diff -r dae829b4de78 -r bc8d925342f0 mercurial/templateutil.py
--- a/mercurial/templateutil.py	Thu Jun 14 22:33:26 2018 +0900
+++ b/mercurial/templateutil.py	Thu Jun 14 23:10:14 2018 +0900
@@ -415,6 +415,7 @@
         raise error.ParseError(_('not comparable'))
 
     def filter(self, context, mapping, select):
+        # implement if necessary; we'll need a wrapped type for a mapping dict
         raise error.ParseError(_('not filterable without template'))
 
     def join(self, context, mapping, sep):