encoding/xml: fix panic in Marshal

Fixes #6341.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13512048

src/pkg/encoding/xml/marshal.go

@@ -655,7 +655,10 @@ func (p *printer) marshalSimple(typ reflect.Type, val reflect.Value) (string, []
 	case reflect.Bool:
 		return strconv.FormatBool(val.Bool()), nil, nil
 	case reflect.Array:
-		// will be [...]byte
+		if typ.Elem().Kind() != reflect.Uint8 {
+			break
+		}
+		// [...]byte
 		var bytes []byte
 		if val.CanAddr() {
 			bytes = val.Slice(0, val.Len()).Bytes()
@@ -665,7 +668,10 @@ func (p *printer) marshalSimple(typ reflect.Type, val reflect.Value) (string, []
 		}
 		return "", bytes, nil
 	case reflect.Slice:
-		// will be []byte
+		if typ.Elem().Kind() != reflect.Uint8 {
+			break
+		}
+		// []byte
 		return "", val.Bytes(), nil
 	}
 	return "", nil, &UnsupportedTypeError{typ}

src/pkg/encoding/xml/marshal_test.go

@@ -904,6 +904,10 @@ type AttrParent struct {
 	X string `xml:"X>Y,attr"`
 }
 
+type BadAttr struct {
+	Name []string `xml:"name,attr"`
+}
+
 var marshalErrorTests = []struct {
 	Value interface{}
 	Err   string
@@ -936,6 +940,10 @@ var marshalErrorTests = []struct {
 		Value: &AttrParent{},
 		Err:   `xml: X>Y chain not valid with attr flag`,
 	},
+	{
+		Value: BadAttr{[]string{"X", "Y"}},
+		Err:   `xml: unsupported type: []string`,
+	},
 }
 
 var marshalIndentTests = []struct {