C++XX requirement/support, which bits of RTT to keep maintaining

[pierrewillenbrockdfki]

In #21 (comment), @doudou said:

@chhtz and @pierrewillenbrockdfki properly migrating to C++XX (we have to choose the target XX here) is not part of this PR. IMO we should first agree on a subset of RTT we want to support and move what we do not support (e.g. scripting for me) to a rtt-extras package (or remove it altogether ... it is after all our RTT)

So, lets discuss and decide.

[pierrewillenbrockdfki]

Did a bit of digging in https://gcc.gnu.org/projects/cxx-status.html.

ubuntu 18.04: gcc-7, gcc-8, defaults to -std=c++14, has support for most of c++17. c++17 abi not stabilized, yet.

ubuntu 20.04: gcc-8, gcc-9, defaults to -std=c++14, has full support for c++17, support of some bits of c++20 with -std=c++2a. No concepts, no operator<=>, no consteval, only early constexpr restriction relaxation bits, no modules, no coroutines, no using enum, no new template deduction features

ubuntu 22.04: gcc-11, gcc-12, defaults to -std=c++17, has support for some bits of c++23, most of c++20.

ubuntu 24.04: gcc-13, gcc-14, defaults to -std=c++17, has support for some bits of c++26, most of c++23.

gentoo: gcc-15, defaults to -std=c++17, has support for most bits of c++26. Notably, no reflection.

gcc-16 defaults to -std=c++20.

Rocks Qt4 port works up to ubuntu 20.04. The choice there would be to limit the required feature set to c++17 or the bits gcc-9 supports of c++20. I would not like to limit support to those, but would keep more recent features/ keywords in mind where possible, and if there are known incompatibilities, check via preprocessor and #error.

If we put ubuntu 20.04 and Qt4 aside, the default becomes -std=c++17 already and one could think about requiring c++20 or c++23. c++26 seems to me to be a bit too ambitous.