bug#31309: 26.1; Customize widget :type 'text reports "bad format"

[Phil Sainty <psainty@orcon.net.nz>]

It seems that the multi-line `text' widget is broken.  e.g.:

(defcustom foo
   "Text value\nspanning multiple\nlines."
   "Multi-line text field."
   :type 'text)

M-x customize-option RET foo RET

Message: custom-variable-value-create: Bad format


wid-edit.el defines the `text' widget like so:

(define-widget 'text 'editable-field
   "A multiline text area."
   :keymap widget-text-keymap)


The `string' widget (an 'editable-field derivative which works fine)
is defined as:

(define-widget 'string 'editable-field
   "A string"
   :tag "String"
   :format "%{%t%}: %v"
   :complete-function 'ispell-complete-word
   :prompt-history 'widget-string-prompt-value-history)

I also see 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)."

If I redefine `text' as follows (by copying the :format from `string')
then the widget seems to work correctly.

(define-widget 'text 'editable-field
   "A multiline text area."
   :format "%{%t%}: %v"
   :keymap widget-text-keymap)


I'm not familiar with the internals of widgets, so I don't know if
this is the best fix, but I see that all the other `define-widget'
derivatives of `editable-field' do have explicit :format strings.

However I would have thought that in the absence of an explicit
:format, a *valid* default would be used, so I think the main bug
is in `editable-field' itself, which does not follow its own rule
when defining its own :format, which is simply "%v" (testing confirms
that this value is used by default for derivative widgets which do
not override :format).

(define-widget 'editable-field 'default
   "An editable text field.
Note: In an `editable-field' widget, the `%v' escape must be preceded
by some other text in the `:format' string (if specified)."
   :convert-widget 'widget-value-convert-widget
   :keymap widget-field-keymap
   :format "%v"
   ...)

Changing that to :format "%{%t%}: %v" proves to successfully resolve
the original problem when the `text' widget definition is reverted.

n.b. In the process of testing I noticed that "preceded by some other
text" is not entirely accurate, as a value of " %v" (with a leading
space) continued to trigger the bug.


I also note that grepping for 'editable-field in general turns up a
pile of uses of (widget-create 'editable-field) which also do not
follow the same rule; either omitting the format:

(setq widget (widget-create 'editable-field :size 15))

or setting a :format which begins with "%v":

(setq comment-widget
       (widget-create 'editable-field
                      :size 60
                      :format "%v "
                      :value (or (image-dired-get-comment file) "")))

I've not attempted to test any of that code, so I don't know
whether the same problem would occur.





In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
  of 2018-04-15 built on shodan
Windowing system distributor 'The X.Org Foundation', version 
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