Default dns addr doesn't work on ubuntu 16.04

[dolanor]

Hi,

I tried to use spilld, but I got this error on ubuntu 16.04, and I don't understand why it does that.
I tried to investigate to understand, and it seems the type switch in *net.ListenConfig.Listen() can't distinguish which type of addr it is.

spilld -dev -debug_addr :22222
spilld, version unknown, starting at 2019-06-30 13:18:52.717845551 +0000 UTC m=+0.005995874
***DEVELOPMENT MODE***
temp dir /tmp/spilld-425601882
listen tcp :53: address :53: unexpected address type

I added some debug fmt.Println in the src/net/dial.go to see what was wrong:

diff /tmp/dial-dbg.go /usr/local/go-1.12.6/src/net/dial.go 
11,12d10
< 	"log"
< 	"reflect"
619,620d616
< 	log.Println("addr:", reflect.TypeOf(la),
< 		reflect.ValueOf(la))

And the output is now:

spilld -debug_addr :22222 -dev -dns_addr :8888 -imap_addr :9943 -smtp_addr :2525 -msa_addr :1465
spilld, version unknown, starting at 2019-06-30 22:38:14.890883754 +0200 CEST m=+0.003851093
***DEVELOPMENT MODE***
temp dir /tmp/spilld-843824628
addr: *net.TCPAddr :9943
addr: *net.TCPAddr :2525
addr: *net.TCPAddr :1465
addr: *net.TCPAddr :8888
listen tcp :8888: address :8888: unexpected address type

I don't get why it fails for this special case.

[dolanor]

I added a test in the log reflect.TypeOf(la) == reflect.TypeOf(&TCPAddr{})

--- dial.go	2019-06-30 23:09:27.680602947 +0200
+++ dial.go.ori	2019-06-11 19:27:51.000000000 +0200
@@ -8,8 +8,6 @@
 	"context"
 	"internal/nettrace"
 	"internal/poll"
-	"log"
-	"reflect"
 	"syscall"
 	"time"
 )
@@ -616,9 +614,6 @@
 	}
 	var l Listener
 	la := addrs.first(isIPv4)
-	log.Println("addr:", reflect.TypeOf(la),
-		reflect.ValueOf(la), reflect.TypeOf(la) == reflect.TypeOf(&TCPAddr{}))
-
 	switch la := la.(type) {
 	case *TCPAddr:
 		l, err = sl.listenTCP(ctx, la)

And now I get:

spilld -debug_addr :22222 -dev -dns_addr :8888 -imap_addr :9943 -smtp_addr :2525 -msa_addr :1465
spilld, version unknown, starting at 2019-06-30 23:09:39.38378492 +0200 CEST m=+0.005742067
***DEVELOPMENT MODE***
temp dir /tmp/spilld-171764129
addr: *net.TCPAddr :9943 true
addr: *net.TCPAddr :2525 true
addr: *net.TCPAddr :1465 true
addr: *net.TCPAddr :8888 true
listen tcp :8888: address :8888: unexpected address type

la is of the right reflect.Type, but the type switch didn't think it was. Could it be a bug in Go directly?
Why on that case?