Inconsistent <cword>, <C-R><C-W>, and keyword behavior

Question

I'm in a Ruby file, and because ! is not an iskeyword character, if I place the cursor on a call to mymethod! and attempt a tag lookup via C - ], Vim searches for mymethod (without the !). Adding ! to the iskeyword option causes other problems, which I won't go into here.

Given the above behavior, I expected hitting C - RC - W from command-line mode would insert mymethod on the command-line, but it actually inserts mymethod! (including the !). Running expand('<cword>'), on the other hand, results in mymethod.

What's going on here? The documentation describes C - RC - W, saying:

Insert the object under the cursor:
...

CTRL-W the Word under the cursor
CTRL-A the WORD under the cursor; see WORD

and <cword>:

<cword> is replaced with the word under the cursor (like star)

This is actually helpful in my case, as it means I can get tag lookups working correctly by putting the following in my .vim/ftplugin/ruby.vim:

nnoremap <buffer><silent> <C-]> :tag <C-R><C-W><CR>

But I'm curious where the nuanced behavior comes from and whether it's intentional, or whether it's likely to be fixed at some point, breaking my solution.

Answer 1

It is probably the result of the mappings in $VIMRUNTIME/ftplugin/ruby.vim, which include a mapping of <C-R><C-W> to <Plug><cword> which is then mapped to a call to RubyCursorIdentifier(). That function contains expressions more complicated than I wished to follow, but it probably sees the ! as belonging to the identifier mymethod and adds it to the result of <C-R><C-W>.