View Compilation Error

0_0_37916697_18500.cpp:40:30: error: 'operator[]' function uses 'auto' type specifier without trailing return type
  inline auto operator[](int x){return a[x];}
                              ^
0_0_37916697_18500.cpp:40:30: note: deduced return type only available with -std=c++1y or -std=gnu++1y