Clang和gcc中的C ++ 14可变参数模板参数推断

Question

I am using clang 3.5.0 and gcc version 4.9.2 (both with C++14 options on, although with trailing return types this could be done in C++11).

The following code compiles in g++ and not in clang++. My question is "which is right?"

#include <iostream>
#include <tuple>
#include <functional>
using namespace std;

template<typename OP, typename F1, typename... Fs>
struct symop {
     OP op;
     tuple<F1,Fs...> fs;

     symop(const OP &oopp, const F1 &f1, const Fs &...ffss)
          : op(oopp), fs(f1,ffss...) {}
};

template<typename OP, typename... Fs>
auto baz(const symop<OP,Fs...> &so) {
     return so.op(get<0>(so.fs),get<1>(so.fs));
}

/* // this version compiles on both compilers
template<typename OP, typename F1, typename... Fs>
auto baz(const symop<OP,F1,Fs...> &so) {
     return so.op(get<0>(so.fs),get<1>(so.fs));
}
*/

int main() {
     symop<plus<int>,int,int> so{plus<int>{},3,4};
     cout << baz(so) << endl;
}

Clang reports

q.cpp:29:10: error: no matching function for call to 'baz'
        cout << baz(so) << endl;
                ^~~
q.cpp:16:6: note: candidate template ignored: couldn't infer template argument
      'OP'
auto baz(const symop<OP,Fs...> &so) {
     ^

Answer 1

You should understand that Clang is still experimental (even though it is "widely" used), and basically is built upon GCC (trough it has some significant changes) and you can be sure that everything (talking C++ code) that is compiled in GCC should compile in Clang, and if not, it is a Clang bug.

GCC had it's time to prove being robust.