diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -51,6 +51,9 @@ def _register(configtable, *args, **kwar
         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):
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -457,11 +457,17 @@ class ui(object):
         if default is _unset:
             if item is None:
                 value = default
+            elif item.default is configitems.dynamicdefault:
+                value = None
+                msg = "config item requires an explicit default value: '%s.%s'"
+                msg %= (section, name)
+                self.develwarn(msg, 2, 'warn-config-default')
             elif callable(item.default):
                     value = item.default()
             else:
                 value = item.default
-        elif item is not None:
+        elif (item is not None
+              and item.default is not configitems.dynamicdefault):
             msg = ("specifying a default value for a registered "
                    "config item: '%s.%s' '%s'")
             msg %= (section, name, default)
diff --git a/tests/test-devel-warnings.t b/tests/test-devel-warnings.t
--- a/tests/test-devel-warnings.t
+++ b/tests/test-devel-warnings.t
@@ -218,7 +218,7 @@ Test warning on config option access and
   $ cat << EOF > ${TESTTMP}/buggyconfig.py
   > """A small extension that tests our developer warnings for config"""
   > 
-  > from mercurial import registrar
+  > from mercurial import registrar, configitems
   > 
   > cmdtable = {}
   > command = registrar.command(cmdtable)
@@ -227,6 +227,7 @@ Test warning on config option access and
   > configitem = registrar.configitem(configtable)
   > 
   > configitem('test', 'some', default='foo')
+  > configitem('test', 'dynamic', default=configitems.dynamicdefault)
   > # overwrite a core config
   > configitem('ui', 'quiet', default=False)
   > configitem('ui', 'interactive', default=None)
@@ -236,6 +237,8 @@ Test warning on config option access and
   >     repo.ui.config('ui', 'quiet', False)
   >     repo.ui.config('ui', 'interactive', None)
   >     repo.ui.config('test', 'some', 'foo')
+  >     repo.ui.config('test', 'dynamic', 'some-required-default')
+  >     repo.ui.config('test', 'dynamic')
   > EOF
 
   $ hg --config "extensions.buggyconfig=${TESTTMP}/buggyconfig.py" buggyconfig
@@ -244,5 +247,6 @@ Test warning on config option access and
   devel-warn: specifying a default value for a registered config item: 'ui.quiet' 'False' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
   devel-warn: specifying a default value for a registered config item: 'ui.interactive' 'None' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
   devel-warn: specifying a default value for a registered config item: 'test.some' 'foo' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
+  devel-warn: config item requires an explicit default value: 'test.dynamic' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
 
   $ cd ..