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

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Drew Adams <drew.adams@oracle.com> writes:

> (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?

It looks like `widget-file-prompt-value' consults it, but I can't tell
when this function is called.