[Development] Qt 6 co-installability with Qt 5

[Kevin Kofler]

Tor Arne Vestbø wrote:
> IMHO, suffixing our binaries is a kludge. It “solves” the major version
> transition, but not any of the other use cases such as minor version
> upgrades, different builds of the same Qt version, etc.

But the major versions are the only case where we need parallel installation 
of different versions of Qt in distributions to begin with.

For minor versions, there is a simple solution: we just ship the latest. 
Some distributions (such as Fedora) even ship new minor feature releases of 
Qt as updates to stable releases when needed. Due to the backwards 
compatibility rules, those upgrades are usually reasonably seamless.

Different builds of the same version are something we just don't do (except 
32-bit multilibs, but there, running the corresponding 64-bit binary is 
usually good enough).

> Qt Creator already provides a way to manage and switch Qt versions:
> https://doc.qt.io/qtcreator/creator-project-qmake.html
> 
> What we need is a command line tool for doing the same. That’s how other
> project solve things too (nvm, rvm, pyenv, etc). And we already have a
> tool for that, qtchooser. If maintenance of that is an issue, then let’s
> solve that.
> 
> Why is that not an acceptable solution for distros?

I have already answered that question in this thread:
https://lists.qt-project.org/pipermail/development/2020-November/040486.html

        Kevin Kofler