patch 9.1.1490: 'wildchar' does not work in search contexts

Problem:  'wildchar' does not work in search contexts
Solution: implement search completion when 'wildchar' is typed
          (Girish Palya).

This change enhances Vim's command-line completion by extending
'wildmode' behavior to search pattern contexts, including:

- '/' and '?' search commands
- ':s', ':g', ':v', and ':vim' commands

Completions preserve the exact regex pattern typed by the user,
appending the completed word directly to the original input. This
ensures that all regex elements — such as '<', '^', grouping brackets
'()', wildcards '\*', '.', and other special characters — remain intact
and in their original positions.

---

**Use Case**

While searching (using `/` or `?`) for lines containing a pattern like
`"foobar"`, you can now type a partial pattern (e.g., `/f`) followed by
a trigger key (`wildchar`) to open a **popup completion menu** showing
all matching words.

This offers two key benefits:

1. **Precision**: Select the exact word you're looking for without
typing it fully.
2. **Memory aid**: When you can’t recall a full function or variable
name, typing a few letters helps you visually identify and complete the
correct symbol.

---

**What’s New**

Completion is now supported in the following contexts:

- `/` and `?` search commands
- `:s`, `:g`, `:v`, and `:vimgrep` ex-commands

---

**Design Notes**

- While `'wildchar'` (usually `<Tab>`) triggers completion, you'll have
to use `<CTRL-V><Tab>` or "\t" to search for a literal tab.
- **Responsiveness**: Search remains responsive because it checks for
user input frequently.

---

**Try It Out**

Basic setup using the default `<Tab>` as the completion trigger:

```vim
set wim=noselect,full wop=pum wmnu
```

Now type:

```
/foo<Tab>
```

This opens a completion popup for matches containing "foo".
For matches beginning with "foo" type `/\<foo<Tab>`.

---

**Optional: Autocompletion**

For automatic popup menu completion as you type in search or `:`
commands, include this in your `.vimrc`:

```vim
vim9script
set wim=noselect:lastused,full wop=pum wcm=<C-@> wmnu

autocmd CmdlineChanged [:/?] CmdComplete()

def CmdComplete()
  var [cmdline, curpos, cmdmode] = [getcmdline(), getcmdpos(),
expand('<afile>') == ':']
  var trigger_char = '\%(\w\|[*/:.-]\)$'
  var not_trigger_char = '^\%(\d\|,\|+\|-\)\+$'  # Exclude numeric range
  if getchar(1, {number: true}) == 0  # Typehead is empty, no more
pasted input
      && !wildmenumode() && curpos == cmdline->len() + 1
      && (!cmdmode || (cmdline =~ trigger_char && cmdline !~
not_trigger_char))
    SkipCmdlineChanged()
    feedkeys("\<C-@>", "t")
    timer_start(0, (_) => getcmdline()->substitute('\%x00', '',
'ge')->setcmdline())  # Remove <C-@>
  endif
enddef

def SkipCmdlineChanged(key = ''): string
  set ei+=CmdlineChanged
  timer_start(0, (_) => execute('set ei-=CmdlineChanged'))
  return key == '' ? '' : ((wildmenumode() ? "\<C-E>" : '') .. key)
enddef

**Optional: Preserve history recall behavior**
cnoremap <expr> <Up> SkipCmdlineChanged("\<Up>")
cnoremap <expr> <Down> SkipCmdlineChanged("\<Down>")

**Optional: Customize popup height**
autocmd CmdlineEnter : set bo+=error | exec $'set ph={max([10,
winheight(0) - 4])}'
autocmd CmdlineEnter [/?] set bo+=error | set ph=8
autocmd CmdlineLeave [:/?] set bo-=error ph&
```

closes: #17570

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/cmdline.txt

@@ -1,4 +1,4 @@
-*cmdline.txt*   For Vim version 9.1.  Last change: 2025 Mar 08
+*cmdline.txt*   For Vim version 9.1.  Last change: 2025 Jun 28
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -415,7 +415,7 @@ CTRL-D		List names that match the pattern in front of the cursor.
 		to the end.
 		The 'wildoptions' option can be set to "tagfile" to list the
 		file of matching tags.
-					*c_CTRL-I* *c_wildchar* *c_<Tab>*
+				*c_CTRL-I* *c_wildchar* *c_<Tab>* */_<Tab>*
 'wildchar' option
 		A match is done on the pattern in front of the cursor.  The
 		match (if there are several, the first match) is inserted
@@ -425,6 +425,10 @@ CTRL-D		List names that match the pattern in front of the cursor.
 		again and there were multiple matches, the next
 		match is inserted.  After the last match, the first is used
 		again (wrap around).
+
+		In search context use <CTRL-V><Tab> or "\t" to search for a
+		literal <Tab> instead of triggering completion.
+
 		The behavior can be changed with the 'wildmode' option.
 							*c_<S-Tab>*
 <S-Tab>		Like 'wildchar' or <Tab>, but begin with the last match and
@@ -458,7 +462,7 @@ CTRL-G		When 'incsearch' is set, entering a search pattern for "/" or
 		"?" and the current match is displayed then CTRL-G will move
 		to the next match (does not take |search-offset| into account)
 		Use CTRL-T to move to the previous match.  Hint: on a regular
-		keyboard T is above G.
+		keyboard G is below T.
 	                                            *c_CTRL-T* */_CTRL-T*
 CTRL-T		When 'incsearch' is set, entering a search pattern for "/" or
 		"?" and the current match is displayed then CTRL-T will move

runtime/doc/options.txt

@@ -9752,7 +9752,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 		:set wc=X
 		:set wc=^I
 		:set wc=<Tab>
-<	NOTE: This option is set to the Vi default value when 'compatible' is
+<	'wildchar' also enables completion in search pattern contexts such as
+	|/|, |?|, |:s|, |:g|, |:v|, and |:vim|.  To insert a literal <Tab>
+	instead of triggering completion, type <C-V><Tab> or "\t".
+	NOTE: This option is set to the Vi default value when 'compatible' is
 	set and to the Vim default value when 'compatible' is reset.
 
 						*'wildcharm'* *'wcm'*

runtime/doc/tags

@@ -1815,6 +1815,7 @@ $quote	eval.txt	/*$quote*
 /\{-	pattern.txt	/*\/\\{-*
 /\~	pattern.txt	/*\/\\~*
 /^	pattern.txt	/*\/^*
+/_<Tab>	cmdline.txt	/*\/_<Tab>*
 /_CTRL-G	cmdline.txt	/*\/_CTRL-G*
 /_CTRL-L	cmdline.txt	/*\/_CTRL-L*
 /_CTRL-T	cmdline.txt	/*\/_CTRL-T*

runtime/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Jun 27
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Jun 28
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -41632,6 +41632,8 @@ Completion: ~
 - add ":filetype" command completion
 - add "filetypecmd" completion type for |getcompletion()|
 - 'smartcase' applies to completion filtering
+- 'wildchar' enables completion in search contexts using |/|, |?|, |:g|, |:v|
+  and |:vimgrep| commands
 
 Options: ~
 - the default for 'commentstring' contains whitespace padding to have