Hi,

I realized that a certain defcustom form is unable to customize
a user option.  To reproduce this, 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.  After some investigations I found this is due to
`widget-default-get', that fails to get the default value of
components within an inlined form (I mean (bar :inline t ...)).

A patch is attached, not tested with all sorts of defcustom
forms though.