Allow layered singletons to be subtyped.

Author: cstancu

substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/ImageSingletonsSupportImpl.java

@@ -228,10 +228,6 @@ private void doAddInternal(Class<?> key, Object value) {
 
                 if (singleton instanceof MultiLayeredImageSingleton || ApplicationLayerOnlyImageSingleton.isSingletonInstanceOf(singleton)) {
 
-                    if (!key.equals(singleton.getClass())) {
-                        throw UserError.abort("The implementation class must be the same as the key class. key: %s, singleton: %s", key, singleton);
-                    }
-
                     if (singleton instanceof MultiLayeredImageSingleton && ApplicationLayerOnlyImageSingleton.isSingletonInstanceOf(singleton)) {
                         throw UserError.abort("Singleton cannot implement both %s and %s. singleton: %s", MultiLayeredImageSingleton.class, ApplicationLayerOnlyImageSingleton.class, singleton);
                     }

substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/imagelayer/LoadImageSingletonFeature.java

@@ -580,7 +580,7 @@ void assignSlots(HostedMetaAccess metaAccess) {
             int slotAssignment;
             LoadImageSingletonDataImpl info = entry.getValue();
             var hType = metaAccess.lookupJavaType(info.getLoadType());
-            if (hType.isInstantiated()) {
+            if (hType.getWrapped().isAnySubtypeInstantiated()) {
                 Class<?> keyClass = entry.getKey();
                 SlotInfo slotInfo = priorKeyToSlotInfoMap.get(entry.getKey());