bug#39680: Test case

["Kévin Le Gouguec" <kevin.legouguec@gmail.com>]

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Finally nailed it, after finding out about ert-simulate-command.

The following patch adds a test that passes on master and emacs-27, and
"successfully fails" when reverting Stefan's fix (commit c1ce9fa7f2).


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From c83fbd50f0f0999814f09706fc908df5c22c5a47 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?K=C3=A9vin=20Le=20Gouguec?= <kevin.legouguec@gmail.com>
Date: Tue, 19 May 2020 23:17:04 +0200
Subject: [PATCH] Add test for bug#39680

* test/lisp/electric-tests.el (electric-pair-undo-unrelated-state):
New test.
---
 test/lisp/electric-tests.el | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/test/lisp/electric-tests.el b/test/lisp/electric-tests.el
index 56d1bdb110..67f474cbd5 100644
--- a/test/lisp/electric-tests.el
+++ b/test/lisp/electric-tests.el
@@ -546,6 +546,24 @@ js-mode-braces-with-layout-and-indent
       (electric-pair-delete-pair 1)
       (should (equal "" (buffer-string))))))
 
+\f
+;;; Undoing
+(ert-deftest electric-pair-undo-unrelated-state ()
+  "Make sure `electric-pair-mode' does not confuse `undo' (bug#39680)."
+  (with-temp-buffer
+    (buffer-enable-undo)
+    (electric-pair-local-mode)
+    (let ((last-command-event ?\())
+      (ert-simulate-command '(self-insert-command 1)))
+    (undo-boundary)
+    (let ((last-command-event ?a))
+      (ert-simulate-command '(self-insert-command 1)))
+    (undo-boundary)
+    (ert-simulate-command '(undo))
+    (let ((last-command-event ?\())
+      (ert-simulate-command '(self-insert-command 1)))
+    (should (string= (buffer-string) "(())"))))
+
 \f
 ;;; Electric newlines between pairs
 ;;; TODO: better tests
-- 
2.26.2