Re: dired-compare-directories

[Juri Linkov]

>   I do not know whether anything relies on this behavior, but before
>   making any incompatible change this close before a release, one would
>   better be really extra careful.

`read-directory-name' inherits its current behavior from `read-file-name'.
The Emacs manual says:

     If DEFAULT is `nil', `read-file-name' tries to find a substitute
     default to use in its place, which it treats in exactly the same
     way as if it had been specified explicitly.  If DEFAULT is `nil',
     but INITIAL is non-`nil', then the default is the absolute file
     name obtained from DIRECTORY and INITIAL.  If both DEFAULT and
     INITIAL are `nil' and the buffer is visiting a file,
     `read-file-name' uses the absolute file name of that file as
                      ========================================
     default.  If the buffer is not visiting a file, then there is no
     default.  In that case, if the user types <RET> without any
     editing, `read-file-name' simply returns the pre-inserted contents
                               ========================================
     of the minibuffer.

Silently using the absolute file name is a very error-prone behavior.

There was a bug report recently about `copy-file' and other similar
command assuming a wrong filename.  This was fixed by introducing
a new interactive specifier "G" (which is not mnemonic).  I think this
workaround was a mistake.  `read-file-name' (and `read-directory-name')
with nil DEFAULT should not return a filename not explicitly
specified by the user in the minibuffer.  They should return the
pre-inserted contents, unless specially programmed to return something
else by specifying the non-nil DEFAULT argument.

-- 
Juri Linkov
http://www.jurta.org/emacs/