Ah, I see I'm not quite right there. PyQt _used_ to include a file listing the usual open source licenses (MIT, BSD, Apache, Python and so on) as ok to use with their GPL code base, but they since removed that, because in the meantime it has become clearer that the GPL is already giving people that freedom.

https://riverbankcomputing.com/pipermail/pyqt/2014-February/...

Your statement sounds like GPL is giving a "freedom", so like you could use GPL code in an MIT licensed project (sorry if I misinterpreted your statement); this is not true, see https://www.tawesoft.co.uk/kb/article/mit-license-faq "Can I use GPL-licensed code in my MIT-licensed project? No. The project as a whole must conform to the terms of the GPL license and must be distributed under the terms of that license. Therefore such a project as a whole must be distributed as GPL, but can still contain MIT-licensed software."

So it is more like MIT licensed projects give you the freedom to use in a GPL project.

The keyword here is "linking exception". It is a hack to sidestep the licensing problems caused by dynamic linking. e.g. If I encase gcc in java, and release gcc.class under GPL3+, but use it extensively in a separate proprietary java program, does that infringe on the terms of the GPL? Does providing copies of the two together, with a single installer make a difference?

https://en.wikipedia.org/wiki/GPL_linking_exception

Free PyQT does not have the linking exception (which would be lgpl); only the commercial version has.

https://riverbankcomputing.com/commercial/license-faq