bug#24089: 25.1; Dired: diff error on = <RET> if the file has a backup inside .~/

[Jorge Peixoto de Morais Neto]

On Fri, 2016-07-29 at 17:43 +0300, Eli Zaretskii wrote:
> I'm not sure why you expect that case to work.  How can Emacs
> reverse-engineer backup-directory-alist, to figure out which of its
> elements was in effect when the original file was backed up?  In your
> example, there's only one element in the alist, but in general there
> could be several.
> 
> It sounds like in this case the users cannot just press RET, they
> should actually type a file name.  Am I missing something?
I understood the intended behavior that way because of the dired-diff
docstring:
    If the file at point is a backup file, use its original.

Perhaps the docstring could be clarified?

Also dired-diff could be tweaked so that, when it is is invoked on a backup
file, the default would be empty.  This would avoid the diff error on = <RET>.
 I am not sure though.

Another possibility (which can be combined with the previous) is to tweak
dired-diff to reverse-engineer backup-directory-alist at least for a few
important easy particular cases, such as the following two:
   1. (("." . ".~"))
   2. (("." . "/ABSOLUTE/PATH"))
and possibly others.

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