case syscall.AF_INET6:
			syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_PORTRANGE, syscall.IPV6_PORTRANGE_HIGH)
		}
	}
	if family == syscall.AF_INET6 && sotype != syscall.SOCK_RAW && supportsIPv4map() {
		// Allow both IP versions even if the OS default
		// is otherwise. Note that some operating systems
		// never admit this option.
		syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY, boolint(ipv6only))

	for _, tt := range dualStackTCPListenerTests {
		if !testableListenArgs(tt.network1, JoinHostPort(tt.address1, "0"), "") {
			t.Logf("skipping %s test", tt.network1+" "+tt.address1)
			continue
		}

		if !supportsIPv4map() && differentWildcardAddr(tt.address1, tt.address2) {
			tt.xerr = nil
		}
		var firstErr, secondErr error
		for i := 0; i < 5; i++ {
			lns, err := newDualStackListener()
			if err != nil {

// functionality.
func supportsIPv6() bool {
	ipStackCaps.Once.Do(ipStackCaps.probe)
	return ipStackCaps.ipv6Enabled
}

// supportsIPv4map reports whether the platform supports mapping an
// IPv4 address inside an IPv6 address at transport layer
// protocols. See RFC 4291, RFC 4038 and RFC 3493.
func supportsIPv4map() bool {
	// Some operating systems provide no support for mapping IPv4
	// addresses to IPv6, and a runtime check is unnecessary.

			return false
		}
		if !supportsIPv6() && cip.To16() != nil && cip.To4() == nil {
			return false
		}
	}

	// Test functionality of IPv4 communication using AF_INET6
	// sockets.
	if !supportsIPv4map() && supportsIPv4() && (network == "tcp" || network == "udp" || network == "ip") && wildcard {
		// At this point, we prefer IPv4 when ip is nil.
		// See favoriteAddrFamily for further information.

	switch network[len(network)-1] {
	case '4':
		return syscall.AF_INET, false
	case '6':
		return syscall.AF_INET6, true
	}

	if mode == "listen" && (laddr == nil || laddr.isWildcard()) {
		if supportsIPv4map() || !supportsIPv4() {
			return syscall.AF_INET6, false
		}
		if laddr == nil {
			return syscall.AF_INET, false
		}
		return laddr.family(), false
	}

}

func TestProhibitionaryDialArg(t *testing.T) {
	testenv.MustHaveExternalNetwork(t)

	switch runtime.GOOS {
	case "plan9":
		t.Skipf("not supported on %s", runtime.GOOS)
	}
	if !supportsIPv4map() {
		t.Skip("mapping ipv4 address inside ipv6 address not supported")
	}

	ln, err := Listen("tcp", "[::]:0")
	if err != nil {
		t.Fatal(err)
	}
	defer ln.Close()