g++ compiler optimization: cannot convert ‘<brace-enclosed initializer list>’ - c++11

Question

Following code not compiling:
A.h
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <fcntl.h>
namespace net {
using Ip = in_addr_t;
using Port = in_port_t;
using SockFd = int;
class Params final {
public:
Ip getIp() const { return ip_; }
Port getPort() const { return port_; }
private:
Ip ip_ {INADDR_ANY};
Port port_ {htons(5)};
};
}
A.cpp
#include <iostream>
#include "A.h"
int main(){
net::Params a {};
std::cout << "Ip=" << a.getIp() << ", Port=" << a.getPort() << std::endl;
return 0;
}
Compilation:
g++-6 -O2 -std=c++11 A.cpp
Error:
In file included from /usr/include/x86_64-linux-gnu/bits/byteswap.h:35:0,
from /usr/include/endian.h:60,
from /usr/include/ctype.h:39,
from /usr/include/c++/6/cctype:42,
from /usr/include/c++/6/bits/localefwd.h:42,
from /usr/include/c++/6/ios:41,
from /usr/include/c++/6/ostream:38,
from /usr/include/c++/6/iostream:39,
from A.cpp:1:
A.h:21:15: error: statement-expressions are not allowed outside functions nor in template-argument lists
Port port_ {htons(5)};
^
In file included from A.cpp:3:0:
A.h:21:23: error: cannot convert ‘<brace-enclosed initializer list>’ to ‘net::Port {aka short unsigned int}’ in initialization
Port port_ {htons(5)};
^
But when I change port_ member variable initialization to: Port port_ {5};, g++ with -O2 compiles fine.
Above code compiles fine without optimization flag, whether port_ initialized as: Port port_ {htons(5)}; or as Port port_ {5};
Whats wrong?

Answer 1

Seems to be a ompiler and/or libstd bug. The compiler tries to optimize away the function call to htons with some macros and compiler magic. That results in some problem I do not understand. But you can define an inline function myhtons that calls htons and use that instead. Works for me with gcc 7.2.
inline Port myhtons( Port v )
{
return htons(v);
}
class Params final {
public:
Ip getIp() const { return ip_; }
Port getPort() const { return port_; }
private:
Ip ip_ {INADDR_ANY};
Port port_ { myhtons(5) };
};