Disallowed to serialize unknown enum members

codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/generators/EnumGenerator.java

@@ -217,12 +217,19 @@ private void generateUnknownVariant() {
             writer.pushState();
             writer.putContext("objects", Objects.class);
             writer.putContext("toString", new ToStringGenerator(writer));
+            writer.putContext("shapeSerializer", ShapeSerializer.class);
+            writer.putContext("unsupportedOperation", UnsupportedOperationException.class);
             var template = """
                     record $$Unknown(${value:T} value) implements ${shape:T} {${?string}
                         public $$Unknown {
                             ${objects:T}.requireNonNull(value, "Value cannot be null");
                         }${/string}
 
+                        @Override
+                        public void serialize(${shapeSerializer:T} serializer) {
+                            throw new ${unsupportedOperation:T}("Cannot serialize enum with unknown value " + this.value);
+                        }
+
                         @Override
                         public ${value:T} getValue() {
                             return value;

codegen/codegen-plugin/src/test/resources/software/amazon/smithy/java/codegen/types/test-cases/enums/expected/EnumType.java

@@ -97,6 +97,11 @@ record $Unknown(String value) implements EnumType {
             Objects.requireNonNull(value, "Value cannot be null");
         }
 
+        @Override
+        public void serialize(ShapeSerializer serializer) {
+            throw new UnsupportedOperationException("Cannot serialize enum with unknown value " + this.value);
+        }
+
         @Override
         public String getValue() {
             return value;

codegen/codegen-plugin/src/test/resources/software/amazon/smithy/java/codegen/types/test-cases/enums/expected/IntEnumType.java

@@ -110,6 +110,11 @@ public String toString() {
 
     record $Unknown(int value) implements IntEnumType {
 
+        @Override
+        public void serialize(ShapeSerializer serializer) {
+            throw new UnsupportedOperationException("Cannot serialize enum with unknown value " + this.value);
+        }
+
         @Override
         public int getValue() {
             return value;

codegen/codegen-plugin/src/test/resources/software/amazon/smithy/java/codegen/types/test-cases/enums/expected/TextType.java

@@ -80,6 +80,11 @@ record $Unknown(String value) implements TextType {
             Objects.requireNonNull(value, "Value cannot be null");
         }
 
+        @Override
+        public void serialize(ShapeSerializer serializer) {
+            throw new UnsupportedOperationException("Cannot serialize enum with unknown value " + this.value);
+        }
+
         @Override
         public String getValue() {
             return value;