diff -r 9e593db5f1a1 -r 6498f0e03526 mercurial/vfs.py
--- a/mercurial/vfs.py	Wed Dec 26 13:44:37 2018 +0100
+++ b/mercurial/vfs.py	Wed Jan 02 10:29:12 2019 +0100
@@ -469,7 +469,7 @@
 
 opener = vfs
 
-class proxyvfs(object):
+class proxyvfs(abstractvfs):
     def __init__(self, vfs):
         self.vfs = vfs
 
@@ -481,7 +481,7 @@
     def options(self, value):
         self.vfs.options = value
 
-class filtervfs(abstractvfs, proxyvfs):
+class filtervfs(proxyvfs, abstractvfs):
     '''Wrapper vfs for filtering filenames with a function.'''
 
     def __init__(self, vfs, filter):
@@ -499,7 +499,7 @@
 
 filteropener = filtervfs
 
-class readonlyvfs(abstractvfs, proxyvfs):
+class readonlyvfs(proxyvfs):
     '''Wrapper vfs preventing any writing.'''
 
     def __init__(self, vfs):