diff -r ab493a8353d1 -r d74141ccfd8b mercurial/configitems.py
--- a/mercurial/configitems.py	Fri Jul 14 16:17:37 2017 +0200
+++ b/mercurial/configitems.py	Wed Jul 12 23:36:10 2017 +0200
@@ -51,6 +51,9 @@
         raise error.ProgrammingError(msg % (item.section, item.name))
     section[item.name] = item
 
+# special value for case where the default is derived from other values
+dynamicdefault = object()
+
 # Registering actual config items
 
 def getitemregister(configtable):