patch 8.2.3127: Vim9: no error when adding number to list of string

Problem:    Vim9: no error when adding number to list of string.
Solution:   Check the value type. (closes #8529)

src/list.c

@@ -597,13 +597,16 @@ list_append(list_T *l, listitem_T *item)
 
 /*
  * Append typval_T "tv" to the end of list "l".  "tv" is copied.
- * Return FAIL when out of memory.
+ * Return FAIL when out of memory or the type is wrong.
  */
     int
 list_append_tv(list_T *l, typval_T *tv)
 {
     listitem_T	*li = listitem_alloc();
 
+    if (l->lv_type != NULL && l->lv_type->tt_member != NULL
+		&& check_typval_arg_type(l->lv_type->tt_member, tv, 0) == FAIL)
+	return FAIL;
     if (li == NULL)
 	return FAIL;
     copy_tv(tv, &li->li_tv);

src/testdir/test_vim9_builtin.vim

@@ -104,6 +104,13 @@ def Test_add_list()
       add(l, 123)
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      var l: list<string> = ['a']
+      l->add(123)
+  END
+  CheckScriptFailure(lines, 'E1012: Type mismatch; expected string but got number', 3)
 enddef
 
 def Test_add_blob()

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3127,
 /**/
     3126,
 /**/