std::jthread & libc++

[andrjohns]

Just some more info on what I mentioned during the meeting - std::jthread isn't universally available when using clang & libc++ until clang20, and is only available from clang18 onwards behind the -fexperimental-library flag: https://en.cppreference.com/cpp/compiler_support/20

Might be worth adding an informative assert when using clang & libc++ < 20 (advising to use libstdc++ or gcc or more recent clang), since clang 20 is less than a year old

[WardBrian]

Do you know if there were substantial changes between the LLVM18 implementation and LLVM20? I'd be happy to conditionally add -fexperimental-library to our cmake config

[andrjohns]

No idea on that! But can have a proper look tomorrow and get back to you

[bob-carpenter]

Also, thanks for letting us know @andrjohns. This is why we're still in an alpha release cycle!

We could also rewrite without std::jthread. The only relevant difference may be that it doesn't use RAII for joining.

But we're also using C++20 concepts, so I doubt we're going to get far with a pre C++20 compiler.

[WardBrian]

The only relevant difference may be that it doesn't use RAII for joining.

stop_token is also part of the jthread API, so we'd have to resort to a global variable for communication without it

[bob-carpenter]

That's a bigger fix. I don't mind specifying requiring a recent version of C++ for Walnutpie given that we're not requiring direct ABI compatibility.