libc++-18-dev explicitly declares the ability to coinstall via
Multi-Arch: same. Attempting to do this practically fails though.

$ mmdebstrap --variant=essential --verbose unstable /dev/null --architectures=amd64,i386 --include=libc++-18-dev,libc++-18-dev:i386
...
Selecting previously unselected package libunwind-18:i386.
Preparing to unpack .../09-libunwind-18_1%3a18.1.8-18_i386.deb ...
Unpacking libunwind-18:i386 (1:18.1.8-18) ...
dpkg: error processing archive /tmp/apt-dpkg-install-869OWv/09-libunwind-18_1%3a18.1.8-18_i386.deb (--unpack):
 trying to overwrite shared '/usr/lib/llvm-18/lib/libunwind.so.1.0', which is different from other instances of package libunwind-18:i386
Selecting previously unselected package libc++abi1-18:i386.
Preparing to unpack .../10-libc++abi1-18_1%3a18.1.8-18_i386.deb ...
Unpacking libc++abi1-18:i386 (1:18.1.8-18) ...
dpkg: error processing archive /tmp/apt-dpkg-install-869OWv/10-libc++abi1-18_1%3a18.1.8-18_i386.deb (--unpack):
 trying to overwrite shared '/usr/lib/llvm-18/lib/libc++abi.so.1.0', which is different from other instances of package libc++abi1-18:i386
Selecting previously unselected package libc++1-18:i386.
Preparing to unpack .../11-libc++1-18_1%3a18.1.8-18_i386.deb ...
Unpacking libc++1-18:i386 (1:18.1.8-18) ...
dpkg: error processing archive /tmp/apt-dpkg-install-869OWv/11-libc++1-18_1%3a18.1.8-18_i386.deb (--unpack):
 trying to overwrite shared '/usr/lib/llvm-18/lib/libc++.so.1.0', which is different from other instances of package libc++1-18:i386
Selecting previously unselected package libc++abi-18-dev:i386.
Preparing to unpack .../12-libc++abi-18-dev_1%3a18.1.8-18_i386.deb ...
Unpacking libc++abi-18-dev:i386 (1:18.1.8-18) ...
dpkg: error processing archive /tmp/apt-dpkg-install-869OWv/12-libc++abi-18-dev_1%3a18.1.8-18_i386.deb (--unpack):
 trying to overwrite shared '/usr/lib/llvm-18/lib/libc++abi.a', which is different from other instances of package libc++abi-18-dev:i386
Selecting previously unselected package libunwind-18-dev:i386.
Preparing to unpack .../13-libunwind-18-dev_1%3a18.1.8-18_i386.deb ...
Unpacking libunwind-18-dev:i386 (1:18.1.8-18) ...
dpkg: error processing archive /tmp/apt-dpkg-install-869OWv/13-libunwind-18-dev_1%3a18.1.8-18_i386.deb (--unpack):
 trying to overwrite shared '/usr/lib/llvm-18/lib/libunwind.a', which is different from other instances of package libunwind-18-dev:i386
Selecting previously unselected package libc++-18-dev:i386.
Preparing to unpack .../14-libc++-18-dev_1%3a18.1.8-18_i386.deb ...
Unpacking libc++-18-dev:i386 (1:18.1.8-18) ...
dpkg: error processing archive /tmp/apt-dpkg-install-869OWv/14-libc++-18-dev_1%3a18.1.8-18_i386.deb (--unpack):
 trying to overwrite shared '/usr/lib/llvm-18/lib/libc++.a', which is different from other instances of package libc++-18-dev:i386
Errors were encountered while processing:
 /tmp/apt-dpkg-install-869OWv/09-libunwind-18_1%3a18.1.8-18_i386.deb
 /tmp/apt-dpkg-install-869OWv/10-libc++abi1-18_1%3a18.1.8-18_i386.deb
 /tmp/apt-dpkg-install-869OWv/11-libc++1-18_1%3a18.1.8-18_i386.deb
 /tmp/apt-dpkg-install-869OWv/12-libc++abi-18-dev_1%3a18.1.8-18_i386.deb
 /tmp/apt-dpkg-install-869OWv/13-libunwind-18-dev_1%3a18.1.8-18_i386.deb
 /tmp/apt-dpkg-install-869OWv/14-libc++-18-dev_1%3a18.1.8-18_i386.deb
E: Sub-process env returned an error code (1)
...
$

Given the freeze timing, I recommend dropping the Multi-Arch
declaration instead of attempting to move files around.

Helmut

Control: found -1 1:18.1.8-17

This is reproducible in testing (replace unstable by trixie in the sample
mmdebstrap command-line) so there's no need for it to prevent the
unrelated RC bug fix in 1:18.1.8-18 from migrating.

     smcv

Dear submitter,

as the package llvm-toolchain-18 has just been removed from the Debian archive
unstable we hereby close the associated bug reports.  We are sorry
that we couldn't deal with your issue properly.

For details on the removal, please see https://bugs.debian.org/1120403

The version of this package that was in Debian prior to this removal
can still be found using https://snapshot.debian.org/.

Please note that the changes have been done on the master archive and
will not propagate to any mirrors until the next dinstall run at the
earliest.

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

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pp.
Thorsten Alteholz (the ftpmaster behind the curtain)