Dear Maintainer,

In a clean debian:trixie environment with a foreign architecture enabled,
libpoco-dev:amd64 and libpoco-dev:arm64 cannot be installed simultaneously:

# docker run --rm debian:trixie bash
dpkg --add-architecture arm64
apt-get update
apt-get install -s libpoco-dev:amd64 libpoco-dev:arm64
...
libpoco-dev : Conflicts: libpoco-dev:arm64 but 1.13.0-6+b2 is to be
installed
libpoco-dev:arm64 : Conflicts: libpoco-dev but 1.13.0-6+b2 is to be
installed

Root cause: the libpoco-dev binary package in trixie (1.13.0-6+b2) is not
marked "Multi-Arch: same". The field was deliberately removed in 1.13.0-6
("Disclaim libpoco-dev to be MA same", closing my earlier bug #1064971).
Closing #1064971 by dropping Multi-Arch instead of resolving the underlying
file layout means the -dev package is, by design, no longer co-installable
across architectures -- which is a regression for cross-arch development.

The good news: this has effectively already been corrected in unstable.
libpoco-dev 1.14.2-6 (sid) restores "Multi-Arch: same", and I have verified
it is genuinely co-installable: all 855 shared headers under /usr/include
are byte-identical between amd64 and arm64, and every architecture-specific
.so development symlink lives in its triplet path (/usr/lib/<triplet>/), so
there are no shared-path file conflicts.

Request: please restore "Multi-Arch: same" for libpoco-dev in trixie via a
stable update (it is a one-line debian/control change, already present in
1.14.2-6). I'm happy to prepare the debdiff / NMU if that helps, given the
package is currently QA-maintained.

Note on a second, independent blocker (not libpoco-dev's bug, flagged here
for completeness): even with Multi-Arch: same restored, co-installing the
two libpoco-dev arches is still blocked by the dependency on
default-libmysqlclient-dev, whose libmariadb-dev-compat is not co-installable
(libmariadb-dev-compat:amd64 Conflicts libmariadb-dev-compat:arm64). That is
a separate issue in the mariadb packaging and I will report it separately.

Thanks,
Dick Hollenbeck

The patch (Blocker 1) — trivial, and exactly what unstable already did.
Against the 1.13.0-6 source, in debian/control, the libpoco-dev stanza:

Package: libpoco-dev
Section: libdevel
Architecture: any
+Multi-Arch: same
Depends: ${misc:Depends},
libpocofoundation... (= ${binary:Version}),
...