bug#38587: base64-decode-region breaks encoding

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: larsi@gnus.org,  38587@debbugs.gnu.org
> Date: Mon, 16 Dec 2019 00:41:48 +0200
> 
> >> But when it's know that the source string was in UTF-8,
> >> shouldn't it be reversible?  What is needed for that?
> >
> > The source string is not in UTF-8, it is in internal Emacs
> > representation of strings.
> 
> Is internal Emacs representation compatible with UTF-8?

It is (currently).  But it isn't UTF-8, strictly speaking.  In
particular, raw bytes are represented there by 2-byte sequences that
aren't valid UTF-8.