diff --git a/sphinx/domains/cpp.py b/sphinx/domains/cpp.py
index 4dac89253..90c3533ef 100644
--- a/sphinx/domains/cpp.py
+++ b/sphinx/domains/cpp.py
@@ -132,6 +132,8 @@ class DefExpr(object):
     def __ne__(self, other):
         return not self.__eq__(other)
 
+    __hash__ = None
+
     def clone(self):
         """Close a definition expression node"""
         return deepcopy(self)