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bug#39233: .elc file - possibly outdated backward compatibility comments

From: Lars Ingebrigtsen
Subject: bug#39233: .elc file - possibly outdated backward compatibility comments
Date: Fri, 24 Jan 2020 15:36:22 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

Stefan Monnier <address@hidden> writes:

>> Will these changes still allow old Emacs binaries to load *.elc files
>> compiled by Emacs after these changes?
> New Emacsen loading old .elc won't be affected.
> Old Emacsen loading .elc files compiled by new Emacsen is something that
> has always been risky, tho it has sometimes worked.  My proposal is to
> make it emit a warning (so it will work as much as it does now, but
> will bring to the attention of the user that it's in murky territory).

Reading the fix for the compilation problem, it was:

-    (search-forward "\n;;; This file uses")

It's quite possible that I misunderstand the patch, but doesn't this
mean that (until a couple of days ago), Emacs assumed that that string
exists in .elc files unconditionally.  And now it does, so if you try to
use an Emacs from last week to load .elc files from this week, that
week-old Emacs will break?

That doesn't seem optimal.

(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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