class LiteParsedDocumentSourceExtension : public LiteParsedDocumentSource {};

class LiteParsedDocumentSourceExtensionDesugar : public LiteParsedDocumentSourceExtension {
   public:
      StringData stageName() const { return _stageName; }
      const BSONObj& originalStageBson() const { return _spec; }

      const std::vector<BSONObj>& getDesugaredPipeline() const {
         return _desugaredPipeline.get([this] { return getDesugaredBSONVec() });
      }

   protected:
      virtual std::vector<BSONObj> getDesugaredBSONVec() const = 0;
      std::string _stageName;
      BSONObj _spec;

   private:
      Deferred<std::vector<BSONObj>> _desugaredPipeline;
};