Because of the use of checked integers in the implementation of QPoint, (pt1-pt2).x() will calculate the y value, too (using checked arithmetic, so we can't expect the optimizer to remove this as dead code). Ditto .y() and x value. This is unnecessary and introduces a failure state where there wasn't one before (e.g. if, while calling x(), pt1.xp - pt2.xp doesn't overflow, but pt1.yp - pt2.yp does). Fix by calculating the difference only for the dimension at hand, which, to keep using checked ints, requires QLine to have access to QPoint::{xp,yp}, so add friendship. Amends 1145e1709d1072f7dd45683e9c25a14615603854. Found in API-review. Pick-to: 6.10 Change-Id: I163beb65da5fc50e5c12e7a140bcfbc1d07a69a6 Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>