patch 8.2.2921: E704 for script local variable is not backwards compatible

Problem:    E704 for script local variable is not backwards compatible.
            (Yasuhiro Matsumoto)
Solution:   Only give the error in Vim9 script.  Also check for function-local
            variable.

src/dict.c

@@ -352,8 +352,9 @@ dict_copy(dict_T *orig, int deep, int copyID)
 dict_wrong_func_name(dict_T *d, typval_T *tv, char_u *name)
 {
     return (d == get_globvar_dict()
-	    || (SCRIPT_ID_VALID(current_sctx.sc_sid)
+		|| (in_vim9script() && SCRIPT_ID_VALID(current_sctx.sc_sid)
-		  && d == &SCRIPT_ITEM(current_sctx.sc_sid)->sn_vars->sv_dict))
+		   && d == &SCRIPT_ITEM(current_sctx.sc_sid)->sn_vars->sv_dict)
+		|| &d->dv_hashtab == get_funccal_local_ht())
 	    && (tv->v_type == VAR_FUNC || tv->v_type == VAR_PARTIAL)
 	    && var_wrong_func_name(name, TRUE);
 }

src/testdir/test_functions.vim

@@ -4,6 +4,7 @@ source shared.vim
 source check.vim
 source term_util.vim
 source screendump.vim
+source vim9.vim
 
 " Must be done first, since the alternate buffer must be unset.
 func Test_00_bufexists()
@@ -2684,8 +2685,13 @@ endfunc
 func Test_builtin_check()
   call assert_fails('let g:["trim"] = {x -> " " .. x}', 'E704:')
   call assert_fails('let g:.trim = {x -> " " .. x}', 'E704:')
-  call assert_fails('let s:["trim"] = {x -> " " .. x}', 'E704:')
+  call assert_fails('let l:["trim"] = {x -> " " .. x}', 'E704:')
-  call assert_fails('let s:.trim = {x -> " " .. x}', 'E704:')
+  call assert_fails('let l:.trim = {x -> " " .. x}', 'E704:')
+  let lines =<< trim END
+    vim9script
+    var s:trim = (x) => " " .. x
+  END
+  call CheckScriptFailure(lines, 'E704:')
 
   call assert_fails('call extend(g:, #{foo: { -> "foo" }})', 'E704:')
   let g:bar = 123

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2921,
 /**/
     2920,
 /**/