diff -r e7966337aed0 -r 23f891164e59 mercurial/configitems.py
--- a/mercurial/configitems.py	Sun Oct 08 20:39:51 2017 +0200
+++ b/mercurial/configitems.py	Tue Oct 10 10:48:47 2017 +0200
@@ -412,6 +412,10 @@
 coreconfigitem('experimental', 'updatecheck',
     default=None,
 )
+coreconfigitem('extensions', '.*',
+    default=None,
+    generic=True,
+)
 coreconfigitem('format', 'aggressivemergedeltas',
     default=False,
 )