c++ - Is it possible to expand a parameter pack with a lambda function? -

i think question corner case, of how far can go parameter pack expansion , it's little hard explain, want. idea easiest express little code, lets have @ following lines:

// <main.cpp> #include <tuple> #include <iostream>  // function supposed return std::tuple of function objects // each template argument type, there should 1 such function object, // takes value of type , returns half. template <class... arg_t> auto make_divide_by_two_lambda_tuple() {     return std::make_tuple(         [](const arg_t& elem) -> arg_t         {             return elem/2;         }...     ); }  int main() {     // lets initialise tuple our function     auto value = make_divide_by_two_lambda_tuple<int, float>();      /*      * if workes planned, tuple has structure      * value<0> := [](const int&){ ... }      * value<1> := [](const float&){ ... }      */      // , able use our functions in expected way     std::cout << "5/2 (int version) = " << std::get<0>(value)(5) << std::endl;     std::cout << "5/2 (float version) = " << std::get<1>(value)(5) << std::endl;      return 0; } 

for clang(4.0.1), example compiles , run expect do:

$ clang++ -std=c++14 ../main.cpp $ ./a.out  5/2 (int version) = 2 5/2 (float version) = 2.5 

for g++(7.1.1), example fail compile:

$ g++ -std=c++14 ../main.cpp     ../main.cpp: in function ‘auto make_divide_by_two_lambda_tuple()’: ../main.cpp:7:28: error: parameter packs not expanded ‘...’:    [](const arg_t& elem) -> arg_t                             ^~~~~ ../main.cpp:7:28: note:         ‘arg_t’ ../main.cpp:7:28: error: parameter packs not expanded ‘...’: ../main.cpp:7:28: note:         ‘arg_t’ ../main.cpp:10:4: error: expansion pattern ‘<lambda>’ contains no argument packs    }...     ^~~ ../main.cpp: in function ‘int main()’: ../main.cpp:15:10: error: ‘void value’ has incomplete type      auto value = make_divide_by_two_lambda_tuple<int, float>(); 

so seems, gcc willing use parameter pack inside argument list of lambda expression. using whole lambda stretching far gcc accept.

is use of lambda expressions undefined behaviour, or 1 compiler (hopefully gcc) wrong?

ps.: checked docs, explains, can use parameter pack expansion, , not statments can expand.