diff --git a/llvm/lib/Demangle/ItaniumDemangle.cpp b/llvm/lib/Demangle/ItaniumDemangle.cpp
index be72e15..d32ebbf 100644
--- a/llvm/lib/Demangle/ItaniumDemangle.cpp
+++ b/llvm/lib/Demangle/ItaniumDemangle.cpp
@@ -19,6 +19,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <functional>
 #include <numeric>
 #include <utility>
 #include <vector>