Make operator<< static
diff --git a/examples/all_features/stringification.cpp b/examples/all_features/stringification.cpp
index 1a100cb..d36d372 100644
--- a/examples/all_features/stringification.cpp
+++ b/examples/all_features/stringification.cpp
@@ -267,7 +267,7 @@
 
 enum class Foo { };
 
-std::ostream& operator<<(std::ostream& os, Foo) {
+static std::ostream& operator<<(std::ostream& os, Foo) {
     return os << "Foo";
 }