tun2: interface check

lib/tun2/client.go

@@ -122,6 +122,10 @@ type smuxListener struct {
 	session *smux.Session
 }
 
+var (
+	_ net.Listener = &smuxListener{} // interface check
+)
+
 func (sl *smuxListener) Accept() (net.Conn, error) {
 	return sl.session.AcceptStream()
 }