The standard states that std::rethrow_if_nested have the signature: template <class E> void rethrow_if_nested(const E& e); // C++14 18.8.6 p9 The library implementation adds a second paramter to its definition of the function for the purpose of utilizing SFINAE. This only affects uses of std::rethrow_if_nested if attempting to describe its signature as a type. As an example, the following attempt to capture the function pointer in a variable will fail to compile: void (*p)(const int&) = &std::rethrow_if_nested<int>;