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#include <iostream>
using namespace std;
 
auto sum() { return {}; }
 
template<typename Head, typename... Tail>
auto sum(Head head, Tail... tail)
{
    return head+sum(tail...);
}
 
int main() {
    cout<< sum(1,2.4) << endl;
    cout<< sum("hello ", "world") << endl;
    return 0;
}

prog.cpp:4:21: error: cannot deduce return type from initializer list
auto sum() { return {}; }
                    ^~
prog.cpp:9:17: error: no matching function for call to 'sum'
    return head+sum(tail...);
                ^~~
prog.cpp:9:17: note: in instantiation of function template specialization 'sum<double>' requested here
prog.cpp:13:12: note: in instantiation of function template specialization 'sum<int, double>' requested here
    cout<< sum(1,2.4) << endl;
           ^
prog.cpp:7:6: note: candidate function template not viable: requires at least argument 'head', but no arguments were provided
auto sum(Head head, Tail... tail)
     ^
prog.cpp:9:17: error: no matching function for call to 'sum'
    return head+sum(tail...);
                ^~~
prog.cpp:9:17: note: in instantiation of function template specialization 'sum<const char *>' requested here
prog.cpp:14:12: note: in instantiation of function template specialization 'sum<const char *, const char *>' requested here
    cout<< sum("hello ", "world") << endl;
           ^
prog.cpp:7:6: note: candidate function template not viable: requires at least argument 'head', but no arguments were provided
auto sum(Head head, Tail... tail)
     ^
3 errors generated.

Standard output is empty