bug#33566: 26; `group' :type for defcustom

[Drew Adams]

> This appears to be the same issue as bug #31309
> 
> https://urldefense.proofpoint.com/v2/url?u=https-
> 3A__debbugs.gnu.org_cgi_bugreport.cgi-3Fbug-
> 3D31309&d=DwIDaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=kI3P6l
> jGv6CTHIKju0jqInF6AOwMCYRDQUmqX22rJ98&m=q_aD3h6_uQ5XqacM7h8h0yyr4bJrYed
> Kx_CcGGTUXRU&s=BlmxwvxgNdZTq6QqgdDRglwd0qa8LpG-SLOp13SLoV4&e=
> 
> 
> At present in wid-edit.el we see:
> 
> (define-widget 'group 'default
>   [...]
>   :format "%v"
> 
> And that format of "%v" is triggering the error.
> 
> In #31309 I noted that:
> 
> > the parent `editable-field' widget says:
> > "Note: In an ‘editable-field’ widget, the ‘%v’ escape must be
> preceded
> > by some other text in the ‘:format’ string (if specified)."
> 
> And so I copied the :format "%{%t%}: %v" used by the 'string type.
> 
> If we make that same change here:
> 
> (define-widget 'group 'default
>   [...]
>   :format "%{%t%}: %v"
> 
> Then the example code from this new bug works.
> 
> 
> Possibly there are other such bugs here as well?
> 
> 8 matches for ":format "%v"" in buffer: wid-edit.el
>     594:  :type '(repeat (cons :format "%v"
>     597:                             (string :format "%v")))))
>    1870:  :format "%v"
>    2214:  :format "%v"
>    2392:  :format "%v"
>    2749:  :format "%v"
>    2905:  :format "%v"
>    3565:                              (cons :format "%v"

Great.  Good catch and follow-up.

A couple things:

This (e.g. the recipe in bug #33566) has apparently been broken since at least 
Emacs 20.  But in Emacs 20 the message in *Messages* is just "Bad format".

But :group is supposed to not show any :tag.  So I think this is what we should 
use:

(define-widget 'group 'default
  "A widget which groups other widgets inside."
  :convert-widget 'widget-types-convert-widget
  :copy 'widget-types-copy
  :format ":\n%v"  ; <================
  :value-create 'widget-group-value-create
  :value-get 'widget-editable-list-value-get
  :default-get 'widget-group-default-get
  :validate 'widget-children-validate
  :match 'widget-group-match
  :match-inline 'widget-group-match-inline)

There doesn't seem to be any way to eliminate the `:' altogether, though.

I think the doc needs to be corrected in (widget) Editable Text Fields.  This 
is not correct/sufficient;

  *Warning:* In an ‘editable-field’ widget, the ‘%v’
  escape must be preceded by some other text in the
  ‘:format’ string (if specified).

It's not sufficient that just any old text precede
the `%v'.  There must be a colon, AFAICT.

It should instead say something like this:

  *Warning:* In an ‘editable-field’ widget, the ‘%v’
  escape must be preceded by a colon (`:') in the
  ‘:format’ string (if specified).  No tag name need
  precede the colon, but the colon must be present.
  (Additional text can follow the colon and precede
  the ‘%v’.)

(Perhaps more testing or checking of just what can
precede the %v needs to be done.)

I'd also drop the "*Warning:*", personally, and
instead say that this is necessary or you get a
bad format message (or some such).