[Bug c++/70810] New: std::function template variadic template arguments do not unpack in function template

["sd.foolegg at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70810

            Bug ID: 70810
           Summary: std::function template variadic template arguments do
                    not unpack in function template
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sd.foolegg at gmail dot com
  Target Milestone: ---

std::function template variadic template arguments do not unpack in function
template, like this:

template<typename ... Ts>
void invoke(std::function<void(Ts ...)> f, Ts ... args) {}

void foo(std::string s, int i) {}

invoke<std::string, int>(foo, "a", 1); // fail

// mismatched types 'std::function<void(Ts ...)>' and 'void
(*)(std::__cxx11::string, int) {aka void (*)(std::__cxx11::basic_string<char>,
int)}'

full code :

#include <iostream>
#include <functional>

template<typename ... Ts>
struct Invoker {
    void invoke(std::function<void(Ts ...)> f, Ts ... args) {
        f(args ...);
    }
};

template<typename ... Ts>
void invoke(std::function<void(Ts ...)> f, Ts ... args) {
    f(args ...);
}

void foo(std::string s, int i) {
    std::cout << "foo " << s << " " << i << std::endl;
}

int main() {
    Invoker<std::string, int> invoker;
    invoker.invoke(foo, "a", 1); // ok

    invoke<std::string, int>(foo, "a", 1); // fail

    return 0;
}

main.cc:24:41: note:   mismatched types 'std::function<void(Ts ...)>' and 'void
(*)(std::__cxx11::string, int) {aka void (*)(std::__cxx11::basic_string<char>,
int)}'

gcc version 5.3.1 20160301 [gcc-5-branch revision 233849] (SUSE Linux)