patch 8.2.4447: Vim9: can still use s:var in a compiled function

Problem:    Vim9: can still use s:var in a compiled function.
Solution:   Disallow using s:var for Vim9 script. (closes #9824)

runtime/doc/vim9.txt

@@ -1,4 +1,4 @@
-*vim9.txt*	For Vim version 8.2.  Last change: 2022 Feb 18
+*vim9.txt*	For Vim version 8.2.  Last change: 2022 Feb 22
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -245,9 +245,11 @@ that the name interferes with builtin functions.
 							*vim9-s-namespace*
 The use of the "s:" prefix is not supported at the Vim9 script level.  All
 functions and variables without a prefix are script-local.
-In :def functions the use of "s:" is optional.  This is because in legacy
-script the "s:" might be needed.  Disallowing the use of "s:" only in a :def
-function in Vim9 script would be a bit confusing.
+
+In :def functions the use of "s:" depends on the script: Script-local
+variables and functions in a legacy script do use "s:", while in a Vim9 script
+they do not use "s:".  This matches what you see in the rest of the file.
+
 In legacy functions the use of "s:" for script items is required, as before.
 
 In all cases the function must be defined before used.  That is when it is
@@ -1467,7 +1469,7 @@ strings: >
 	# typename(mylist) == "list<string>", no error
 
 There is a subtle difference between using a list constant directly and
-through a variable declaraiton.  Because of type inference, when using a list
+through a variable declaration.  Because of type inference, when using a list
 constant to initialize a variable, this also sets the declared type: >
 	var mylist = [1, 2, 3]
 	# typename(mylist) == "list<number>"

src/testdir/test_vim9_assign.vim

@@ -220,7 +220,7 @@ def Test_assignment()
     enddef
     defcompile
   END
-  v9.CheckScriptFailure(lines, 'E1089:')
+  v9.CheckScriptFailure(lines, 'E1268:')
 
   g:inc_counter += 1
   assert_equal(2, g:inc_counter)
@@ -2460,6 +2460,49 @@ def Run_Test_declare_command_line()
   g:StopVimInTerminal(buf)
 enddef
 
+def Test_using_s_var_in_function()
+  var lines =<< trim END
+      vim9script
+      var scriptlevel = 123
+      def SomeFunc()
+        echo s:scriptlevel
+      enddef
+      SomeFunc()
+  END
+  v9.CheckScriptFailure(lines, 'E1268:')
+
+  # OK in legacy script
+  lines =<< trim END
+      let s:scriptlevel = 123
+      def s:SomeFunc()
+        echo s:scriptlevel
+      enddef
+      call s:SomeFunc()
+  END
+  v9.CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      var scriptlevel = 123
+      def SomeFunc()
+        s:scriptlevel = 456
+      enddef
+      SomeFunc()
+  END
+  v9.CheckScriptFailure(lines, 'E1268:')
+
+  # OK in legacy script
+  lines =<< trim END
+      let s:scriptlevel = 123
+      def s:SomeFunc()
+        s:scriptlevel = 456
+      enddef
+      call s:SomeFunc()
+      call assert_equal(456, s:scriptlevel)
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 
 
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4447,
 /**/
     4446,
 /**/

src/vim9compile.c

@@ -1331,6 +1331,12 @@ compile_lhs(
 		    char_u	*rawname = lhs->lhs_name
 					   + (lhs->lhs_name[1] == ':' ? 2 : 0);
 
+		    if (script_namespace && current_script_is_vim9())
+		    {
+			semsg(_(e_cannot_use_s_colon_in_vim9_script_str),
+								    var_start);
+			return FAIL;
+		    }
 		    if (is_decl)
 		    {
 			if (script_namespace)

src/vim9expr.c

@@ -422,8 +422,15 @@ compile_load(
 	    {
 		case 'v': res = generate_LOADV(cctx, name, error);
 			  break;
-		case 's': if (is_expr && ASCII_ISUPPER(*name)
-				       && find_func(name, FALSE) != NULL)
+		case 's': if (current_script_is_vim9())
+			  {
+			      semsg(_(e_cannot_use_s_colon_in_vim9_script_str),
+									 *arg);
+			      vim_free(name);
+			      return FAIL;
+			  }
+			  if (is_expr && ASCII_ISUPPER(*name)
+					     && find_func(name, FALSE) != NULL)
 			      res = generate_funcref(cctx, name, FALSE);
 			  else
 			      res = compile_load_scriptvar(cctx, name,