class Optional<T> where T : notnull { public T? Value; }
class User { string Id; }
class UserDto { string Id; }
class Document { Optional<User> ModifiedBy; }
class DocumentDto { Optional<UserDto> ModifiedBy; }

[Mapper]
public partial class Mapper
{
    public partial DocumentDto MapDocument(Document source);
    
    private Optional<T> MapOptional<TTarget, TSource>(Optional<TSource> source)
    {
        return new Optional<TTarget> { Value = Map<TTarget>(source.Value) };
    }

    private partial T Map(object source);

    private partial UserDto MapUser(User source);
}