Variadic templates: unfold arguments in groups

You can accomplish it with overloads.

template <typename T1, typename T2>
void bar(T1&& arg1, T2&& arg2) {
    foo( std::forward<T1>(arg1), std::forward<T2>(arg2) ); // (until) sends (the last) two arguments to foo
}

template <typename T1, typename T2, typename... Args>
void bar(T1&& arg1, T2&& arg2, Args&&... args) {
    foo( std::forward<T1>(arg1), std::forward<T2>(arg2) ); // sends the 1st two arguments to foo
    bar( std::forward<Args>(args)... );                    // call bar with remaining elements recursively
}

LIVE

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^{Note that with the minimum snippet above when calling bar with 0 or odd arguments you'll get no matching function error. If you want more clear compilation message with static_assert you can start from this snippet.}

C++17 generalization for n-ary functors:

namespace impl
{
    template<std::size_t k, class Fn, class Tuple, std::size_t... js>
    void unfold_nk(Fn fn, Tuple&& tuple, std::index_sequence<js...>) {
        fn(std::get<k + js>(std::forward<Tuple>(tuple))...);
    }

    template<std::size_t n, class Fn, class Tuple, std::size_t... is>
    void unfold_n(Fn fn, Tuple&& tuple, std::index_sequence<is...>) {
        (unfold_nk<n * is>(fn, std::forward<Tuple>(tuple), 
            std::make_index_sequence<n>{}), ...);
    }
}

template<std::size_t n, class Fn, typename... Args>
void unfold(Fn fn, Args&&... args) {
    static_assert(sizeof...(Args) % n == 0);
    impl::unfold_n<n>(fn, std::forward_as_tuple(std::forward<Args>(args)...), 
        std::make_index_sequence<sizeof...(Args) / n>{});
}

int main() {
    auto fn = [](auto... args) { 
        (std::cout << ... << args) << ' ';
    };

    unfold<2>(fn, 1, 2, 3, 4, 5, 6);   // Output: 12 34 56
    unfold<3>(fn, 1, 2, 3, 4, 5, 6);   // Output: 123 456
}

Simple recursion using if constexpr:

// print as many pairs as we can
template<class T, class U, class... Args>
void foo(T t, U u, Args&&... args)
{
    std::cout << t << " + " << u << "\n";
    if constexpr(sizeof...(Args) > 0 && sizeof...(Args) % 2 == 0)
        foo(std::forward<Args>(args)...);
}

template<class... Args>
void bar(Args&&... args)
{
    static_assert(sizeof...(Args) % 2 == 0);
    foo(std::forward<Args>(args)...);
}

Call it like so:

bar(1, 2, 3, 4);

Demo

I would say that songyanyao's answer is pretty canonical pre-C++17. Afterwards, if constexpr has allowed us to move logic into the bodies of our functions instead of using overloading tricks.