[Development] Raising the minimum to C++20

[Marc Mutz]

Hi Maurice,

On 03.05.23 08:01, Maurice Kalinowski wrote:
> Basically, the idea from our end has been to take a two-step approach by first enabling every developer to use C++20 in their projects and potentially add helpers/functionality where possible. Only at a later stage we can then start using it in Qt itself. Vladimir Minenko has all the details, as he has been laying out the rough plan with the foundation engineering.

This is the status quo. Qt users can use C++20 with all active Qt 
versions, incl. 5.15, but it's a best-effort thing atm, not something we 
check on the CI.

Having taken this first step, the next one is, indeed, to make C++20 
mandatory (for building Qt and therefore for using Qt[1]).

Thanks,
Marc

[1] I also heard the idea to make C++20 mandatory for building Qt, but 
user projects could continue to use C++17. That would require _forward_ 
binary compatibility between stdlib implementations. Given that a C++20 
stdlib must needs contain more symbols (C++20-only features), it's 
almost guaranteed that forward BC isn't maintained by stdlibs.

-- 
Marc Mutz <marc.mutz at qt.io>
Principal Software Engineer

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