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.

2 files changed, 65 insertions, 0 deletions

diff --git a/test/data/themes/faces-test-dark-theme.el b/test/data/themes/faces-test-dark-theme.el
new file mode 100644
index 00000000000..2c114069f9c
--- /dev/null
+++ b/test/data/themes/faces-test-dark-theme.el
@@ -0,0 +1,33 @@
+;;; faces-test-dark-theme.el --- A dark theme from tests ;;; -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(deftheme faces-test-dark
+  "")
+
+(custom-theme-set-faces
+ 'faces-test-dark
+ '(diff-added ((t (:foreground "Green" :extend t))))
+ '(diff-changed-face ((t (:foreground "Khaki"))))
+ '(diff-file-header-face ((t (:background "grey20" :foreground "ivory1")))))
+
+(provide-theme 'faces-test-dark)
+
+;;; faces-test-dark-theme.el ends here
diff --git a/test/data/themes/faces-test-light-theme.el b/test/data/themes/faces-test-light-theme.el
new file mode 100644
index 00000000000..fc22d47cf8d
--- /dev/null
+++ b/test/data/themes/faces-test-light-theme.el
@@ -0,0 +1,32 @@
+;;; faces-test-light-theme.el --- A dark theme from tests ;;; -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(deftheme faces-test-light
+  "")
+
+(custom-theme-set-faces
+ 'faces-test-light
+ '(diff-added ((t (:inherit diff-changed :background "light green" :extend t))))
+ '(diff-changed ((t (:background "light steel blue")))))
+
+(provide-theme 'faces-test-light)
+
+;;; faces-test-light-theme.el ends here