bug#7617: 24.0.50; `expand-file-name': removal of slashes

[Drew Adams]

> > Besides a workable but ugly replace-regexp-in-string hack that saves
> > and restores consecutive slashes around a call to
> > `expand-file-name', I don't see a good way to do that.
> 
> If you seek suggestions for how to solve your problem in a non-ugly
> way, please tell more about the problem.  It's clear that one
> _solution_ to that problem is to have the old behavior of
> expand-file-name.  But that behavior is gone and will not come back.
> To suggest other solutions, we need to understand the problem.

The behavior is no longer provided by `expand*'.
But that behavior is what I want to achieve, in Lisp.
Achieving that behavior is the problem.
I understand that `expand*' is no longer the solution.

> > And we should say what the function
> > does/returns otherwise (e.g., say that it is undefined - i.e., that
> > it is defined only in the case where the args are acceptable names
> > for the file system).  If we tell users that the args must be of a
> > form acceptable by the current file system, then we should also tell
> > them what happens otherwise (raise an error?...).
> 
> If the argument doesn't have a valid file-name semantics, all bets are
> off.

Then that's what we should tell users: If either arg is not a file name
acceptable to the file system then the behavior is undefined (aka we have no
idea what will happen).  This is not obvious - since we demand valid file names
and produce a canonical name, one might well wonder what happens otherwise,
whether we raise an error, for example.