bug#43351: 27.1; [PATCH] Change ASCII handling in mm-charset-to-coding-system to us-ascii

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Sat, 12 Sep 2020 16:13:43 +0200
> Cc: 43351@debbugs.gnu.org
> 
> > In the fix for bug#5458 (2011-06-30), a change was made to
> > mm-charset-to-coding-system to support "ansi.x3.4*" as an alias for
> > 'ascii. As part of that patch 'us-ascii was also mapped to 'ascii. This
> > is problematic because decode-coding-string does not recognize 'ascii as
> > a coding system and throws an "Invalid coding system: ascii" exception.
> 
> Indeed; ascii isn't a valid coding system...  but poking around here, I
> think the function (despite its name) isn't really returning a coding
> system.  I mean, it does in most cases, but not for ascii.  :-/

Maybe the simplest solution is to define a coding-system-alias named
'ascii'.