Use C++20 instead of C++17.

[3405691582]

As discussed elsewhere; upgrade the version. Hopefully this doesn't break any interesting edge cases we care about.

[Databean]

The question was whether if we switched off the clang prebuilt to the distro clang, if using c++20 features would break anything.

Looking at popular distros, comparing against the clang feature set for c++20: https://clang.llvm.org/cxx_status.html

• Debian bookworm (current oldstable) is at clang 1.14
• Debian trixie (current stable) is at clang 1.19, same as our prebuilt
• Debian forky (current testing) is at clang 1.21, ahead of our prebuilt

Ubuntu offers multiple clang versions: https://documentation.ubuntu.com/ubuntu-for-developers/reference/availability/llvm/

Clang 19 has been supported since 24.04 and was made the default in 24.10.

https://packages.fedoraproject.org/pkgs/llvm/clang/index.html

Fedora 42 uses clang 20.

I think the conclusion is that at worst we'll break bookworm users when we use some of the c++20 features that were implemented later:

• Lambdas in unevaluated contexts
• Structured binding extensions
• Concepts (partial)
• Modules
• Class template argument deduction for aggregates
• Permit conversions to arrays of unknown bound