diff -r 9f529abcaebf -r 703ebf476a1d lemon/concepts/maps.h
--- a/lemon/concepts/maps.h	Thu Jun 11 23:13:24 2009 +0200
+++ b/lemon/concepts/maps.h	Thu Aug 20 20:34:30 2009 +0200
@@ -182,7 +182,8 @@
 
       template<typename _ReferenceMap>
       struct Constraints {
-        void constraints() {
+        typename enable_if<typename _ReferenceMap::ReferenceMapTag, void>::type
+        constraints() {
           checkConcept<ReadWriteMap<K, T>, _ReferenceMap >();
           ref = m[key];
           m[key] = val;