    //=============================================================================
    //  MuseScore
    //  Music Composition & Notation
    //
    //  Copyright (C) 2010-2011 Werner Schweer
    //
    //  This program is free software; you can redistribute it and/or modify
    //  it under the terms of the GNU General Public License version 2
    //  as published by the Free Software Foundation and appearing in
    //  the file LICENCE.GPL
    //=============================================================================

    #ifndef __STAFFSTATE_H__
    #define __STAFFSTATE_H__

    #include "element.h"
    #include "elementlayout.h"
    #include "instrument.h"

    namespace Ms {

    enum class StaffStateType : char {
          INSTRUMENT,
          TYPE,
          VISIBLE,
          INVISIBLE
          };

    //---------------------------------------------------------
    //   @@ StaffState
    //---------------------------------------------------------

    class StaffState : public Element {
          Q_GADGET

          StaffStateType _staffStateType;
          qreal lw;
          QPainterPath path;

          Instrument* _instrument;

          virtual void draw(QPainter*) const;
          virtual void layout();

       public:
          StaffState(Score*);
          StaffState(const StaffState&);
          ~StaffState();

          virtual StaffState* clone() const  { return new StaffState(*this); }
          virtual ElementType type() const { return ElementType::STAFF_STATE; }

          void setStaffStateType(const QString&);
          void setStaffStateType(StaffStateType st) { _staffStateType = st; }
          StaffStateType staffStateType() const     { return _staffStateType; }
          QString staffStateTypeName() const;

          virtual bool acceptDrop(EditData&) const override;
          virtual Element* drop(EditData&);
          virtual void write(XmlWriter&) const;
          virtual void read(XmlReader&);
          Instrument* instrument() const           { return _instrument; }
          void setInstrument(const Instrument* i)  { *_instrument = *i;    }
          void setInstrument(const Instrument&& i) { *_instrument = i;    }
          Segment* segment()                       { return (Segment*)parent(); }
          };

    }     // namespace Ms
    #endif

↑ V690 The 'StaffState' class implements a copy constructor, but lacks the '=' operator. It is dangerous to use such a class.