问Qt 5.7.1 / gcc 6.3.0:错误: constexpr函数'static constexpr int QMetaTypeId2<T>的主体

EN

namespace Config {
  class Value {
  public:
    enum Type {
      Null,
      Integer,
      String,
      Map,
      List,
      Boolean
    };

    Value();
    Value(int v);
    Value(const QString &v);
    Value(const QMap<QString, Value> &v);
    Value(const QList<Value> &v);
    Value(bool v);

    template<class T> T get() const {
      return value.value<T>();
    }

    enum Type type() const;
    enum Type listType() const;
    bool isNull() const;

    void setListType(enum Type t);

    operator QString() const;
    QString toString() const;

  private:
    QVariant value;
    enum Type value_type;
    enum Type list_elements_type;
  };
}

Q_DECLARE_METATYPE(Config::Value)

../../../../include/x86_64-linux-gnu/qt5/QtCore/qglobal.h:746:47: error: static assertion failed: Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system

../../../../include/x86_64-linux-gnu/qt5/QtCore/qmetatype.h:1604:100: error: body of constexpr function 'static constexpr int QMetaTypeId2<T>::qt_metatype_id() [with T = std::nullptr_t]' not a return-statement