Constexpr CRC for C++23

[jaskij]

This ended up a bit bigger than I originally intended, but all the tests pass.

What's added:

• basic C++23 support wired: CI, ETL_CONSTEXPR23, CI runs for C++23
• CRC checksums should be fully constexpr for C++23 builds

C++23

Since my feature requires C++23, I added basic support for it, including:

• ETL_CONSTEXPR23
• fixing tests not building because of std::aligned_storage deprecation
• very basic C++23 support detection (which fails for GCC 13)
• CI

Constexpr CRC

I wanted to created some binary headers for use with a bootloader, and for those headers to be guarded with CRC. Since I'm using ETL's implementation, I updated the library to actually allow using CRCs in a constant evaluated context.

The requirement for C++23 comes from P2647R1, which permits putting constexpr on etl::crc_table::add(). The limitation being the static part of the actual table. This could probably be relaxed, but I don't exactly see how at the moment, not with support for C++98 or C++03.

[semanticdiff-com]

Review changes with SemanticDiff.

[jaskij]

Sorry for bundling it like this, the PR was written in several hours of sleep-deprivation-fueled-mania. I can split the C++23 support later on.

I can also tell already that the check for C++23 will probably need to be refined in the future.

[jaskij]

That last push should fix the C++11 builds (my fault for using ETL_CONSTEXPR instead of ETL_CONSTEXPR14).

Right now I have absolutely no clue though what's going on in those C++23 test on clang. The include stack would point to something in UnitTest++?

[  8%] Building CXX object test/CMakeFiles/etl_tests.dir/test_algorithm.cpp.o
In file included from /home/runner/work/etl/etl/test/test_algorithm.cpp:29:
In file included from /home/runner/work/etl/etl/test/unit_test_framework.h:32:
In file included from /home/runner/work/etl/etl/test/UnitTest++/UnitTest++.h:1:
In file included from /home/runner/work/etl/etl/test/UnitTest++/UnitTestPP.h:5:
In file included from /home/runner/work/etl/etl/test/UnitTest++/TestMacros.h:7:
In file included from /home/runner/work/etl/etl/test/UnitTest++/ExecuteTest.h:8:
In file included from /home/runner/work/etl/etl/test/UnitTest++/MemoryOutStream.h:9:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/sstream:40:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/istream:40:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/ios:44:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/ios_base.h:41:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/locale_classes.h:40:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/string:67:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/memory_resource.h:41:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/uses_allocator_args.h:39:

[jwellbelove]

It looks like the clang checks are failing at the point where clang is being upgraded to version 17.
The messages seem to indicate that it is already at v17.

[jaskij]

Sorry about the multiple pushes, hopefully it's not much of an issue.

I don't have a Ubuntu system on hand, but removing the installation and changing the builds to clang/clang++ 18 should work, hopefully

[jaskij]

In light of #912 , I marked this PR as a draft. My current plan is:

• Improve ci #912
• general C++23 enablement work as a separate PR (right now it's in here)
• rebase or rewrite this PR