Strange compiler behaviour (compiler passes wrong argument)

[SasaVilic]

I have following class declaration:

class GraphicsArc : public GraphicsComponent
{
public:
    enum { Type = Arc };
 
    explicit GraphicsArc();
    explicit GraphicsArc(const ArcF &arc);
    explicit GraphicsArc(qreal cx, qreal cy, qreal radius, qreal startAngle, qreal spanAngle);
    ~GraphicsArc();
 
    QRectF boundingRect() const;
    QPainterPath shape() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
 
    ArcF arc() const;
    void setArc(const ArcF &arc);
 
    int type() const { return Type; }
 
private:
    void updateBoundingRect();
    QPointF endPoint(const QPointF &center, qreal radius, qreal angle);
 
    ArcF m_arc;
    QRectF bounding_rect;
};

class GraphicsArc : public GraphicsComponent
{
public:
    enum { Type = Arc };

    explicit GraphicsArc();
    explicit GraphicsArc(const ArcF &arc);
    explicit GraphicsArc(qreal cx, qreal cy, qreal radius, qreal startAngle, qreal spanAngle);
    ~GraphicsArc();

    QRectF boundingRect() const;
    QPainterPath shape() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);

    ArcF arc() const;
    void setArc(const ArcF &arc);

    int type() const { return Type; }

private:
    void updateBoundingRect();
    QPointF endPoint(const QPointF &center, qreal radius, qreal angle);

    ArcF m_arc;
    QRectF bounding_rect;
};

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and problem is here:

...
QPainterPath GraphicsArc::shape() const
{
    QPainterPath path;
 
    QPointF sp1 (endPoint(m_arc.center(), m_arc.radius() - 5, -m_arc.startAngle()));
    QPointF sp2 (endPoint(m_arc.center(), m_arc.radius() + 5, -m_arc.startAngle()));
...

...
QPainterPath GraphicsArc::shape() const
{
    QPainterPath path;

    QPointF sp1 (endPoint(m_arc.center(), m_arc.radius() - 5, -m_arc.startAngle()));
    QPointF sp2 (endPoint(m_arc.center(), m_arc.radius() + 5, -m_arc.startAngle()));
...

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Compiler says:

..\TestGraphicView\graphicsarc.cpp: In member function 'virtual QPainterPath GraphicsArc::shape() const':
..\TestGraphicView\graphicsarc.cpp:42: error: passing 'const GraphicsArc' as 'this' argument of 'QPointF GraphicsArc::endPoint(const QPointF&, qreal, qreal)' discards qualifiers
..\TestGraphicView\graphicsarc.cpp:43: error: passing 'const GraphicsArc' as 'this' argument of 'QPointF GraphicsArc::endPoint(const QPointF&, qreal, qreal)' discards qualifiers

I do not understand that, because there also a function updateBoundingRect, which also use endPoint(), and does not have same problem. I think it has something to do with virtual function, but I am not sure what.