vim-patch:8.2.1673: complete_info() selected index has an invalid value

Problem:    complete_info() selected index has an invalid value. (Ben Jackson)
Solution:   Set the index when there is only one match. (closes vim/vim#6945)
            Add test for complete_info().
b806aa5bd9

src/nvim/edit.c

@@ -5242,6 +5242,7 @@ static int ins_complete(int c, bool enable_pum)
     } else if (compl_curr_match->cp_next == compl_curr_match->cp_prev) {
       edit_submode_extra = (char_u *)_("The only match");
       edit_submode_highl = HLF_COUNT;
+      compl_curr_match->cp_number = 0;
     } else {
       // Update completion sequence number when needed.
       if (compl_curr_match->cp_number == -1) {

src/nvim/testdir/test_ins_complete.vim

@@ -312,6 +312,24 @@ func Test_completefunc_args()
   delfunc CompleteFunc
 endfunc
 
+func CompleteTest(findstart, query)
+  if a:findstart
+    return col('.')
+  endif
+  return ['matched']
+endfunc
+
+func Test_completefunc_info()
+  new
+  set completeopt=menuone
+  set completefunc=CompleteTest
+  call feedkeys("i\<C-X>\<C-U>\<C-R>\<C-R>=string(complete_info())\<CR>\<ESC>", "tx")
+  call assert_equal("matched{'pum_visible': 1, 'mode': 'function', 'selected': -1, 'items': [{'word': 'matched', 'menu': '', 'user_data': '', 'info': '', 'kind': '', 'abbr': ''}]}", getline(1))
+  bwipe!
+  set completeopt&
+  set completefunc&
+endfunc
+
 " Check that when using feedkeys() typeahead does not interrupt searching for
 " completions.
 func Test_compl_feedkeys()