bug#75328: 31.0.50; use-package cannot override a built in package

[Philip Kaludercic <philipk@posteo.net>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: "Mark A. Hershberger" <mah@everybody.org>
>> Cc: 75328@debbugs.gnu.org
>> Date: Sat, 04 Jan 2025 10:50:21 -0500
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > Did you try customizing package-install-upgrade-built-in?
>> 
>> Yes.  I also reported this to Jonas[1] and in the following discussion
>> he suggested that and I tried it[2].
>> 
>> The following does not check out the transient repository into
>> ~/.emacs.d/elpa/:
>> 
>>     rm -rf ~/.emacs.d &&
>>     echo '(setq package-install-upgrade-built-in t)
>>     (use-package transient :vc (:url "https://github.com/magit/transient" :rev :newest) :ensure t)' > ~/.emacs &&
>>     ~/bin/emacs
>
> Philip, shouldn't package-install-upgrade-built-in allow upgrading a
> built-in package via use--package's :vc keyword?

Built-in packages are not supported by package-vc right now, as
emacs.git is not a emacs package the way ELPA intends it.  There is also
the annoyance that without custom code (which would be doable if we had
to) we would have a separate checkout of emacs.git for every built-in
package that one wishes to clone that way.  This would be a lot easier
if Git would support subtree or single-file checkouts...