戻り値のautoが決まらないってことでしょ？autoをdouble固定にしてReturnをなくせば通る
#include <iostream>
auto sum() {return 0;}
template<typename Head> auto sum(Head head) {return head;}
template<typename Head, typename... Tails> auto sum(Head head, Tails ...tails) {return head + sum(tails...);}
template<typename Return, typename... Args> Return wrap(Return (*f)(Args...), Args ...args) {return f(args...);}
int main( int argc, char *argv[] ) {
std::cout << sum(1,2.1) << std::endl;
std::cout << wrap(static_cast<double(*)(int,double)>(sum), 1,2.1) << std::endl;
std::cout << wrap(sum<int,double>, 1,2.1) << std::endl;
std::cout << wrap(sum, 1,2.1) << std::endl;
return 0;
}
// /usr/bin/g++ -fdiagnostics-color=always -g /home/user/cpp/autoreturn/sample.cpp -o /home/user/cpp/autoreturn/sample
// /home/user/cpp/autoreturn/sample.cpp: In function ‘int main(int, char**)’:
// /home/user/cpp/autoreturn/sample.cpp:10:29: error: no matching function for call to ‘wrap(<unresolved overloaded function type>, int, double)’
// 10 | std::cout << wrap(sum, 1,2.1) << std::endl;
// | ^
// /home/user/cpp/autoreturn/sample.cpp:5:52: note: candidate: ‘template<class Return, class ... Args> Return wrap(Return (*)(Args ...), Args ...)’
// 5 | template<typename Return, typename... Args> Return wrap(Return (*f)(Args...), Args ...args) {return f(args...);}
// | ^~~~
// /home/user/cpp/autoreturn/sample.cpp:5:52: note: template argument deduction/substitution failed:
// /home/user/cpp/autoreturn/sample.cpp:10:29: note: couldn’t deduce template parameter ‘Return’
// 10 | std::cout << wrap(sum, 1,2.1) << std::endl;
// | ^