BlueWolf_SK

That was it. After removing the cache folder (after backing it up of course), recreating the project and building it everything ran correctly. Thank you very much. I wasn't aware gradle cached data outside the project folder. Useful to know.

First things first. I tried the latest forge mdk (1.15.2 - 31.1.46) and it works. I felt like poking around it a little bit more to understand what I was doing wrong, so I tried few other things. I uninstalled all JDKs I had except one version of the adoptopenjdk 8 (tried jdk8u252-b09, jdk8u242-b08 and jdk8u232-b09 and settled on the last one for the remainder of my tests). The problem persisted. I tried it with eclipse instead of intellij (clean sepparate folders were used for eclipse tests). The problem persisted. I tried running it with these versions of forge mdks: forge-1.15.2-31.0.0-mdk forge-1.15.2-31.0.11-mdk forge-1.15.2-31.0.15-mdk forge-1.15.2-31.0.19-mdk forge-1.15.2-31.1.0-mdk forge-1.15.2-31.1.1-mdk forge-1.15.2-31.1.5-mdk forge-1.15.2-31.1.12-mdk forge-1.15.2-31.1.25-mdk forge-1.15.2-31.1.44-mdk forge-1.15.2-31.1.46-mdk It worked on all versions except the 31.1.0. I used the exact same steps to create the projects, always in clean folders. No idea what could be affecting it since the build configuration seem to be the same for instance between 31.1.0 and 31.1.1.

I'm not sure how to state this more plainly but: Doesn't work when compiling with JDK 8. Works when compiling with JDK 11. At least from what I can tell (making sure correct environment variables are set, making sure correct options are set in IntelliJ, making sure to clean everything before rebuilding). That's what has me confused. Is there any way to make sure what JDK the code was actually built with (log during build process or inspecting resulting files somehow)? I'll try to google around some more myself as well and try few things. Maybe I'm missing something and it's actually compiling with 11 when I think it's compiling with 8 and vice versa but that seems unlikely. Thanks for the workaround, I'll give it a try.

I'm getting the exact same error. forge-1.15.2-31.1.0-mdk IntelliJ IDEA 2020.1 (Community Edition) on both AdoptOpenJDK jdk-8.0.252.09-hotspot and Oracle jdk1.8.0_251. But it works on AdoptOpenJDK jdk-11.0.7.10-hotspot when I set the project language level to 8. I checked JAVA_HOME in the windows environment variables before building. I ran gradle clean before building, invalidatec caches and restarted IntelliJ and even created entirely new project. Always made sure SDK is set properly in the IntelliJ Project Structure menu. Same result. Doesn't work with jdk 8s, works on jdk11. Not sure what I'm missing. I do not have JDK9 installed anywhere and never had. I do have another (older) Oracle openjdk version of 11 and latest version of Oracle 14. With 14 it crashes on something different which leads me believe setting JDK in the IntelliJ Project Structure menu does set it properly for building but perhaps that's something different? Full crash just to be sure but it's pretty much same as Fallout's: