[QGIS-Developer] Minimal version of supported Qt6 version in QGIS 3.99

[Greg Troxel]

DelazJ via QGIS-Developer <qgis-developer at lists.osgeo.org> writes:

> Thanks Nyall for the information (and the pointer).
> If QGIS seems unusable with the minimal supported Qt version, should we not
> raise that min value?
> I see that Debian, Ubuntu (and their derivatives?) latest versions come
> with Qt 6.8 LTS. QGIS users may encounter bugs due to their specific
> version or do you think the risk is highly mitigated?

This is a messy question, but from a packaging viewpoint, it is best for
things like QGIS to require recent versions only when QGIS really
requires it, because of a used API that is not in older versions, and to
be careful and restrained in raising it.

Of course QGIS users may encounter bugs with old Qt, old kernels, old
gdal, and everything else.  Adopting that means that QGIS should require
the most recent formal release of all dependencies, at all times.
That's bonkers.

Entirely separately from what QGIS needs to build are the issues that
come with each Qt version.  It's not QGIS's place to break compatibility
with older ones.  Everyone should make their own decision about which
version of everything to run, and generally the right answer is the
latest stable release, or for upstreams that aren't really stable, the
latest micro release that's .1, meaning disqualifying x.y.0, or z.0.0).

It's important to realize that any particular packaging system with an
old release series might adopt bugfixes that resolve the trouble.
Insisting on new versions because somebody might not have bugfixes is
rude and causes unnecessary work.

If 6.8 LTS is problematic (note that it's LTS, so presumably fixes are
pulled up), then that's on Debian and Debian derivatives to address it.
It's not a QGIS problem.

If there are problems with some particular older QT versions, that are
problems with QGIS only, and not generally recognized outside of the
QGIS world, then it makes sense to have a comment in the release notes
about this.  I suspect that this isn't what's going on, and that it's
just that QT6 is moving fast and you really want to be on the latest
release.

As always, the LTS concept is problematic.  If someone wants to run old
QT, they should be ok with old QGIS, is my take on the world.  Or if
their LTS company wants to do maintenance.  But it's not ok for LTS
users/companies to demand that upstreams accomodate LTS.  (It is ok to
ask for min versions to be about used APIs and to refrain from crossing
into "you shouldn't choose that" judgement.  It is of course ok to offer
judgement and advice in a way that doesn't break the build!)