/in/foo.cc:57:10: error: expected primary-expression before 'auto'
 decltype(auto) get_val_and_pop (stack<T>& s)
          ^~~~
/in/foo.cc:57:10: error: expected ')' before 'auto'
/in/foo.cc:57:1: error: expected unqualified-id before 'decltype'
 decltype(auto) get_val_and_pop (stack<T>& s)
 ^~~~~~~~
/in/foo.cc: In function 'int64_t calc(const string&, int64_t)':
/in/foo.cc:82:41: error: 'get_val_and_pop' was not declared in this scope
    for(string op;(op=get_val_and_pop(ops))!="(";)
                                         ^
/in/foo.cc:91:44: error: 'get_val_and_pop' was not declared in this scope
     vars.push(functions[get_val_and_pop(ops)](get_val_and_pop(vars),get_val_and_pop(vars)));
                                            ^
/in/foo.cc:99:42: error: 'get_val_and_pop' was not declared in this scope
   vars.push(functions[get_val_and_pop(ops)](get_val_and_pop(vars),get_val_and_pop(vars)));
                                          ^