Fix 'move-to-column' when invisible text follows a TAB

* src/indent.c (scan_for_column): Accept 2 more arguments, and
report through them the position corresponding to PREVCOL.  All
callers changed.
(Fmove_to_column): Use the prev_col's position to test for a TAB
instead of assuming that the TAB is just before point (which is
false when there's invisible text around).  (Bug#43587)

* test/src/indent-tests.el: New file.

1 files changed, 59 insertions, 0 deletions

diff --git a/test/src/indent-tests.el b/test/src/indent-tests.el
new file mode 100644
index 00000000000..7d1a6ce6dc3
--- /dev/null
+++ b/test/src/indent-tests.el
@@ -0,0 +1,59 @@
+;;; indent-tests.el --- tests for src/indent.c  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2020 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; 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:
+
+(ert-deftest indent-tests-move-to-column-invis-1tab ()
+  "Test `move-to-column' when a TAB is followed by invisible text."
+  (should
+   (string=
+    (with-temp-buffer
+      (insert "\tLine starting with INVISIBLE text after TAB\n")
+      (add-text-properties 2 21 '(invisible t))
+      (goto-char (point-min))
+      (move-to-column 7 t)
+      (buffer-substring-no-properties 1 8))
+    "       ")))
+
+(ert-deftest indent-tests-move-to-column-invis-2tabs ()
+  "Test `move-to-column' when 2 TABs are followed by invisible text."
+  (should
+   (string=
+    (with-temp-buffer
+      (insert "\t\tLine starting with INVISIBLE text after TAB\n")
+      (add-text-properties 3 22 '(invisible t))
+      (goto-char (point-min))
+      (move-to-column 12 t)
+      (buffer-substring-no-properties 1 11))
+    "\t    \tLine")))
+
+(ert-deftest indent-tests-move-to-column-invis-between-tabs ()
+  "Test `move-to-column' when 2 TABs are mixed with invisible text."
+  (should
+   (string=
+    (with-temp-buffer
+      (insert "\txxx\tLine starting with INVISIBLE text after TAB\n")
+      (add-text-properties 6 25 '(invisible t))
+      (add-text-properties 2 5 '(invisible t))
+      (goto-char (point-min))
+      (move-to-column 12 t)
+      (buffer-substring-no-properties 1 14))
+    "\txxx    \tLine")))