Revert "net/url: disallow raw IPv6 addresses in host"

This reverts commit e3be2d1b2b.

Reason for revert: Causes extensive failures in Google-internal testing.

Change-Id: I232f547fc326dff7df959d25f3a89777ea33b201
Reviewed-on: https://go-review.googlesource.com/c/go/+/711800
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/net/url/url.go

@@ -698,9 +698,7 @@ func parseHost(host string) (string, error) {
 			return "", errors.New("invalid IP-literal")
 		}
 		return "[" + unescapedHostname + "]" + unescapedColonPort, nil
-	} else if i := strings.Index(host, ":"); i != -1 {
+	} else if i := strings.LastIndex(host, ":"); i != -1 {
-		// IPv4address / reg-name
-		// E.g. 1.2.3.4, 1.2.3.4:80, example.com, example.com:80
 		colonPort := host[i:]
 		if !validOptionalPort(colonPort) {
 			return "", fmt.Errorf("invalid port %q after host", colonPort)

src/net/url/url_test.go

@@ -506,6 +506,26 @@ var urltests = []URLTest{
 		},
 		"",
 	},
+	{
+		// Malformed IPv6 but still accepted.
+		"http://2b01:e34:ef40:7730:8e70:5aff:fefe:edac:8080/foo",
+		&URL{
+			Scheme: "http",
+			Host:   "2b01:e34:ef40:7730:8e70:5aff:fefe:edac:8080",
+			Path:   "/foo",
+		},
+		"",
+	},
+	{
+		// Malformed IPv6 but still accepted.
+		"http://2b01:e34:ef40:7730:8e70:5aff:fefe:edac:/foo",
+		&URL{
+			Scheme: "http",
+			Host:   "2b01:e34:ef40:7730:8e70:5aff:fefe:edac:",
+			Path:   "/foo",
+		},
+		"",
+	},
 	{
 		"http://[2b01:e34:ef40:7730:8e70:5aff:fefe:edac]:8080/foo",
 		&URL{
@@ -715,9 +735,6 @@ var parseRequestURLTests = []struct {
 	{"https://[0:0::test.com]:80", false},
 	{"https://[2001:db8::test.com]", false},
 	{"https://[test.com]", false},
-	{"https://1:2:3:4:5:6:7:8", false},
-	{"https://1:2:3:4:5:6:7:8:80", false},
-	{"https://example.com:80:", false},
 }
 
 func TestParseRequestURI(t *testing.T) {