bug#39143: 26.3; `widget-complete' in `customize-option'

[Eli Zaretskii]

> Date: Wed, 15 Jan 2020 09:54:58 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> emacs -Q
> 
> `M-x customize-option default-frame-alist'
> 
> Click `INS'.
> 
> Enter `cursor-color' for field `Parameter: '.
> 
> Put cursor at beginning of field `Value: ', and delete the default
> value, `nil'.
> 
> Use `M-TAB' (or `ESC TAB').
> 
> You get this error/message: "Not in an editable field".

In Emacs 27 get the message "No completions available for this field"
instead.  So this problem is already solved.

> 1. That message is incorrect.  The cursor is definitely in an editable
> field.  (Same incorrect message if you first type `Blu', then `M-TAB'.)

This part is already solved.

> 2. Emacs should provide reasonable completion for both fields,
> `Parameter' and `Value', for `default-frame-alist'.

It does provide completions for Parameter, although it's debatable
whether they are "reasonable".