QImageReader reader(fn);
    QImage image;
    for (int i = 0; true; ++i) {
        QDebug d = qDebug();
        d << '#' << i << "QImageReader::size()=" << reader.size();
        if (!reader.read(&image))
            break;
        d << "QImage::size()=" << image.size() << "  \n" << image;
        icon.addPixmap(QPixmap::fromImage(image));
        if (!reader.jumpToNextImage())
            break;
    }

debug\qtbug48103.exe ..\qtbug48103\ok.ico
5.6.0
loadWithImageReader "..\\qtbug48103\\ok.ico" supportsOption(QImageIOHandler::Size)= true
# 0 QImageReader::size()= QSize(48, 48) QImage::size()= QSize(48, 48)
 QImage(QSize(48, 48),format=6,depth=32,devicePixelRatio=1,bytesPerLine=192,byteCount=9216)
# 1 QImageReader::size()= QSize(0, 0) QImage::size()= QSize(32, 32)
 QImage(QSize(32, 32),format=6,depth=32,devicePixelRatio=1,bytesPerLine=128,byteCount=4096)
# 2 QImageReader::size()= QSize(0, 0) QImage::size()= QSize(16, 16)
 QImage(QSize(16, 16),format=6,depth=32,devicePixelRatio=1,bytesPerLine=64,byteCount=1024)
# 3 QImageReader::size()= QSize(82, 71) QImage::size()= QSize(256, 256)
 QImage(QSize(256, 256),format=24,depth=8,devicePixelRatio=1,bytesPerLine=256,byteCount=65536)
# 4 QImageReader::size()= QSize(39, 0) QImage::size()= QSize(128, 128)
 QImage(QSize(128, 128),format=6,depth=32,devicePixelRatio=1,bytesPerLine=512,byteCount=65536)

QIcon::availableSizes= (QSize(48, 48), QSize(32, 32), QSize(16, 16), QSize(256, 256), QSize(128, 128))