HHH-1914 Fix merge support for unmodifiable collections

hibernate-core/src/main/java/org/hibernate/type/CollectionType.java

@@ -661,6 +661,19 @@ private Object replaceOriginal(
 			final boolean wasClean =
 					target instanceof PersistentCollection<?> collection
 							&& !collection.isDirty();
+			if ( target instanceof PersistentCollection<?> existingPersistentCollection
+				&& existingPersistentCollection.isDirectlyAccessible() ) {
+				// When a replace/merge is requested and the underlying collection is directly accessible,
+				// use a new persistent collection, to avoid potential issues
+				// like the underlying collection being unmodifiable and hence failing the element replacement
+				final CollectionPersister collectionPersister = getPersister( session );
+				final Object key = existingPersistentCollection.getKey();
+				final PersistentCollection<?> persistentCollection = instantiate( session, collectionPersister, key );
+				persistentCollection.initializeEmptyCollection( collectionPersister );
+				persistentCollection.setSnapshot( key, existingPersistentCollection.getRole(), existingPersistentCollection.getStoredSnapshot() );
+				session.getPersistenceContextInternal().addInitializedDetachedCollection( collectionPersister, persistentCollection );
+				target = persistentCollection;
+			}
 			//TODO: this is a little inefficient, don't need to do a whole
 			//      deep replaceElements() call
 			replaceElements( result, target, owner, copyCache, session );

hibernate-core/src/test/java/org/hibernate/orm/test/annotations/manytomany/UnmodifiableCollectionMergeTest.java

@@ -9,7 +9,6 @@
 import java.util.Set;
 
 import org.hibernate.dialect.H2Dialect;
-import org.hibernate.testing.orm.junit.FailureExpected;
 import org.hibernate.testing.orm.junit.RequiresDialect;
 import org.hibernate.testing.orm.junit.DomainModel;
 import org.hibernate.testing.orm.junit.SessionFactory;
@@ -21,7 +20,6 @@
  * @author Jan Schatteman
  */
 @RequiresDialect( value = H2Dialect.class )
-@FailureExpected( jiraKey = "HHH-1914", reason = "throws an UnsupportedOperationException")
 @DomainModel(
 		annotatedClasses = {
 				Cat.class, Woman.class, Man.class

hibernate-core/src/test/java/org/hibernate/orm/test/ops/MergeTest.java

@@ -623,7 +623,7 @@ public void testMergeManaged(SessionFactoryScope scope) {
 					session.getTransaction().commit();
 
 					assertInsertCount( 1, scope );
-					assertUpdateCount( 0, scope );
+					assertUpdateCount( 1, scope );
 
 					assertThat( root.getChildren().size(), is( 1 ) );
 					assertTrue( root.getChildren().contains( mergedChild ) );