[Development] Raising the minimum to C++20

[Ville Voutilainen]

On Wed, 3 May 2023 at 03:41, Thiago Macieira <thiago.macieira at intel.com> wrote:
>
> C++23 is on the way, so maybe it's time for us to raise our minimum to the one
> version before that. Let's aim for Qt 6.7, because feature-freeze for 6.6 is
> within one month, and lets us warn our users this is coming.
>
> By this, I mean to:
> * modify our build system so Qt compiles with -std=c++20 or equivalent
> * require that user code compiling Qt headers be similarly done
> * remove the requirement for #if checks for C++17 Standard Library features
> * make a couple of C++20 features mandatory (see below)
>
> Of the C++20 features I currently see a good reason to make mandatory:
> * feature-test macros (no change: we're already using them)
> * spaceship operator and <compare> header
> * char8_t
> * std::is_constant_evaluated()
> * constinit
> * <bit> header
> * (maybe) designated initialisers
> * (maybe) constexpr from <algorithm> and <utility>

I don't see any of these as worth breaking embedded users who want new
Qt versions but don't yet
have the compilers that can give them these facilities.