@Override
    public TypeMirror proxyFieldType() {
      // If this is an Optional<String> then the proxy field type is String.
      // If this is an Optional<Foo>, and the proxy field type for Foo is Bar, then the proxy field
      // type for Optional<Foo> is Bar.
      return containedTypeSerializer.proxyFieldType();
    }
Creates a {@link Serializer} that supports {@link Optional} types.
value/src/main/java/com/google/auto/value/extension/serializable/serializer/impl/OptionalSerializerExtension.java