diff -r 45a0047c0ebc -r 41b6245c3fc4 mercurial/localrepo.py
--- a/mercurial/localrepo.py	Thu Nov 22 21:00:13 2018 +0100
+++ b/mercurial/localrepo.py	Thu Nov 22 18:44:07 2018 +0100
@@ -1031,7 +1031,7 @@
             if path.startswith('cache/'):
                 msg = 'accessing cache with vfs instead of cachevfs: "%s"'
                 repo.ui.develwarn(msg % path, stacklevel=2, config="cache-vfs")
-            if path.startswith('journal.'):
+            if path.startswith('journal.') or path.startswith('undo.'):
                 # journal is covered by 'lock'
                 if repo._currentlock(repo._lockref) is None:
                     repo.ui.develwarn('write with no lock: "%s"' % path,