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I'm developing an free and open source application that uses Qt. I'm not making any changes to Qt library itself. I wanted to know:

  1. On what license can I publish my application sources.
  2. On what license can I distribute app's executables (that are linked with Qt)
  3. Are there any other requirements that I need to fulfill in order to be compliant with Qt licenses?

Here's what I believe is the answer:

  1. I can publish my application sources on any license that I want (MIT for example).
  2. I need to distribute app's executables with Qt compliant licence (LGPL in that case) as the executable linked with Qt library is a derivative work of Qt library.
  3. I need to put Qt compliant license in the same package I distribute the executables with. Since I don't display any copyright notices of my own during the application execution, I don't need to put any Qt's copyright notices either. Additionally since I use plain Qt library without any modifications I don't need to distribute its source code as it's publicly available on their website.

I have also read Qt FAQ, but I think Qt's requirements are not 100% compliant with the LGPL itself. Those parts I have doubts about (this is part of "3.7. What are my obligations when using Qt under the LGPL?"):

You will need to deliver the complete source code of Qt (including all modifications you did or applied) to your users/customers. Alternatively you need to provide a written offer with instructions on how to get the source code. Please also note that this has to be under your control, so a link to the source code provided by the Qt Project or Qt Company is not sufficient.

I haven't seen anything in LGPL that could back up this claim (that a link to Qt source code from their website isn't enough).

The user of an application or device using LGPL licensed software has to be notified of their rights by providing a copy of the LGPL license to the end user and displaying a prominent notice about your usage of LGPL licensed software.

I understand this as apart from adding LGPL license to the package I use to distribute the app I need explicitly display proper notice to the end user in the application, while LGPL says:

If the work during execution displays copyright notices, you must include the copyright notice for the Library among them, as well as a reference directing the user to the copy of this License.

and my work doesn't display any copyright notices during the execution, so according to this there's no need to display Qt's copyright notice either.

I would be very grateful if someone competent could review my thinking.

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According to the FAQ that you link, the relevant LGPL is LGPLv3. As LPGLv3 makes clear, it is a set of permissions hacks to GPLv3 ("This license is a set of additional permissions added to version 3 of the GNU General Public License"). So to answer questions it may be necessary to refer to one or both of those licences.

On what license can I publish my application sources?

I agree with you that it can be under any licence (LGPLv3 s4: "You may convey a Combined Work under terms of your choice...").

On what license can I distribute app's executables (that are linked with Qt)?

I disagree with your answer; you can distribute the app's executable under any licence that you choose; again, see LGPLv3 s4. However, there are additional obligations, as your next question suggests.

Are there any other requirements that I need to fulfill in order to be compliant with Qt licenses?

LGPLv3 s4a and b require you to inform users that your app contains the Qt library, and you must give them copies of the GPLv3 and LPGLv3 so they can know their rights thereunder. I agree with your reading of LGPLv3 s4c as regards not needing to display Qt's copyright notice.

As for s4d obligations, s4(d)0 says you must "Convey the Minimal Corresponding Source under the terms of this License". Since this Licence includes GPLv3 except where LGPLv3 explicitly says otherwise, I contend that your obligations under this section must be considered in the context of GPLv3 s6.

I heard a very interesting talk at Copyleft Conference 2019, given by a chap from no less a monolith than Intel. My writeup can be read in LWN, but the relevant part was that Intel had satisfied themselves that outsourcing the hosting of corresponding source was perfectly lawful under GPL. That is, for GPLv3 s6d conveyances of your app, where you offer users the ability to download the executable, you don't have to host the relevant corresponding source yourself; it suffices to point to the source on someone else's server. You note that Qt say this doesn't satisfy your obligations; I think Qt are wrong about this.

However, the obligation to ensure that the corresponding source is complete and of the appropriate version remains yours, as does the responsibility for it remaining available. Given that Qt are actively hostile to your strategy, they could perfectly lawfully keep changing the URL that points to their source, or update the version without telling anyone, or in some other way frustrate you. You may find that it is less effort to host it yourself; but if you're willing to go to the effort of constantly checking that the source you've linked to on Qt's servers is still correct and available, then I think you're OK to do it, and so do some others.

IANAL/IANYL; you should get proper legal advice before relying on this strategy in your actual business.

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