I have a proprietary closed-source project. In this project I use Qt5 under LGPL. Now I want to add documentation to my project. Can I just add Qt Assistant into my distribution (under GPL3-EXCEPT, see bellow)?
According to the website and the source code Qt Assistant is available under Commercial License and GPL with exceptions (aka Qt Company GPL Exception 1.0):
The Qt Company GPL Exception 1.0
As a special exception you may create a larger work which contains the output of this application and distribute that work under terms of your choice, so long as the work is not otherwise derived from or based on this application and so long as the work does not in itself generate output that contains the output from this application in its original or modified form.
As a special exception, you have permission to combine this application with Plugins licensed under the terms of your choice, to produce an executable, and to copy and distribute the resulting executable under the terms of your choice. However, the executable must be accompanied by a prominent notice offering all users of the executable the entire source code to this application, excluding the source code of the independent modules, but including any changes you have made to this application, under the terms of this license.
The first exception is similar to the GCC Runtime Library Exception. Let's focus on the exception no.2. Does it allow me to use Qt Assistant "under the terms of my choice"?
Here's my sequence of thoughts: I take "this application"(Qt Assistant), I add a "plugin" to it(my doc files) and produce an "executable". Now I ship it under the terms of my choice.
The thing that stops me is the word "Plugins". What is a Plugin in this particular context? Qt Assistant doesn't have a plugin system in C++/Qt meaning - dll/so/dylib library. Qt uses the exact same terms for other tools: Designer, Linguist with lupdate, lrelease etc. https://github.com/qt/qttools