[#3991] Fixed returnByReference in optional.h

diff --git a/src/lib/util/optional.h b/src/lib/util/optional.h
index 40c6df221d0bcffc0910a92434294ecd8cb19221..a9cea63343078b6855985b512109a0738e9c6ebf 100644 (file)
--- a/src/lib/util/optional.h
+++ b/src/lib/util/optional.h
@@ -10,6 +10,7 @@
 #include <exceptions/exceptions.h>
 #include <ostream>
 #include <string>
+#include <type_traits>
 
 namespace isc {
 namespace util {
@@ -161,6 +162,10 @@ public:
 protected:
     T default_;         ///< Encapsulated value.
     bool unspecified_;  ///< Flag which indicates if the value is specified.
+
+private:
+    // Refuse instantiation by a reference type.
+    static_assert(!std::is_reference<T>::value, "!std::is_reference<T>::value");
 };
 
 /// @brief Specialization of the default @c Optional constructor for