bug#51595: 26.3; Face attribute value `unspecified' in `defface' breaks Customize UI

[Stefan Kangas]

tags 51595 + confirmed
found 51595 29.0.50
thanks

Drew Adams <drew.adams@oracle.com> writes:

> emacs -Q
>
> (defface foo '((t :background "LightBlue")) "DOC ..." :group 'faces)
> (defface bar '((t :foreground unspecified :background "LightBlue"))
>   "DOC..." :group 'faces)
>
> `M-x customize-face foo' shows the UI for the attributes of the face.
>
> `M-x customize-face bar' shows only the Lisp value of the face spec.  A
> user has no way to get from there to showing all of the attributes.
> There's no way to edit the face.
>
> The doc (Elisp manual) draws no such distinction - speaks of no such
> limitation.  It just lists `unspecified' as one of the possible face
> attributes:
>
>   "Apart from the values given below, each face attribute can have the
>    value 'unspecified'."

(That's in (info "(elisp) Face Attributes").)

> This looks like a lacuna in the UI.  You can specify `unspecified' in a
> `defface', and the face is perfectly usable and well-defined, but users
> can't edit it - can't manipulate its attributes.

I can reproduce this on current master.