clang++-21: massive failures in Perl 5 test suite

[jkeenan]

Smoke-test results, mainly from @cjg-cguevara, indicate that as various Linux distributions roll out upgrades of LLVM to include clang version 21, we are running into massive test failures when we compile perl with clang++-21. Consider these smoke test reports run at c0b8ab1ec6:

linux 6.12.60_1 [Void Linux rolling] clang++ Clang 21.1.4 https://perl.develop-help.com/db/5522079

linux 6.17.9-300.fc43.x86_64 [Fedora Linux 43 (Server Edition)] clang++ version Clang 21.1.6 (Fedora 21.1.6-1.fc43) https://perl.develop-help.com/db/5522092

linux 6.17.9-arch1-1 [Arch Linux] clang++ version Clang 21.1.6 (*1) https://perl.develop-help.com/db/5522090

linux 6.18.0-3-lts [Alpine Linux v3.23 3.23.0] clang++ version Alpine Clang 21.1.2 https://perl.develop-help.com/db/5522075

It appears that we don't yet have any smoke-test reports from other OSes, e.g., FreeBSD, run with clang++-21. On my own machines, clang++-19 is as new as I can get. I suspect, however, that this problem will not be confined just to Linux.

(We're also getting lots of build-time warnings on these, but at least the build is completing.)

Can we get this investigated?

Thank you very much.