patch 8.2.4682: Vim9: can use :unlockvar for const variable

Problem:    Vim9: can use :unlockvar for const variable. (Ernie Rael)
Solution:   Check whether the variable is a const.

src/eval.c

@@ -1065,7 +1065,7 @@ get_lval(
 		&& lp->ll_tv == &v->di_tv
 		&& ht != NULL && ht == get_script_local_ht())
 	{
-	    svar_T  *sv = find_typval_in_script(lp->ll_tv, 0);
+	    svar_T  *sv = find_typval_in_script(lp->ll_tv, 0, TRUE);
 
 	    // Vim9 script local variable: get the type
 	    if (sv != NULL)

src/evalvars.c

@@ -1951,16 +1951,33 @@ do_lock_var(
 								  lp->ll_name);
 		ret = FAIL;
 	    }
-	    else if ((di->di_flags & DI_FLAGS_FIX)
+	    else
+	    {
+		if ((di->di_flags & DI_FLAGS_FIX)
 			    && di->di_tv.v_type != VAR_DICT
 			    && di->di_tv.v_type != VAR_LIST)
 		{
-		// For historic reasons this error is not given for a list or
-		// dict.  E.g., the b: dict could be locked/unlocked.
+		    // For historic reasons this error is not given for a list
+		    // or dict.  E.g., the b: dict could be locked/unlocked.
 		    semsg(_(e_cannot_lock_or_unlock_variable_str), lp->ll_name);
 		    ret = FAIL;
 		}
 		else
+		{
+		    if (in_vim9script())
+		    {
+			svar_T  *sv = find_typval_in_script(&di->di_tv,
+								     0, FALSE);
+
+			if (sv != NULL && sv->sv_const != 0)
+			{
+			    semsg(_(e_cannot_change_readonly_variable_str),
+								  lp->ll_name);
+			    ret = FAIL;
+			}
+		    }
+
+		    if (ret == OK)
 		    {
 			if (lock)
 			    di->di_flags |= DI_FLAGS_LOCK;
@@ -1970,6 +1987,8 @@ do_lock_var(
 			    item_lock(&di->di_tv, deep, lock, FALSE);
 		    }
 		}
+	    }
+	}
 	*name_end = cc;
     }
     else if (deep == 0)
@@ -2812,7 +2831,7 @@ eval_variable(
 	    if (ht != NULL && ht == get_script_local_ht()
 		    && tv != &SCRIPT_SV(current_sctx.sc_sid)->sv_var.di_tv)
 	    {
-		svar_T *sv = find_typval_in_script(tv, 0);
+		svar_T *sv = find_typval_in_script(tv, 0, TRUE);
 
 		if (sv != NULL)
 		    type = sv->sv_type;
@@ -3557,7 +3576,7 @@ set_var_const(
 	    if (var_in_vim9script && (flags & ASSIGN_FOR_LOOP) == 0)
 	    {
 		where_T where = WHERE_INIT;
-		svar_T  *sv = find_typval_in_script(&di->di_tv, sid);
+		svar_T  *sv = find_typval_in_script(&di->di_tv, sid, TRUE);
 
 		if (sv != NULL)
 		{

src/ex_docmd.c

@@ -303,7 +303,6 @@ static void	ex_tag_cmd(exarg_T *eap, char_u *name);
 # define ex_throw		ex_ni
 # define ex_try			ex_ni
 # define ex_unlet		ex_ni
-# define ex_unlockvar		ex_ni
 # define ex_while		ex_ni
 # define ex_import		ex_ni
 # define ex_export		ex_ni

src/proto/vim9script.pro

@@ -16,7 +16,7 @@ int find_exported(int sid, char_u *name, ufunc_T **ufunc, type_T **type, cctx_T
 char_u *vim9_declare_scriptvar(exarg_T *eap, char_u *arg);
 void update_vim9_script_var(int create, dictitem_T *di, char_u *name, int flags, typval_T *tv, type_T **type, int do_member);
 void hide_script_var(scriptitem_T *si, int idx, int func_defined);
-svar_T *find_typval_in_script(typval_T *dest, scid_T sid);
+svar_T *find_typval_in_script(typval_T *dest, scid_T sid, int must_find);
 int check_script_var_type(svar_T *sv, typval_T *value, char_u *name, where_T where);
 int check_reserved_name(char_u *name);
 /* vim: set ft=c : */

src/testdir/test_vim9_cmd.vim

@@ -1650,6 +1650,23 @@ def Test_lockvar()
       LockIt()
   END
   v9.CheckScriptFailure(lines, 'E1246', 1)
+
+  lines =<< trim END
+      vim9script
+      const name = 'john'
+      unlockvar name
+  END
+  v9.CheckScriptFailure(lines, 'E46', 3)
+
+  lines =<< trim END
+      vim9script
+      const name = 'john'
+      def UnLockIt()
+        unlockvar name
+      enddef
+      UnLockIt()
+  END
+  v9.CheckScriptFailure(lines, 'E46', 1)
 enddef
 
 def Test_substitute_expr()

src/userfunc.c

@@ -1697,7 +1697,7 @@ deref_func_name(
 	{
 	    if (!did_type && type != NULL && ht == get_script_local_ht())
 	    {
-		svar_T  *sv = find_typval_in_script(tv, 0);
+		svar_T  *sv = find_typval_in_script(tv, 0, TRUE);
 
 		if (sv != NULL)
 		    *type = sv->sv_type;

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4682,
 /**/
     4681,
 /**/

src/vim9script.c

@@ -956,7 +956,7 @@ update_vim9_script_var(
     }
     else
     {
-	sv = find_typval_in_script(&di->di_tv, 0);
+	sv = find_typval_in_script(&di->di_tv, 0, TRUE);
     }
     if (sv != NULL)
     {
@@ -1053,10 +1053,11 @@ hide_script_var(scriptitem_T *si, int idx, int func_defined)
 /*
  * Find the script-local variable that links to "dest".
  * If "sid" is zero use the current script.
+ * if "must_find" is TRUE and "dest" cannot be found report an internal error.
  * Returns NULL if not found and give an internal error.
  */
     svar_T *
-find_typval_in_script(typval_T *dest, scid_T sid)
+find_typval_in_script(typval_T *dest, scid_T sid, int must_find)
 {
     scriptitem_T    *si = SCRIPT_ITEM(sid == 0 ? current_sctx.sc_sid : sid);
     int		    idx;
@@ -1076,6 +1077,7 @@ find_typval_in_script(typval_T *dest, scid_T sid)
 	if (sv->sv_name != NULL && sv->sv_tv == dest)
 	    return sv;
     }
+    if (must_find)
 	iemsg("find_typval_in_script(): not found");
     return NULL;
 }