bug#51270: 28.0.50; xref core package 1.3.0 published, breaks etags

[Ingo Lohmar]

This appears to be a packaging bug: The xref core package has changed
its version number from 1.2.2 to 1.3.0 in
35a752863afc9f9075473e34c395d36e0bd18bff.

The breakage happens because xref 1.3.0 has been published on GNU ELPA
https://elpa.gnu.org/packages/ (although the details page shows 1.2.2 as
the latest version, don't know why).  I am using the "eglot" package,
which requires xref (at a lower minimum version), and when upgrading
packages this morning, I got xref 1.3.0.

After that, everything related to etags fell apart (company backends,
after-save hooks to regenerate tags etc) with error messages about
undefined xref-location classes (and more).

Brief analysis: The new xref version switches from eieio to
cl-defstruct, which means that older etags (not a core package) code
breaks, because it relies on the xref-location class.  This seems to be
in violation of the comment in xref.el: "Avoid functionality that is not
compatible with the version of Emacs recorded above." (the version
"recorded above" is 26.1)

I am not really clear on how the "core package" idea is supposed to
work.  One solution would be to "un-publish" xref 1.3.0.  A more general
approach would introduce semantic versioning to package.el, and the
above change would require a version 2.0.0 in that world.


In GNU Emacs 28.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo 
version 1.16.0)
 of 2021-09-02 built on t14s-il
Repository revision: 4c49ec7f865bdad1629d2f125f71f4e506b258f2
Repository branch: feature/pgtk
Windowing system distributor 'System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --without-gsettings --with-pgtk'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM HARFBUZZ JPEG JSON LCMS2
LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PGTK
PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS XIM GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: ELisp/l

[...]

Memory information:
((conses 16 1269822 164531)
 (symbols 48 46201 11)
 (strings 32 209533 58772)
 (string-bytes 1 7112283)
 (vectors 16 94542)
 (vector-slots 8 1961389 54375)
 (floats 8 959 1792)
 (intervals 56 129413 1921)
 (buffers 992 56))