c++ - variadic templates sum operation left associative -

the code below works the: goal left associative sum operation: sum(1,2,3,4);

however, won't work correctly sum(1,2,3,4,5) or sum(1,2,3,4,5,...). more 4 arguments gives error:

error: no matching function call sum(int, int, int, int, int)

=================================

template <typename t> t sum(const t& v) { return v; }  template <typename t1, typename t2> auto sum(const t1& v1, const t2& v2) -> decltype( v1 + v2) { return v1 + v2; }  template <typename t1, typename t2, typename... ts> auto sum(const t1& v1, const t2& v2, const ts&... rest) -> decltype( v1 + v2 +      sum(rest...) ) { return v1 + v2 + sum(rest... ); }  int main() {     cout << sum(1,2,3,4); //works correctly     //cout << sum(1,2,3,4,5); //compile error  } 

that seems bug in gcc, when working variadic templates, auto return types , recursive reference same variadic template in trailing return type.

it solvable, through old template meta programming:

//first metafunction calculate result type of sum(ts...) template <typename...> struct sumts; template <typename t1> struct sumts<t1> { typedef t1 type; }; template <typename t1, typename... ts> struct sumts<t1, ts...> {   typedef typename sumts<ts...>::type rhs_t;   typedef decltype(std::declval<t1>() + std::declval<rhs_t>()) type; };  //now sum function template <typename t> t sum(const t& v) {   return v; }  template <typename t1, typename... ts> auto sum(const t1& v1, const ts&... rest)    -> typename sumts<t1,ts...>::type //instead of decltype {   return v1 + sum(rest... ); }  #include <iostream> using std::cout;  int main() {   cout << sum(1,2,3,4,5);     } 

ps: more generic, whole thing pimped "universal references" , std::forward.

update: used std::declval instead of handwritten functions