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

[Volker Hilsheimer]

> On 27 Nov 2024, at 02:18, Thiago Macieira <thiago.macieira at intel.com> wrote:
> 
> On Tuesday 26 November 2024 16:32:20 Pacific Standard Time Tor Arne Vestbø via 
> Development wrote:
>>> Someone who cares about Windows/MSVC may want to take the mantle here,
>>> because cpp-winrt will be disabled for C++17.
>> 
>> Please elaborate. Disabled by who, in what configuration, and why?
> 
> Disabled by https://codereview.qt-project.org/c/qt/qtbase/+/606908, which I've 
> just submitted to fix that bug. Because cpp-winrt/C++17 is binary-incompatible 
> with cpp-winrt/C++20, I am making a choice to only support the one with the 
> official <coroutine> support.
> 
> Alternative analyses of the problem are welcome too.


As per the comments to that change, it’s a) not going to fly and b) not the choice a single person can make, even if that person is the maintainer of Qt Core :)

A more pragmatic and scalable choice seems to be to not try to support mixing C++ versions in a static build of Qt. I see no reason for doing that, and explicitly disallowing that option would create clarity for anyone trying to do so.

Volker