patch 8.2.3682: Vim9: assigning to a script variable drops the type

Problem:    Vim9: assigning to a script variable drops the required type.
Solution:   Lookup the type of the variable and use it. (closes #9219)

src/evalvars.c

@@ -3206,13 +3206,14 @@ set_var(
     void
 set_var_const(
     char_u	*name,
-    type_T	*type,
+    type_T	*type_arg,
     typval_T	*tv_arg,
     int		copy,	    // make copy of value in "tv"
     int		flags_arg,  // ASSIGN_CONST, ASSIGN_FINAL, etc.
     int		var_idx)    // index for ":let [a, b] = list"
 {
     typval_T	*tv = tv_arg;
+    type_T	*type = type_arg;
     typval_T	bool_tv;
     dictitem_T	*di;
     typval_T	*dest_tv = NULL;
@@ -3334,13 +3335,18 @@ 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);
 
+		    if (sv != NULL)
+		    {
 			// check the type and adjust to bool if needed
 			where.wt_index = var_idx;
 			where.wt_variable = TRUE;
-		    if (check_script_var_type(&di->di_tv, tv, name, where)
+			if (check_script_var_type(sv, tv, name, where) == FAIL)
-								       == FAIL)
 			    goto failed;
+			if (type == NULL)
+			    type = sv->sv_type;
+		    }
 		}
 
 		if ((flags & ASSIGN_FOR_LOOP) == 0

src/proto/vim9script.pro

@@ -16,6 +16,6 @@ char_u *vim9_declare_scriptvar(exarg_T *eap, char_u *arg);
 void update_vim9_script_var(int create, dictitem_T *di, 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);
-int check_script_var_type(typval_T *dest, typval_T *value, char_u *name, where_T where);
+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_assign.vim

@@ -322,6 +322,16 @@ def Test_skipped_assignment()
   CheckDefAndScriptSuccess(lines)
 enddef
 
+def Test_assign_keep_type()
+  var lines =<< trim END
+      vim9script
+      var l: list<number> = [123]
+      l = [123]
+      l->add('string')
+  END
+  CheckScriptFailure(lines, 'E1012:', 4)
+enddef
+
 def Test_assign_unpack()
   var lines =<< trim END
     var v1: number

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3682,
 /**/
     3681,
 /**/

src/vim9script.c

@@ -999,16 +999,13 @@ find_typval_in_script(typval_T *dest)
  */
     int
 check_script_var_type(
-	typval_T    *dest,
+	svar_T	    *sv,
 	typval_T    *value,
 	char_u	    *name,
 	where_T	    where)
 {
-    svar_T  *sv = find_typval_in_script(dest);
     int	    ret;
 
-    if (sv != NULL)
-    {
     if (sv->sv_const != 0)
     {
 	semsg(_(e_cannot_change_readonly_variable_str), name);
@@ -1025,9 +1022,6 @@ check_script_var_type(
 	value->vval.v_number = val ? VVAL_TRUE : VVAL_FALSE;
     }
     return ret;
-    }
-
-    return OK; // not really
 }
 
 // words that cannot be used as a variable