Bug#1109174: libfst-dev: fails to upgrade from bookworm because of libfst26

[Lucas Nussbaum]

Package: libfst-dev
Version: 1.7.9-5
Severity: serious

While testing upgrades from bookworm to trixie, I ran into the following
problem.
If I:
- install libfst-dev in bookworm
- upgrade to trixie (apt update && apt dist-upgrade)
Then libfst-dev is not upgraded.

MWE:
mmdebstrap --chrooted-customize-hook='apt -y install libfst-dev && sed -e s/bookworm/trixie/ -i /etc/apt/sources.list && apt update && apt dist-upgrade -y && apt dist-upgrade -y -o Debug::pkgProblemResolver=true' bookworm /dev/null

Relevant output:
Investigating (0) libfst-dev:amd64 < 1.7.9-5 -> 1.8.4-2 @ii umU Ib >
Broken libfst-dev:amd64 Depends on libfst26:amd64 < none | 1.8.4-2 @un uH > (= 1.8.4-2)
  Considering libfst26:amd64 0 as a solution to libfst-dev:amd64 0
  Holding Back libfst-dev:amd64 rather than change libfst26:amd64
 Try to Re-Instate (1) libfst-dev:amd64
Done
Entering ResolveByKeep
The following packages were automatically installed and are no longer required:
  libapt-pkg6.0  libargon2-1  libgnutls30  libtasn1-6  libunistring2
Use 'apt autoremove' to remove them.

Not upgrading:
  libfst-dev

What happens is that libfst22 and libfst26 cannot be co-installed,
because libfst22 Conflicts with libfst, which is Provided by libfst26.

I'm not sure of how to untangle this.

This affects upgrades of the following packages:
hfst
libfst-dev
libfst-tools
libhfst-dev
libhfst55
python3-hfst
python3-libhfst

Lucas