<C-x><C-o> "Pattern not found" even with a Ctags file (C/C++)

Question

<C-x><C-o> returns Pattern not found in C/C++ files even with a ctags file.

• It used to work and I haven't made any configuration/plugin changes that I know of
• Starting vim with vim -u NONE -U NONE and manually setting omnifunc=ccomplete#Complete results in the same problem (as does vim --clean), so it doesn't seem to be caused by my vim config
• Python and HTML omnicomplete still works, so the problem seems to be with the tags file
• <C-]> and <C-x><C-]> still work and complain when the tags file is removed, so the tags file seems to be fine??
• I'm using Gutentags with Universal-Ctags in Vim 8.2 to automatically generate the tags file (tried a stock Neovim config as well and still the same error)

Any ideas what the problem could be or how I could go about debugging it?

Answer 1

This is an old question, but since it is getting bumped without an answer, I will provide an explanation anyway.

The reason <C-x><C-o> (omni-completion) returns "Pattern not found" can be found in the documentation:

CTRL-X CTRL-O           Guess what kind of item is in front of the cursor and
                        find the first match for it

The important word here is Guess. It cannot guess if you haven't typed at least a single letter, which I suspect that's what OP was trying to do.

but it definitely works using a prefix (in this case, the letter "f")

Most other completion functions work by searching, even though the description might lead you to believe that it shouldn't work either. For example,

CTRL-X CTRL-]           Search for the first tag that starts with the same
                        characters as before the cursor. 

Contrary to the most complex omni-complete (see :h complete-functions) which requires a prefix, this type of autocompletion (tag completion) defaults to "show everything" as if nothing was filtered.

However, this doesn't explain why other filetypes seemed to work. As far as I know, other filetypes should work in exactly the same way, unless custom functions are set for the omnifunc option.

Answer 2

In the end I just gave up and installed the clang-complete plugin, which just worked without any fuss. I get the feeling omni-complete may not do what I think it does. The documentation suggests it should be able to complete struct member names but I simply wasn't able to get it to do so.