vim-patch:9.1.0112: Remove undo information, when cleaning quickfix buffer

Problem:  When the quickfix buffer has been modified an autocommand
          may invalidate the undo stack (kawarimidoll)
Solution: When clearing the quickfix buffer, also wipe the undo stack

fixes: vim/vim#13905
closes: vim/vim#13928

https://github.com/vim/vim/commit/f0d3d4a42657dca996e790aa829de3c6be7fdb63

Co-authored-by: Christian Brabandt <cb@256bit.org>

1 files changed, 40 insertions, 0 deletions

diff --git a/test/functional/legacy/autocmd_spec.lua b/test/functional/legacy/autocmd_spec.lua
new file mode 100644
index 0000000000..97051f3d3f
--- /dev/null
+++ b/test/functional/legacy/autocmd_spec.lua
@@ -0,0 +1,40 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local write_file = helpers.write_file
+local command = helpers.command
+local feed = helpers.feed
+local api = helpers.api
+local eq = helpers.eq
+
+before_each(clear)
+
+-- oldtest: Test_autocmd_invalidates_undo_on_textchanged()
+it('no E440 in quickfix window when autocommand invalidates undo', function()
+  write_file(
+    'XTest_autocmd_invalidates_undo_on_textchanged',
+    [[
+    set hidden
+    " create quickfix list (at least 2 lines to move line)
+    vimgrep /u/j %
+
+    " enter quickfix window
+    cwindow
+
+    " set modifiable
+    setlocal modifiable
+
+    " set autocmd to clear quickfix list
+
+    autocmd! TextChanged <buffer> call setqflist([])
+    " move line
+    move+1
+    ]]
+  )
+  finally(function()
+    os.remove('XTest_autocmd_invalidates_undo_on_textchanged')
+  end)
+  command('edit XTest_autocmd_invalidates_undo_on_textchanged')
+  command('so %')
+  feed('G')
+  eq('', api.nvim_get_vvar('errmsg'))
+end)