To use QPoint in a QSet or QHash, you have to have a qHash function for it:

https://doc.qt.io/qt-5/qhash.html#the-qhash-hashing-function

For some reason, Qt doesn't define a qHash function for QPoint. This leads to people defining their own:

https://stackoverflow.com/questions/24498993/cannot-use-qpoint-as-qhash-key

Incidentally, in my case, another library that I'm using defines its own qHash for QPoint in a header that I'd rather not include (because it's for unrelated stuff). Yet if I try to define my own, I get an error about qHash being redefined. This would not be an issue if Qt defined hash functions for all of its applicable types.