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

[Vladimir Minenko]

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.

Effectively, I don't see much difference from the current situation, where we "pretend" that we require C++17 but in reality we just check that we can pass `-std=c++17` to the compiler. Any usage in Qt code of individual language/library features still requires protection, because they're not universally implemented.

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/nightly1728750903. 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 ;-)

--
Vladimir

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