Faces: How to clear an attribute while continuing to inherit other attributes?

[Dave Goel <deego3@gmail.com>]

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How to unset an inherited face attribute while continuing to inherit other
attributes?

Face 'a inherits from 'b, which sets foreground to yellow. How do I keep 'a
inheriting from 'b, but set the foreground to (nothing), so that actually
rendering 'a goes to the default foreground (white) instead of yellow?

Emacs allows us to set a's foreground to nil (instead of 'unspecified), but
it magically becomes 'unspecified when queried. That's not what I was
aiming for. That is because 'unspecified means actually rendering face 'a
would yield yellow, the color inherited from 'b instead of the default
foreground.

Here's a test:

(progn
  (defface a nil "")
  (defface b nil "")
  (set-face-attribute  'b nil :foreground "yellow")
  (set-face-attribute 'a nil :inherit 'b)
  (set-face-attribute 'a nil :foreground nil)
  ;; let's chase how 'a would render:
  (face-attribute 'a :foreground nil t))

=> "yellow"

We expected either "white", the default foreground.


So, unfortunately, setting a's foreground to nil seems to be equivalent to
setting a's foreground to 'unspecified.

Once I set it to nil, I edebugged (face-attribute), and it calls  '
internal-get-lisp-face-attribute which yields 'unspecified instead of the
expected nil.

(Please cc replies if you can. I tested all this with emacs24.)

Thanks
Dave

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