vim-patch:9.1.0762: 'cedit', 'termwinkey' and 'wildchar' may not be parsed correctly (#30704)

Problem:  'cedit', 'termwinkey' and 'wildchar' may not be parsed
          correctly
Solution: improve string_to_key() function in option.c
          (Milly)

- Problem: `^@` raises an error.
  Solution: Store as `<Nul>`.
- Problem: `<t_xx` does not raise an error.
  Solution: Raise an error if closing `>` is missing.
- Problem: Single `<` or `^` raises an error. It is inconvenient for users.
  Solution: They are stored as a single character.

closes: vim/vim#15811

https://github.com/vim/vim/commit/a9c6f90918d0012d1b8c8c5c1dccb77407f553fb

Co-authored-by: Milly <milly.ca@gmail.com>

4 files changed, 29 insertions, 8 deletions

diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 2c2d67c3f0..4248560851 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7076,7 +7076,12 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Some keys will not work, such as CTRL-C, <CR> and Enter.
 	<Esc> can be used, but hitting it twice in a row will still exit
 	command-line as a failsafe measure.
-	Although 'wc' is a number option, you can set it to a special key: >vim
+	Although 'wc' is a number option, it can be specified as a number, a
+	single character, a |key-notation| (e.g. <Up>, <C-F>) or a letter
+	preceded with a caret (e.g. `^F` is CTRL-F): >vim
+		:set wc=27
+		:set wc=X
+		:set wc=^I
 		set wc=<Tab>
 <
 
diff --git a/runtime/lua/vim/_meta/options.lua b/runtime/lua/vim/_meta/options.lua
index 27384dbccc..2bff777899 100644
--- a/runtime/lua/vim/_meta/options.lua
+++ b/runtime/lua/vim/_meta/options.lua
@@ -7699,9 +7699,14 @@ vim.go.ww = vim.go.whichwrap
 --- Some keys will not work, such as CTRL-C, <CR> and Enter.
 --- <Esc> can be used, but hitting it twice in a row will still exit
 --- command-line as a failsafe measure.
---- Although 'wc' is a number option, you can set it to a special key:
+--- Although 'wc' is a number option, it can be specified as a number, a
+--- single character, a `key-notation` (e.g. <Up>, <C-F>) or a letter
+--- preceded with a caret (e.g. `^F` is CTRL-F):
 ---
 --- ```vim
+--- 	:set wc=27
+--- 	:set wc=X
+--- 	:set wc=^I
 --- 	set wc=<Tab>
 --- ```
 ---
diff --git a/src/nvim/option.c b/src/nvim/option.c
index aa6f330dfc..f54da54a4b 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -1525,7 +1525,9 @@ static int find_key_len(const char *arg_arg, size_t len, bool has_lt)
   // Don't use get_special_key_code() for t_xx, we don't want it to call
   // add_termcap_entry().
   if (len >= 4 && arg[0] == 't' && arg[1] == '_') {
-    key = TERMCAP2KEY((uint8_t)arg[2], (uint8_t)arg[3]);
+    if (!has_lt || arg[4] == '>') {
+      key = TERMCAP2KEY((uint8_t)arg[2], (uint8_t)arg[3]);
+    }
   } else if (has_lt) {
     arg--;  // put arg at the '<'
     int modifiers = 0;
@@ -1539,14 +1541,18 @@ static int find_key_len(const char *arg_arg, size_t len, bool has_lt)
 }
 
 /// Convert a key name or string into a key value.
-/// Used for 'wildchar' and 'cedit' options.
+/// Used for 'cedit', 'wildchar' and 'wildcharm' options.
 int string_to_key(char *arg)
 {
-  if (*arg == '<') {
+  if (*arg == '<' && arg[1]) {
     return find_key_len(arg + 1, strlen(arg), true);
   }
-  if (*arg == '^') {
-    return CTRL_CHR((uint8_t)arg[1]);
+  if (*arg == '^' && arg[1]) {
+    int key = CTRL_CHR((uint8_t)arg[1]);
+    if (key == 0) {  // ^@ is <Nul>
+      key = K_ZERO;
+    }
+    return key;
   }
   return (uint8_t)(*arg);
 }
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index 0d3f7154af..db04de1e3a 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -9620,7 +9620,12 @@ return {
         Some keys will not work, such as CTRL-C, <CR> and Enter.
         <Esc> can be used, but hitting it twice in a row will still exit
         command-line as a failsafe measure.
-        Although 'wc' is a number option, you can set it to a special key: >vim
+        Although 'wc' is a number option, it can be specified as a number, a
+        single character, a |key-notation| (e.g. <Up>, <C-F>) or a letter
+        preceded with a caret (e.g. `^F` is CTRL-F): >vim
+        	:set wc=27
+        	:set wc=X
+        	:set wc=^I
         	set wc=<Tab>
         <
       ]=],