Fix issue wrt [databind#2780] change (#803)

Author: cowtowncoder

src/main/java/tools/jackson/dataformat/xml/ser/XmlBeanSerializerBase.java

@@ -307,6 +307,13 @@ public void serializeWithType(Object bean, JsonGenerator gen, SerializationConte
         throws JacksonException
     {
         if (_objectIdWriter != null) {
+            // [dataformat-xml#81]: XML cannot represent typed object id references
+            // as arrays ([type, id]) inline within lists, unlike JSON. So for
+            // back-references, write just the id without type wrapping.
+            WritableObjectId oid = ctxt.findObjectId(bean, _objectIdWriter.generator);
+            if (oid.writeAsReference(gen, ctxt, _objectIdWriter)) {
+                return;
+            }
             _serializeWithObjectId(bean, gen, ctxt, typeSer);
             return;
         }

src/test/java/tools/jackson/dataformat/xml/misc/PolymorphicTypesTest.java

@@ -15,19 +15,19 @@
 public class PolymorphicTypesTest extends XmlTestUtil
 {
     @JsonTypeInfo(use=JsonTypeInfo.Id.CLASS, include=JsonTypeInfo.As.PROPERTY)
-    static class BaseTypeWithClassProperty { }
+    public static class BaseTypeWithClassProperty { }
 
-    static class SubTypeWithClassProperty extends BaseTypeWithClassProperty {
+    public static class SubTypeWithClassProperty extends BaseTypeWithClassProperty {
         public String name;
 
         public SubTypeWithClassProperty() { }
         public SubTypeWithClassProperty(String s) { name = s; }
     }
 
     @JsonTypeInfo(use=JsonTypeInfo.Id.CLASS, include=JsonTypeInfo.As.WRAPPER_OBJECT)
-    protected static class BaseTypeWithClassObject { }
+    public static class BaseTypeWithClassObject { }
 
-    protected static class SubTypeWithClassObject extends BaseTypeWithClassObject {
+    public static class SubTypeWithClassObject extends BaseTypeWithClassObject {
         public String name;
 
         public SubTypeWithClassObject() { }
@@ -36,14 +36,14 @@ public SubTypeWithClassObject() { }
 
     @JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY)
     @JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
-    protected static class TypeWithClassPropertyAndObjectId {
+    public static class TypeWithClassPropertyAndObjectId {
         public String id;
 
         public TypeWithClassPropertyAndObjectId() {}
         public TypeWithClassPropertyAndObjectId(String id) { this.id = id; }
     }
 
-    protected static class Wrapper {
+    public static class Wrapper {
         public List<TypeWithClassPropertyAndObjectId> data;
 
         public Wrapper(){}