--- a/mercurial/templatefilters.py	Sun Mar 15 15:12:44 2020 +0900
+++ b/mercurial/templatefilters.py	Sun Mar 15 22:01:38 2020 +0900
@@ -18,6 +18,7 @@
     node,
     pycompat,
     registrar,
+    smartset,
     templateutil,
     url,
     util,
@@ -108,6 +109,9 @@
 @templatefilter(b'cbor')
 def cbor(obj):
     """Any object. Serializes the object to CBOR bytes."""
+    if isinstance(obj, smartset.abstractsmartset):
+        # cborutil is stricter about type than json() filter
+        obj = list(obj)
     return b''.join(cborutil.streamencode(obj))