Stop splicing anonymous faces in font-lock-append-text-property

This is the same fix as 2019-04-29 "Refrain from splicing anonymous
faces in text properties", which was only applied to
font-lock-prepend-text-property.

* lisp/font-lock.el (font-lock-append-text-property): Distinguish list
of faces from property list.
* test/lisp/font-lock-tests.el: New test suite.

(Bug#35476)

2 files changed, 47 insertions, 1 deletions

diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index 7ff4e606fa6..95ca2f99c21 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -1417,7 +1417,12 @@ Optional argument OBJECT is the string or buffer containing the text."
 Arguments PROP and VALUE specify the property and value to append to the value
 already in place.  The resulting property values are always lists.
 Optional argument OBJECT is the string or buffer containing the text."
-  (let ((val (if (listp value) value (list value))) next prev)
+  (let ((val (if (and (listp value) (not (keywordp (car value))))
+                 ;; Already a list of faces.
+                 value
+               ;; A single face (e.g. a plist of face properties).
+               (list value)))
+        next prev)
     (while (/= start end)
       (setq next (next-single-property-change start prop object end)
             prev (get-text-property start prop object))
diff --git a/test/lisp/font-lock-tests.el b/test/lisp/font-lock-tests.el
new file mode 100644
index 00000000000..5d127039ff9
--- /dev/null
+++ b/test/lisp/font-lock-tests.el
@@ -0,0 +1,41 @@
+;;; font-lock-tests.el --- Test suite for font-lock. -*- lexical-binding: t -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs 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.
+
+;; GNU Emacs 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 GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+(require 'ert)
+
+(ert-deftest font-lock-test-append-anonymous-face ()
+  "Ensure `font-lock-append-text-property' does not splice anonymous faces."
+  (with-temp-buffer
+    (insert "foo")
+    (add-text-properties 1 3 '(face italic))
+    (font-lock-append-text-property 1 3 'face '(:strike-through t))
+    (should (equal (get-text-property 1 'face (current-buffer))
+                   '(italic (:strike-through t))))))
+
+(ert-deftest font-lock-test-prepend-anonymous-face ()
+  "Ensure `font-lock-prepend-text-property' does not splice anonymous faces."
+  (with-temp-buffer
+    (insert "foo")
+    (add-text-properties 1 3 '(face italic))
+    (font-lock-prepend-text-property 1 3 'face '(:strike-through t))
+    (should (equal (get-text-property 1 'face (current-buffer))
+                   '((:strike-through t) italic)))))
+
+;; font-lock-tests.el ends here