Setting PageOrientation in Qt 5.x - Qt6.x

Pre 6.x settting printer's page orienation is easy:

QPrinter printer;
printer.setPageOrientation (QPrinter::Landscape);

This member is removed in 6.x. Code should be replaced by

  QPrinter printer(QPrinter::HighResolution);
  QPageLayout l (printer.pageLayout ());
  // the margins of l are zero here, too small for the next statement:
  l.setOrientation (QPageLayout::Landscape);
  if (printer.setPageLayout (l))
    qDebug () << "Landscape succeeded";
  else
    qDebug () << "Landscape failed";

setPageLayout fails, for no explicit printer margins are given and

QPlatformPrintDevice::isValidPageLayout

checks the bounds for the margin during setPageLayout.

QPrinter::setFullPage, one can virtually print on the whole page.

Retrieving the minimal or sufficient printmargins for the printer, is done internally, but doesn't seem easily available by public members.

I don't understand why QPlatformPrintDevice::isValidPageLayout need to be checked here.