QPainter::drawImage does not draw in certain circumstances

The problem was encountered using QGraphicsView when zooming in on an image; the test program appears to trigger the same issue. Depending on the scale value entered on the command line, the QPainter will either produce a red surface (the expected result) or nothing. In the test program, the scale value of 6 works, 7-12 trigger the issue, and 13 works again.

In src/gui/painting/qpaintengine_raster.cpp, the function

    void QRasterPaintEngine::drawImage(const QRectF &r, const QImage &img, const QRectF &sr, Qt::ImageConversionFlags)

gets called, where the test

    if (s->matrix.type() > QTransform::TxTranslate || stretch_sr)

succeeds and the code block is entered. For the lower values of the scale factor, the first

    if (func && (!clip || clip->hasRectClip))

test (that's a few lines below) succeeds, and eventually

    template <class SrcT, class DestT, class Blender>
    void qt_transform_image_rasterize(...)

in qblendfunctions_p.h is called. There, the line

    int x_r = int((topRight.x + (qreal(0.5) + fromY - topRight.y) * rightSlope + qreal(0.5)) * 0x10000);

seems to overflow, causing 'x_r' to become negative, which in turn causes 'toX' to be smaller than 'fromX', and no drawing occurs.

For higher values of the scale factor, that do work again, a different code path is followed in the first function because an 'exceedsPrecision' flag becomes true.