patch 8.2.3911: Vim9: type check for filter() does not accept unknown

Problem:    Vim9: type check for filter() does not accept unknown.
Solution:   Also accept unknown for the return type. (closes #9413)

src/evalfunc.c

@@ -494,6 +494,7 @@ arg_filter_func(type_T *type, argcontext_T *context)
     if (type->tt_type == VAR_FUNC
 	    && !(type->tt_member->tt_type == VAR_BOOL
 		|| type->tt_member->tt_type == VAR_NUMBER
+		|| type->tt_member->tt_type == VAR_UNKNOWN
 		|| type->tt_member->tt_type == VAR_ANY))
     {
 	arg_type_mismatch(&t_func_bool, type, context->arg_idx + 1);

src/testdir/test_vim9_builtin.vim

@@ -1253,6 +1253,12 @@ def Test_filter()
   assert_equal([], filter([1, 2, 3], '0'))
   assert_equal([1, 2, 3], filter([1, 2, 3], '1'))
   assert_equal({b: 20}, filter({a: 10, b: 20}, 'v:val == 20'))
+
+  def GetFiltered(): list<number>
+    var Odd: func = (_, v) => v % 2
+    return range(3)->filter(Odd)
+  enddef
+  assert_equal([1], GetFiltered())
 enddef
 
 def Test_filter_wrong_dict_key_type()

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3911,
 /**/
     3910,
 /**/