[Development] Switching Qt default builds to C++20 where supported

[Thiago Macieira]

On Monday 14 October 2024 07:56:35 GMT-7 Vladimir Minenko via Development 
wrote:
> 6) when should we do this?
> The last time we discussed C++20, it was made clear that we can only do such
> changes on LTS+1 releases. That's 6.9.
> 
> The last time we discussed this was at QtCS24
> (https://wiki.qt.io/QtCS2024_C%2B%2B20), where IIRC the conclusion was that
> ATM, there is no reason to switch to C++20. So, it is definitely not in
> 6.9.

That was "not require C++20 for users". I'm not asking for that. I am asking 
that we switch how we build Qt. Users won't be affected.

> And because of this, I once asked to set up a nightly build with forced
> `-std=c++17`, e.g.
> https://testresults.qt.io/coin/integration/qt/qtbase/tasks/nightly172875090

Building Qt with strict C++ conformance is not supported. You can have that 
nightly, but I will neither fix issues find in it nor accept patches that fix 
only issues found in it unless they're trivial.

> 3. We already had two cases when C++20-specific code was merged and the
> build failed only because of this. There is a notification email set on
> failures for this build. There are only people from The Qt Company so far
> on it. If someone else is interested, I would gladly change this and
> include other people ;-)

I seriously doubt that. Every time I mistakenly forget a typename or use 
another C++20 feature, the build fails immediately in the CI.

I am not proposing we change that.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Principal Engineer - Intel DCAI Platform & System Engineering
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