2025-08-17: the main fedora-clang checks have been migrated to LLVM 21. As have the clang-ASAN and clang-UBSAN checks. Tests as for the main fedora-clang checks but using LLVM 21 including its flang as the Fortran compiler. Other details as for https://www.stats.ox.ac.uk/pub/bdr/Rconfig/r-devel-linux-x86_64-fedora-clang . This was released on Aug 26 and planned to be included in Fedora 43. Releave notes are now available at https://releases.llvm.org/ Current results are from LLVM 21.1.0-rc3. Some binary distributions are available at https://github.com/llvm/llvm-project/releases and in Debian experimental. The LLVM Linux biaries do not support OpenMP. Although undocumentesd clang-21 seems to have made -Wtautological-overlap-compare the default. This warning could be added for earlier versions of clang. However, the release notes say - Improved the ``-Wtautological-overlap-compare`` diagnostics to warn about overlapping and non-overlapping ranges involving character literals and floating-point literals. The warning message for non-overlapping cases has also been improved (#GH13473"