patch 9.0.0789: dummy buffer ends up in a windowv9.0.0789

Problem:    Dummy buffer ends up in a window.
Solution:   Disallow navigating to a dummy buffer.

3 files changed, 29 insertions, 0 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 0849b7099..5a4825feb 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1351,6 +1351,13 @@ do_buffer_ext(
     if ((flags & DOBUF_NOPOPUP) && bt_popup(buf) && !bt_terminal(buf))
 	return OK;
 #endif
+    if ((action == DOBUF_GOTO || action == DOBUF_SPLIT)
+						  && (buf->b_flags & BF_DUMMY))
+    {
+	// disallow navigating to the dummy buffer
+	semsg(_(e_buffer_nr_does_not_exist), count);
+	return FAIL;
+    }
 
 #ifdef FEAT_GUI
     need_mouse_correct = TRUE;
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 0706e7307..6ba1b02df 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -3857,4 +3857,24 @@ func Test_autocmd_delete()
   call assert_true(autocmd_delete([test_null_dict()]))
 endfunc
 
+func Test_autocmd_split_dummy()
+  " Autocommand trying to split a window containing a dummy buffer.
+  auto BufReadPre * exe "sbuf " .. expand("<abuf>") 
+  " Avoid the "W11" prompt
+  au FileChangedShell * let v:fcs_choice = 'reload'
+  func Xautocmd_changelist()
+    cal writefile(['Xtestfile2:4:4'], 'Xerr')
+    edit Xerr
+    lex 'Xtestfile2:4:4'
+  endfunc
+  call Xautocmd_changelist()
+  call assert_fails('call Xautocmd_changelist()', 'E86:')
+
+  au! BufReadPre
+  au! FileChangedShell
+  delfunc Xautocmd_changelist
+  bwipe! Xerr
+  call delete('Xerr')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/version.c b/src/version.c
index 1c511f94b..05e619f06 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@ static char *(features[]) =
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    789,
+/**/
     788,
 /**/
     787,