Make ':extend' inherited from default spec unless overridden

* lisp/faces.el (face-spec-recalc): Handle the :extend attribute
specially and always inherit it from the default spec unless
overwritten in a theme (bug#37774).

* test/lisp/faces-tests.el (faces--test-data-dir): New variable.
(faces--test-extend-with-themes): Use test themes instead of ones
from etc/themes.  Update expected values.

* test/data/themes/faces-test-dark-theme.el: New file.

* test/data/themes/faces-test-light-theme.el: New file.

* doc/lispref/display.texi (Face Attributes):
Update the description of ':extend'.

* etc/NEWS: Update the entry for ':extend'.

* etc/themes/adwaita-theme.el:
* etc/themes/deeper-blue-theme.el:
* etc/themes/dichromacy-theme.el:
* etc/themes/leuven-theme.el:
* etc/themes/light-blue-theme.el:
* etc/themes/manoj-dark-theme.el:
* etc/themes/misterioso-theme.el:
* etc/themes/tango-dark-theme.el:
* etc/themes/tango-theme.el:
* etc/themes/tsdh-dark-theme.el:
* etc/themes/tsdh-light-theme.el:
* etc/themes/wheatgrass-theme.el:
* etc/themes/wombat-theme.el: Remove the now-redundant ':extend'
attribute in all the themes.

1 files changed, 13 insertions, 6 deletions

diff --git a/lisp/faces.el b/lisp/faces.el
index dc5bcca7609..0f31628f5f9 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -1669,7 +1669,7 @@ The following sources are applied in this order:
   ;; `theme-face' records.
   (let ((theme-faces (get face 'theme-face))
         (no-match-found 0)
-        face-attrs theme-face-applied)
+        default-attrs face-attrs theme-face-applied)
     (if theme-faces
         (dolist (elt (reverse theme-faces))
           (setq face-attrs (face-spec-choose (cadr elt) frame no-match-found))
@@ -1677,13 +1677,20 @@ The following sources are applied in this order:
             (face-spec-set-2 face frame face-attrs)
             (setq theme-face-applied t))))
     ;; If there was a spec applicable to FRAME, that overrides the
-    ;; defface spec entirely (rather than inheriting from it).  If
-    ;; there was no spec applicable to FRAME, apply the defface spec
-    ;; as well as any applicable X resources.
+    ;; defface spec entirely rather than inheriting from it, with the
+    ;; exception of the :extend attribute (which is inherited).
+    ;;
+    ;; If there was no spec applicable to FRAME, apply the defface
+    ;; spec as well as any applicable X resources.
+    (setq default-attrs (face-spec-choose (face-default-spec face) frame))
     (unless theme-face-applied
-      (setq face-attrs (face-spec-choose (face-default-spec face) frame))
-      (face-spec-set-2 face frame face-attrs)
+      (face-spec-set-2 face frame default-attrs)
       (make-face-x-resource-internal face frame))
+    (when (and theme-face-applied
+               (eq 'unspecified (face-attribute face :extend frame t)))
+      (let ((tail (plist-member default-attrs :extend)))
+        (and tail (face-spec-set-2 face frame
+                                   (list :extend (cadr tail))))))
     (setq face-attrs (face-spec-choose (get face 'face-override-spec) frame))
     (face-spec-set-2 face frame face-attrs)))