vim-patch:8.0.1806: InsertCharPre causes problems for autocomplete

Problem:    InsertCharPre causes problems for autocomplete. (Lifepillar)
Solution:   Check for InsertCharPre before calling vpeekc(). (Christian
            Brabandt, closes vim/vim#2876)
39de952577

src/nvim/edit.c

@@ -5541,13 +5541,15 @@ insertchar (
   // 'paste' is set)..
   // Don't do this when there an InsertCharPre autocommand is defined,
   // because we need to fire the event for every character.
+  // Do the check for InsertCharPre before the call to vpeekc() because the
+  // InsertCharPre autocommand could change the input buffer.
   if (!ISSPECIAL(c)
       && (!has_mbyte || (*mb_char2len)(c) == 1)
+      && !has_event(EVENT_INSERTCHARPRE)
       && vpeekc() != NUL
       && !(State & REPLACE_FLAG)
       && !cindent_on()
-      && !p_ri
-      && !has_event(EVENT_INSERTCHARPRE)) {
+      && !p_ri) {
 #define INPUT_BUFLEN 100
     char_u buf[INPUT_BUFLEN + 1];
     int i;

src/nvim/testdir/test_popup.vim

@@ -746,6 +746,36 @@ func Test_popup_complete_backwards()
   bwipe!
 endfunc
 
+fun! Test_complete_o_tab()
+  throw 'skipped: Nvim does not support test_override()'
+  let s:o_char_pressed = 0
+
+  fun! s:act_on_text_changed()
+    if s:o_char_pressed
+      let s:o_char_pressed = 0
+      call feedkeys("\<c-x>\<c-n>", 'i')
+    endif
+  endf
+
+  set completeopt=menu,noselect
+  new
+  imap <expr> <buffer> <tab> pumvisible() ? "\<c-p>" : "X"
+  autocmd! InsertCharPre <buffer> let s:o_char_pressed = (v:char ==# 'o')
+  autocmd! TextChangedI <buffer> call <sid>act_on_text_changed()
+  call setline(1,  ['hoard', 'hoax', 'hoarse', ''])
+  let l:expected = ['hoard', 'hoax', 'hoarse', 'hoax', 'hoax']
+  call cursor(4,1)
+  call test_override("char_avail", 1)
+  call feedkeys("Ahoa\<tab>\<tab>\<c-y>\<esc>", 'tx')
+  call feedkeys("oho\<tab>\<tab>\<c-y>\<esc>", 'tx')
+  call assert_equal(l:expected, getline(1,'$'))
+
+  call test_override("char_avail", 0)
+  bwipe!
+  set completeopt&
+  delfunc s:act_on_text_changed
+endf
+
 func Test_popup_complete_info_01()
   new
   inoremap <buffer><F5> <C-R>=complete_info().mode<CR>