Checks using LLVM 23, released 2026-08-25.

Other details as https://www.stats.ox.ac.uk/pub/bdr/Rconfig/r-devel-linux-x86_64-fedora-clang.

Release notes are not yet available.  However, the release notes for pre-24 at
https://libcxx.llvm.org/ReleaseNotes.html say

"In LLVM 23, libc++ dropped a lot of transitive includes in all language modes.
This improves compile time significantly, but causes programs which rely
on these includes to not compile anymore.
The _LIBCPP_KEEP_TRANSITIVE_INCLUDES_LLVM23 macro that was provided
in LLVM 23 to ease the transition has been removed in this release."

So if declaration(s) (especially in std:) are reported as missing, do ensure
that the header(s) which declare them are included.  Most commonly
<algorithm> or <iterator> is missing.

This is about libc++: some people build clang++ to by default link to
GCC's libstdc++.

LLVM usually provides binary builds (those for -rc3 are avaialble,
and suffice) at https://github.com/llvm/llvm-project/releases