean Client" for example.  And I get this error:

> Task :clean:Main.main() FAILED
Error: LinkageError occurred while loading main class net.minecraft.client.main.Main
    java.lang.UnsupportedClassVersionError: net/minecraft/client/main/Main has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only recognizes class file versions up to 55.0

LinkageError occurred while loading main class net.minecraft.client.main.Main
 

It compiled the classes as Java 17, not 11. Sure enough I find the .class files at:  \projects\clean\build\classes\java\main.  I decompile any of them and they are all bytecode 61, not 55.

I have JDK 11 installed.  I have JDK 11 set in all places I can find in Gradle, module settings, SDKs and javac settings.  I made sure I don't have a JDK 17 installed anywhere on my PC either.  Anybody know why its doing this?

2 hours ago, diesieben07 said:

1.18 requires Java 17. The Gradle Toolchain feature is used to set the Java version. Gradle will install Java 17 if necessary automatically.

Hmmm, At first I tried 17 and then got errors during the build about one of the jars, don't remember which was compiled with 55 and I was using 61.   OK, let me re-install 17 and see how far I get again with that route.  Thanks.

It works!  That first error was because I still had a JAVA_HOME env-var set to JDK 11 and gradle by default used that and not 17 like it should.  All good.  Thanks.