bug#21931: 25.0.50; behaviour of read-directory-name with double slashes

[Nicolas Richard <youngfrog@members.fsf.org>]

After I eval:
(let ((default-directory "/tmp"))
  (list (read-directory-name "foo" "/tmp/src/")
        (read-directory-name "foo" "/tmp/src//")
        (read-directory-name "foo" "src/")
        (read-directory-name "foo" "src//")
        (read-directory-name "foo" "foo//src/")
        (read-directory-name "foo" "foo//src//")))
and hit RET RET RET RET, I get :
("/tmp/src/" "/" "src/" "/" "/src/" "/")

Is this intended ?

We could normalize the directory name via expand-file-name in all cases,
e.g. :

modified   lisp/files.el
@@ -648,8 +648,7 @@ read-directory-name
   (unless dir
     (setq dir default-directory))
   (read-file-name prompt dir (or default-dirname
-				 (if initial (expand-file-name initial dir)
-				   dir))
+				 (expand-file-name (or initial "") dir))
 		  mustmatch initial
 		  'file-directory-p))

but the docstring states "Value is not expanded---you must call
`expand-file-name' yourself." so I guess the behaviour is important (and
I guess e.g. for tramp).

Should read-file-name be fixed or should the callers make sure to not
use double slashes ?

In GNU Emacs 25.0.50.1 (i686-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2015-11-14
Repository revision: ed2e7e20ae0945288c98091f308f5460c3453873
Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
System Description:	Ubuntu 14.04.3 LTS