如何检查给定的类是否包含在参数包中

Question

给出：

class foo_t { };

template<class ... args_t> class bar_t
{
    static_assert(/*fire if foo_t is not part of args_t*/);
};

bar_t<foo_t, int, std::string> bar1; // ok
bar_t<char> bar2; // triggers static_assert above

如何实现有问题的static_assert？