vim-patch:9.1.1933: completion: complete_match() is not useful (#36726)

Problem:  completion: complete_match() Vim script function and
          'isexpand' option are not that useful and confusing
          (after v9.1.1341)
Solution: Remove function and option and clean up code and documentation
          (Girish Palya).

complete_match() and 'isexpand' add no real functionality to Vim. They
duplicate what `strridx()` already does, yet pretend to be part of the
completion system. They have nothing to do with the completion mechanism.

* `f_complete_match()` in `insexpand.c` does not call any completion code.
   It’s just a `STRNCMP()` wrapper with fluff logic.
* `'isexpand'` exists only as a proxy argument to that function.
   It does nothing on its own and amounts to misuse of a new option.

The following Vim script function can be used to implement the same
functionality:

```vim
  func CompleteMatch(triggers, sep=',')
    let line = getline('.')->strpart(0, col('.') - 1)
    let result = []
    for trig in split(a:triggers, a:sep)
      let idx = strridx(line, trig)
      if l:idx >= 0
        call add(result, [idx + 1, trig])
      endif
    endfor
    return result
  endfunc
```

related: vim/vim#16716
fixes: vim/vim#18563
closes: vim/vim#18790

cbcbff8712

Co-authored-by: Girish Palya <girishji@gmail.com>

test/old/testdir/gen_opt_test.vim

@@ -265,7 +265,6 @@ let test_values = {
       \ 'helplang': [['', 'de', 'de,it'], ['xxx']],
       "\ 'highlight': [['', 'e:Error'], ['xxx']],
       "\ 'imactivatekey': [['', 'S-space'], ['xxx']],
-      \ 'isexpand': [['', '.,->', '/,/*,\\,'], [',,', '\\,,']],
       \ 'isfname': [['', '@', '@,48-52'], ['xxx', '@48']],
       \ 'isident': [['', '@', '@,48-52'], ['xxx', '@48']],
       \ 'iskeyword': [['', '@', '@,48-52'], ['xxx', '@48']],

test/old/testdir/test_ins_complete.vim

@@ -4919,99 +4919,6 @@ func Test_nearest_cpt_option()
   delfunc PrintMenuWords
 endfunc
 
-func Test_complete_match()
-  set isexpand=.,/,->,abc,/*,_
-  func TestComplete()
-    let res = complete_match()
-    if res->len() == 0
-      return
-    endif
-    let [startcol, expandchar] = res[0]
-
-    if startcol >= 0
-      let line = getline('.')
-
-      let items = []
-      if expandchar == '/*'
-        let items = ['/** */']
-      elseif expandchar =~ '^/'
-        let items = ['/*! */', '// TODO:', '// fixme:']
-      elseif expandchar =~ '^\.' && startcol < 4
-        let items = ['length()', 'push()', 'pop()', 'slice()']
-      elseif expandchar =~ '^\.' && startcol > 4
-        let items = ['map()', 'filter()', 'reduce()']
-      elseif expandchar =~ '^\abc'
-        let items = ['def', 'ghk']
-      elseif expandchar =~ '^\->'
-        let items = ['free()', 'xfree()']
-      else
-        let items = ['test1', 'test2', 'test3']
-      endif
-
-      call complete(expandchar =~ '^/' ? startcol : startcol + strlen(expandchar), items)
-    endif
-  endfunc
-
-  new
-  inoremap <buffer> <F5> <cmd>call TestComplete()<CR>
-
-  call feedkeys("S/*\<F5>\<C-Y>", 'tx')
-  call assert_equal('/** */', getline('.'))
-
-  call feedkeys("S/\<F5>\<C-N>\<C-Y>", 'tx')
-  call assert_equal('// TODO:', getline('.'))
-
-  call feedkeys("Swp.\<F5>\<C-N>\<C-Y>", 'tx')
-  call assert_equal('wp.push()', getline('.'))
-
-  call feedkeys("Swp.property.\<F5>\<C-N>\<C-Y>", 'tx')
-  call assert_equal('wp.property.filter()', getline('.'))
-
-  call feedkeys("Sp->\<F5>\<C-N>\<C-Y>", 'tx')
-  call assert_equal('p->xfree()', getline('.'))
-
-  call feedkeys("Swp->property.\<F5>\<C-Y>", 'tx')
-  call assert_equal('wp->property.map()', getline('.'))
-
-  call feedkeys("Sabc\<F5>\<C-Y>", 'tx')
-  call assert_equal('abcdef', getline('.'))
-
-  call feedkeys("S_\<F5>\<C-Y>", 'tx')
-  call assert_equal('_test1', getline('.'))
-
-  set ise&
-  call feedkeys("Sabc \<ESC>:let g:result=complete_match()\<CR>", 'tx')
-  call assert_equal([[1, 'abc']], g:result)
-
-  call assert_fails('call complete_match(99, 0)', 'E966:')
-  call assert_fails('call complete_match(1, 99)', 'E964:')
-  call assert_fails('call complete_match(1)', 'E474:')
-
-  set ise=你好,好
-  call feedkeys("S你好 \<ESC>:let g:result=complete_match()\<CR>", 'tx')
-  call assert_equal([[1, '你好'], [4, '好']], g:result)
-
-  set ise=\\,,->
-  call feedkeys("Sabc, \<ESC>:let g:result=complete_match()\<CR>", 'tx')
-  call assert_equal([[4, ',']], g:result)
-
-  set ise=\ ,=
-  call feedkeys("Sif true  \<ESC>:let g:result=complete_match()\<CR>", 'tx')
-  call assert_equal([[8, ' ']], g:result)
-  call feedkeys("Slet a = \<ESC>:let g:result=complete_match()\<CR>", 'tx')
-  call assert_equal([[7, '=']], g:result)
-  set ise={,\ ,=
-  call feedkeys("Sif true  \<ESC>:let g:result=complete_match()\<CR>", 'tx')
-  call assert_equal([[8, ' ']], g:result)
-  call feedkeys("S{ \<ESC>:let g:result=complete_match()\<CR>", 'tx')
-  call assert_equal([[1, '{']], g:result)
-
-  bw!
-  unlet g:result
-  set isexpand&
-  delfunc TestComplete
-endfunc
-
 func Test_register_completion()
   let @a = "completion test apple application"
   let @b = "banana behavior better best"