[Development] Raising the minimum to C++20

[Maurice Kalinowski]

> 
> Can you provide more details on what the difficulties are and when relief should
> be expected for this?
> 
> When you say "supply chain issues" for C++17, I am thinking that those
> customers are buying compilers in a DVD in a box and that is stuck in a
> container still sailing from China. That obviously can't be the case.
> 
[Maurice Kalinowski] 

I would not go that far, but in some industries you are somewhat in that situation.
When we prepared scoping for Qt 6, we were discussing with the OS vendors on the C++17 situation and when we can expect all RTOSes to have C++17 support. Most of them somewhat met their roadmaps and estimates.

However, there is the situation that OEMs rely on Tier1 suppliers and those Tier1s then provide the BSP including toolchain, OS version etc.
Unless those have an incentive to upgrade their toolchain, OS vendors can push updates as they want, those are stuck in between and do not reach device makers desks.

This is the situation we experience in multiple industries still, with an increasing pressure from multiple angles to get those finally supporting Qt 6. Hence, things are getting better for C++17 _now_.

To summarize, we learned to add a delta of at least 2-3 years between OS (especially RTOS) compiler and toolchain availability until those reach out to users and customers, who then start building Qt applications and devices on top.

BR,
Maurice


> My proposal was that C++20 wouldn't be required until March 2024 for non-LTS
> users. The first of your LTS to require it would be the one from October 2024,
> and it wouldn't become the oldest LTS until 3 years after 6.5 (March 2026).
>