[vim/vim] complete_info().selected is -2 when 'menuone' is set and only 1 completion (#6945)

[Ben Jackson]

Describe the bug

When completeopt contains menuone and there is only a single completion candidate, and that completion candidate is 'selected', complete_info().selected is -2.

The docs for complete_info().selected suggest that -1 is the only valid negative value.

To Reproduce

Detailed steps to reproduce the behavior:

1. Run vim --clean -S test.vim test.vim

test.vim:

function! Compl( findstart, query )
  if a:findstart
    return col( '.' )
  endif
  return [ 'test' ]
endfunction

set completeopt=menuone
set completefunc=Compl

"here

2. Go to the end of the file
3. Type <C-x><C-u> (see that test is inserted/selected)
4. Remaining in insert mode, type <C-r>=string( complete_info().selected )
5. Observe that -2 is inserted.

Expected behavior

complete_inf().selected is 0

Environment (please complete the following information):

• Vim version 8.2.1456
• OS: macOS 10.15
• Terminal: iTerm2

Additional context

I trapped it in the debugger, and unsurprisingly in the following:

    if (ret == OK && (what_flag & CI_WHAT_SELECTED))
	ret = dict_add_number(retdict, "selected", (compl_curr_match != NULL) ?
			compl_curr_match->cp_number - 1 : -1);

compl_curr_match->cp_number is -1. Scanning the code, this seems to suggest the current compl match is "invalid" in some way, but if I'm honest I didn't dig too deeply into it.

I'm not totally sure this is.a bug exactly, but in my use case I was writing a test and using complete_info() to confirm that the selected completion item is correct.

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[Bram Moolenaar]

Closed #6945 via b806aa5.

[csh]

hello，when I use coc.nvim and vim(8.2.1767)，and set txt inoremap <expr> <cr> cimplete_info()["selected"] != "-1" ? "\<CR>" : '\<C-g>u\<CR> . If only 1 completion and I select the only one, it will insert a ， but it works well more completions. I don't understand your test(step 4)，can you test again?

[csh]

@puremourning I use vim(patch 8.2.1987 ubuntu20.10)，and I use your test.vim, complete_info().selected is -1 when 'menuone' is set and only 1 completion, would you mind test it again?

[puremo...@gmail.com]

I get the same behaviour. we get `-1` when it should be `0`.

The original report was that it would return `-2` which was invalid by the spec. `-1` is just incorrect I think.