bug#6401:

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Cc: tim@tenkan.org,  6401@debbugs.gnu.org
> Date: Tue, 22 Nov 2011 12:32:49 -0500
> 
> Eli Zaretskii wrote:
> 
> > Building Emacs is just one use case, even if we find a solution for
> > it.  The problem is much broader: Emacs running as a child of another
> > Emacs gets EMACSDATA etc. variables that could well point to a wrong
> > place, because they were set by a different Emacs version.
> 
> But I would imagine that building Emacs from inside Emacs accounts for
> the majority of cases where Emacs is run as a child of another Emacs.
> (I can't think of any other reason one would want to do it.)

The other important use case is running Emacs under a debugger invoked
through GUD.  This actually runs a higher risk of having two different
Emacs versions: imagine that a newer Emacs has a grave bug that
doesn't allow it to be used to run a debugger.