Automatically register nested Kotlin classes for reflection

Users have come to expect that @RegisterForReflection fixes
a lot of native image issues and use it a lot.
For Kotlin classes however, registering a class is often not
enough - the companion classes are need to be registered.
These companion classes are nested classes, so we automatically
set the property to true for Kotlin classes

Closes: quarkusio#28167

core/deployment/src/main/java/io/quarkus/deployment/steps/RegisterForReflectionBuildStep.java

@@ -18,6 +18,8 @@
 import org.jboss.jandex.Type.Kind;
 import org.jboss.logging.Logger;
 
+import io.quarkus.deployment.Capabilities;
+import io.quarkus.deployment.Capability;
 import io.quarkus.deployment.annotations.BuildProducer;
 import io.quarkus.deployment.annotations.BuildStep;
 import io.quarkus.deployment.builditem.CombinedIndexBuildItem;
@@ -32,8 +34,10 @@ public class RegisterForReflectionBuildStep {
 
     private static final Logger log = Logger.getLogger(RegisterForReflectionBuildStep.class);
 
+    private static final DotName KOTLIN_METADATA_ANNOTATION = DotName.createSimple("kotlin.Metadata");
+
     @BuildStep
-    public void build(CombinedIndexBuildItem combinedIndexBuildItem,
+    public void build(CombinedIndexBuildItem combinedIndexBuildItem, Capabilities capabilities,
             BuildProducer<ReflectiveClassBuildItem> reflectiveClass,
             BuildProducer<ReflectiveHierarchyBuildItem> reflectiveClassHierarchy,
             BuildProducer<LambdaCapturingTypeBuildItem> lambdaCapturingTypeProducer) {
@@ -63,6 +67,12 @@ public void build(CombinedIndexBuildItem combinedIndexBuildItem,
             ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
             if (targetsValue == null && classNamesValue == null) {
                 ClassInfo classInfo = i.target().asClass();
+                if (capabilities.isPresent(Capability.KOTLIN) && ignoreNested) {
+                    // for Kotlin classes, we need to register the nested classes as well because companion classes are very often necessary at runtime
+                    if (isKotlinClass(classInfo)) {
+                        ignoreNested = false;
+                    }
+                }
                 registerClass(classLoader, classInfo.name().toString(), methods, fields, ignoreNested, serialization,
                         reflectiveClass, reflectiveClassHierarchy, processedReflectiveHierarchies, registerFullHierarchyVaue,
                         builder);
@@ -88,6 +98,10 @@ public void build(CombinedIndexBuildItem combinedIndexBuildItem,
         }
     }
 
+    private static boolean isKotlinClass(ClassInfo classInfo) {
+        return classInfo.hasDeclaredAnnotation(KOTLIN_METADATA_ANNOTATION);
+    }
+
     /**
      * BFS Recursive Method to register a class and it's inner classes for Reflection.
      *
@@ -213,4 +227,4 @@ private static Type getMethodReturnType(IndexView indexView, DotName initialName
     private static boolean getBooleanValue(AnnotationInstance i, String name) {
         return i.value(name) == null || i.value(name).asBoolean();
     }
-}
+}