[5.15.0 alpha] deprecated iterator operator used when QSet used as a signal/slot parameter

Using a QSet as signal/slot parameter results in a warning:

C:\Qt\5.15.0\mingw73_64\include\QtCore\qset.h:177: warning: 'QHash<K, V>::const_iterator& QHash<K, V>::const_iterator::operator--() [with Key = QString; T = QHashDummyValue]' is deprecated [-Wdeprecated-declarations]
inline QT_DEPRECATED const_iterator &operator--() { --i; return *this; }
^~~

Steps to reproduce:

1. Build the attached project
2. Observe compile output