This reverts commit 92acc94fa98d19929d28feb5d543acf8c50c2290. The change broke QSet ordering guarantees: The documentation clearly states that each item from `other` that isn't already in `*this` is inserted ("STL insertion behavior"). Swapping *this and other breaks this. Independent of STL vs. Qt insertion behavior, making the picking of elements from containers "random" in the sense that the size of the container is now important, and not merely LHS vs. RHS, is a bad idea. [ChangeLog][QtCore][QSet] Fixed a regression in unite() that caused equivalent elements of `*this` to be overwritten by elements of `other` if `other.size()` was larger than `this->size()`. Pick-to: 6.9 6.8 Fixes: QTBUG-132500 Change-Id: Ia636b62325139d618b5467a643ff710716324296 Reviewed-by: Thiago Macieira <thiago.macieira@intel.com> Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>