bug#11562: 24.0.97; Error "Not in an editable field" when `M-TAB' in editable Customize field

[Drew Adams]

emacs -Q
 
(defcustom foo '(current-time-string)
  "@@@@@@@@@@@@@@@"
  :type 'sexp)
 
M-x customize-option foo
 
Type this into the editable field:
 
(current-
 
Then hit `M-TAB'.  You get this inappropriate error message:
 
"Not in an editable field"
 
If completion is not available, the message should say that.  It should
not claim that the field is not editable when it is.
 

In GNU Emacs 24.0.97.1 (i386-mingw-nt5.1.2600)
 of 2012-05-16 on MARVIN
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.6) --no-opt --enable-checking --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include'