Implement HTTP server using libevent
Not too much to review as the code uses evhttp to handle HTTP and provide a basic, static reply for each request.

Why is host constexpr, but not the port?

All those using statements, combined with traits and decltype one after the other makes the code very hard to read, almost obscuring the fact that evhttp is being initialized at that point. I would try to rewrite that part in a simpler way.

Why auto main() -&gt; int instead of the simple, classic int main()?