diff -r ff72bd52d56a -r 9d2b2df2c2ba mercurial/hbisect.py
--- a/mercurial/hbisect.py	Fri Mar 06 10:52:44 2020 +0100
+++ b/mercurial/hbisect.py	Fri Mar 06 13:27:41 2020 -0500
@@ -137,7 +137,7 @@
             side = state[b'bad']
         else:
             side = state[b'good']
-        num = len(set(i.node() for i in parents) & set(side))
+        num = len({i.node() for i in parents} & set(side))
         if num == 1:
             return parents[0].ancestor(parents[1])
     return None