diff -r 3a7efcbdf288 -r f0fa0fc4900a mercurial/branchmap.py
--- a/mercurial/branchmap.py	Fri Apr 12 09:41:08 2019 -0700
+++ b/mercurial/branchmap.py	Wed Mar 27 21:33:04 2019 +0300
@@ -305,7 +305,7 @@
 
     def copy(self):
         """return an deep copy of the branchcache object"""
-        return branchcache(
+        return type(self)(
             self._entries, self.tipnode, self.tiprev, self.filteredhash,
             self._closednodes)