bug#12670: 24.2.50; Uncustomizable defcustom

[Chong Yidong]

Katsumi Yamaoka <yamaoka@jpl.org> writes:

> evaluate the following form, perform M-x customize-option, and press
> the [INS] button.
>
> (defcustom foo nil
>   "Doc"
>   :type '(repeat (group (group :inline t (choice string function)))))
>
> Then you will find a funny thing there and see it is unable to
> edit.

I can't reproduce this on Emacs 23.4, the emacs-24 branch, or trunk.
After opening the customization buffer for `foo', I click on INS and it
opens up

INS DEL Choice: Value Menu invalid (nil)

and clicking on "Value Menu" allows me to insert a function or string.