patch 8.2.1471: :const only locks the variable, not the value

Problem:    :const only locks the variable, not the value.
Solution:   Lock the value as ":lockvar 1 var" would do. (closes #6719)

src/evalvars.c

@@ -3087,7 +3087,7 @@ set_var_const(
     }
 
     if (flags & LET_IS_CONST)
-	di->di_tv.v_lock |= VAR_LOCKED;
+	item_lock(&di->di_tv, 1, TRUE);
 }
 
 /*

src/testdir/test_const.vim

@@ -41,6 +41,7 @@ func Test_define_var_with_lock()
     call assert_fails('let s = "vim"', 'E741:')
     call assert_fails('let F = funcref("s:noop")', 'E741:')
     call assert_fails('let l = [1, 2, 3]', 'E741:')
+    call assert_fails('call filter(l, "v:val % 2 == 0")', 'E741:')
     call assert_fails('let d = {"foo": 10}', 'E741:')
     if has('channel')
       call assert_fails('let j = test_null_job()', 'E741:')
@@ -276,13 +277,16 @@ func Test_lock_depth_is_1()
     const l = [1, 2, 3]
     const d = {'foo': 10}
 
-    " Modify list
+    " Modify list - setting item is OK, adding/removing items not
-    call add(l, 4)
     let l[0] = 42
+    call assert_fails('call add(l, 4)', 'E741:')
+    call assert_fails('unlet l[1]', 'E741:')
 
-    " Modify dict
+    " Modify dict - changing item is OK, adding/removing items not
-    let d['bar'] = 'hello'
+    let d['foo'] = 'hello'
     let d.foo = 44
+    call assert_fails("let d['bar'] = 'hello'", 'E741:')
+    call assert_fails("unlet d['foo']", 'E741:')
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1471,
 /**/
     1470,
 /**/