justbuild currently hard-codes use of clang-19 from llvm-toolchain-19
(which has several RC bugs and is likely to be removed from testing).
Comments in debian/justbuild.makefile suggest that this is because the
current version of fmtlib in unstable only works with older clang:

By default, Debian builds most packages with gcc, and I see that
justbuild is compiled with gcc on riscv64. Would it be possible to
build justbuild with gcc on all architectures? That would mean it's no
longer blocked by #1114025, and no longer in danger of autoremoval.

Thanks,
    smcv

Dear maintainer,

I've prepared an NMU for justbuild (versioned as 1.6.5-0.2) and uploaded
it to DELAYED/2. Please feel free to tell me if I should cancel it.

cu
Adrian

We believe that the bug you reported is fixed in the latest version of
justbuild, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1147663@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Adrian Bunk <bunk@debian.org> (supplier of updated justbuild package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Sat, 19 Sep 2026 21:15:00 +0300
Source: justbuild
Architecture: source
Version: 1.6.5-0.2
Distribution: unstable
Urgency: medium
Maintainer: Oliver Reiche <oliver.reiche@gmail.com>
Changed-By: Adrian Bunk <bunk@debian.org>
Closes: 1147663
Changes:
 justbuild (1.6.5-0.2) unstable; urgency=medium
 .
   * Non-maintainer upload.
   * Build with gcc until the fixed fmtlib is in unstable.
     (Closes: #1147663)
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