View Compilation Error

0_0_17385915_22689.cpp:11:10: error: expected primary-expression before 'auto'
 decltype(auto) add(T a, T b)
          ^
0_0_17385915_22689.cpp:11:10: error: expected ')' before 'auto'
0_0_17385915_22689.cpp:11:1: error: expected unqualified-id before 'decltype'
 decltype(auto) add(T a, T b)
 ^
0_0_17385915_22689.cpp: In function 'int main()':
0_0_17385915_22689.cpp:21:25: error: 'add' was not declared in this scope
         cout << add(a, b) << endl;
                         ^