test: to reproduce regression or broken by vim#12971

// to repro and test 'index out of range' err

Signed-off-by: shane.xb.qian <shane.qian@foxmail.com>

src/testdir/test_ins_complete.vim

@@ -2279,8 +2279,16 @@ func Test_complete_info_index()
   call feedkeys("Go\<C-X>\<C-P>\<C-P>\<F5>\<Esc>_dd", 'tx')
   call assert_equal("eee", g:compl_info['items'][g:compl_info['selected']]['word'])
 
+  " Check if index out of range
+  " https://github.com/vim/vim/pull/12971
+  call feedkeys("Go\<C-X>\<C-N>\<C-P>\<C-P>\<F5>\<Esc>_dd", 'tx')
+  call assert_equal("fff", g:compl_info['items'][g:compl_info['selected']]['word'])
+  call feedkeys("Go\<C-X>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<F5>\<Esc>_dd", 'tx')
+  call assert_equal("aaa", g:compl_info['items'][g:compl_info['selected']]['word'])
+
   " Add 'noselect', check that 'selected' is -1 when nothing is selected.
   set completeopt+=noselect
+
   " Search forward.
   call feedkeys("Go\<C-X>\<C-N>\<F5>\<Esc>_dd", 'tx')
   call assert_equal(-1, g:compl_info['selected'])
@@ -2289,6 +2297,13 @@ func Test_complete_info_index()
   call feedkeys("Go\<C-X>\<C-P>\<F5>\<Esc>_dd", 'tx')
   call assert_equal(-1, g:compl_info['selected'])
 
+  " Check if index out of range
+  " https://github.com/vim/vim/pull/12971
+  call feedkeys("Go\<C-X>\<C-N>\<C-P>\<F5>\<Esc>_dd", 'tx')
+  call assert_equal("fff", g:compl_info['items'][g:compl_info['selected']]['word'])
+  call feedkeys("Go\<C-X>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<C-N>\<F5>\<Esc>_dd", 'tx')
+  call assert_equal("aaa", g:compl_info['items'][g:compl_info['selected']]['word'])
+
   set completeopt&
   bwipe!
 endfunc