Adds check that plugin class is accessible from mod class

testproject/jijtest/src/main/java/jijtest/AccessPluginClass.java

@@ -0,0 +1,15 @@
+package jijtest;
+
+import cpw.mods.modlauncher.TransformingClassLoader;
+import jijtestplugin.Plugin;
+import net.neoforged.fml.common.Mod;
+
+@Mod("jijtest")
+public class AccessPluginClass {
+    public AccessPluginClass() {
+        // Validate that Plugin.class is not loaded via the transforming classloader
+        if (Plugin.class.getClassLoader() instanceof TransformingClassLoader) {
+            throw new IllegalStateException("Expected Plugin to be loaded as a plugin!");
+        }
+    }
+}

testproject/jijtest/src/plugin/java/jijtestplugin/Plugin.java

@@ -1,5 +1,4 @@
 package jijtestplugin;
 
 public class Plugin {
-
 }