apt upgrade problem with libgcc4 and libgcc-s4

Hi all,

I just ran into an issue during upgrading an NFS root FS with Debian Sid
not touched for quite a while:

I originally had gcc-7 and gcc-8 installed and `apt upgrade` gave me the
following list for "NEW packages" to be installed:

```
cpp-9 gcc-10-base gcc-9 gcc-9-base gfortran-9 libapt-pkg6.0 libcbor0
libcrypt-dev libcrypt1 libctf-nobfd0 libctf0
   libdns-export1109 libevent-2.1-7 libffi7 libfido2-1 libgcc-9-dev
libgcc-s4 libgfortran-9-dev libhogweed5 libicu63
   libip4tc2 libip6tc2 libisc-export1105 libisl22 libjson-c4 libnettle7
libnftables1 libperl5.30 libprocps8
   libpython3.8-minimal libpython3.8-stdlib libreadline8
linux-image-5.4.0-4-parisc logsave pci.ids perl-modules-5.30
   python3.8 python3.8-minimal
```

...but during upgrading the packages a problem surfaced:

```
[...]
Unpacking libgcc-s4:hppa (10-20200222-1) ...
dpkg: error processing archive
/var/cache/apt/archives/libgcc-s4_10-20200222-1_hppa.deb (--unpack):
  trying to overwrite '/lib/hppa-linux-gnu/libgcc_s.so.4', which is also
in package libgcc4:hppa 1:8.3.0-3
Errors were encountered while processing:
  /var/cache/apt/archives/libgcc-s4_10-20200222-1_hppa.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
[...]
```

...which I was unable to solve with `apt`.

In the end I could solve it by manually installing libgcc4 from [1]
together with libgcc-s4 from [2] with `dpkg -i [...]`.

[1]:
http://snapshot.debian.org/archive/debian-ports/20200229T090821Z/pool-hppa/main/g/gcc-10/libgcc4_10-20200222-1_hppa.deb

[2]:
http://snapshot.debian.org/archive/debian-ports/20200229T090821Z/pool-hppa/main/g/gcc-10/libgcc-s4_10-20200222-1_hppa.deb

To me it looks like libgcc-s4 is related to gcc-10 and is a dependency
of newer libgcc4 versions related to gcc-10 and if dependencies are
installed earlier then their dependents - which would make sense - this
problem seems to be unsolvable if the originally installed libgcc4
version "shares" files with a dependency of its newer version. Am I
correct here?

OTOH gcc-10 wasn't installed during this upgrade - I only installed it
manually later for a test - so maybe some other package out of:

```
root@c8000:~# apt-cache rdepends --installed libgcc-s4
libgcc-s4
Reverse Depends:
   libgcc-10-dev
   zstd
   libstdc++6
   libgcc4
   libcc1-0
   libapt-pkg6.0
   gcc-10
   apt
   apt-utils
```

...was pulling in libgcc-s4 during the upgrade process (zstd,
libstdc++6, libcc1-0, apt and apt-utils were upgraded in this process,
libapt-pkg6.0 was newly installed). Not sure if that is actually a bug
in the dependencies of one of these packages, as an upgrade seems to not
work for a case like mine then.

Did anybody else run into this issue?

Cheers
Frank