bug#25678: 25.1; defcustom type `(file :must-match t)' seems not to work

[Drew Adams <drew.adams@oracle.com>]

(defcustom foo-file ""
  ""
  :type '(file :must-match t) :group 'convenience)

M-x customize-option foo-file

I don't see anything that prevents you from using a file name that
corresponds to no existing file.  I don't see that completion (`M-TAB') 
enforces it to name an existing file.  And I don't see that simply 
inserting a bad file name is prevented, nor is setting the option to a
bad file name or saving it prevented.

What effect does (file :must-match t) actually have, if any?

If this is just confusion on my part, please consider clarifying the
description of the behavior in the doc: (elisp) `Simple Types'.

(I see the same behavior if `(file :must-match t)' is used within a
complex type, e.g. as the :value-type of an alist entry.)


In GNU Emacs 25.1.1 (x86_64-w64-mingw32)
 of 2016-11-15
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --without-compress-install 'CFLAGS=-O2
 -static -g3''