encoding/json: respect json.Marshaler when encoding byte kind slices

Fixes #13783.

Change-Id: I0122c1f0cf4075acabf5f58241bded1835699dc1
Reviewed-on: https://go-review.googlesource.com/19725Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/encode.go

@@ -679,7 +679,9 @@ func (se *sliceEncoder) encode(e *encodeState, v reflect.Value, _ bool) {
 
 func newSliceEncoder(t reflect.Type) encoderFunc {
 	// Byte slices get special treatment; arrays don't.
-	if t.Elem().Kind() == reflect.Uint8 {
+	if t.Elem().Kind() == reflect.Uint8 &&
+		!t.Elem().Implements(marshalerType) &&
+		!t.Elem().Implements(textMarshalerType) {
 		return encodeByteSlice
 	}
 	enc := &sliceEncoder{newArrayEncoder(t)}

src/encoding/json/encode_test.go

@@ -6,6 +6,7 @@ package json
 
 import (
 	"bytes"
+	"fmt"
 	"math"
 	"reflect"
 	"testing"
@@ -537,6 +538,60 @@ func TestEncodeString(t *testing.T) {
 	}
 }
 
+type jsonbyte byte
+
+func (b jsonbyte) MarshalJSON() ([]byte, error) { return tenc(`{"JB":%d}`, b) }
+
+type textbyte byte
+
+func (b textbyte) MarshalText() ([]byte, error) { return tenc(`TB:%d`, b) }
+
+type jsonint int
+
+func (i jsonint) MarshalJSON() ([]byte, error) { return tenc(`{"JI":%d}`, i) }
+
+type textint int
+
+func (i textint) MarshalText() ([]byte, error) { return tenc(`TI:%d`, i) }
+
+func tenc(format string, a ...interface{}) ([]byte, error) {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, format, a...)
+	return buf.Bytes(), nil
+}
+
+// Issue 13783
+func TestEncodeBytekind(t *testing.T) {
+	testdata := []struct {
+		data interface{}
+		want string
+	}{
+		{byte(7), "7"},
+		{jsonbyte(7), `{"JB":7}`},
+		{textbyte(4), `"TB:4"`},
+		{jsonint(5), `{"JI":5}`},
+		{textint(1), `"TI:1"`},
+		{[]byte{0, 1}, `"AAE="`},
+		{[]jsonbyte{0, 1}, `[{"JB":0},{"JB":1}]`},
+		{[][]jsonbyte{{0, 1}, {3}}, `[[{"JB":0},{"JB":1}],[{"JB":3}]]`},
+		{[]textbyte{2, 3}, `["TB:2","TB:3"]`},
+		{[]jsonint{5, 4}, `[{"JI":5},{"JI":4}]`},
+		{[]textint{9, 3}, `["TI:9","TI:3"]`},
+		{[]int{9, 3}, `[9,3]`},
+	}
+	for _, d := range testdata {
+		js, err := Marshal(d.data)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		got, want := string(js), d.want
+		if got != want {
+			t.Errorf("got %s, want %s", got, want)
+		}
+	}
+}
+
 func TestTextMarshalerMapKeysAreSorted(t *testing.T) {
 	b, err := Marshal(map[unmarshalerText]int{
 		{"x", "y"}: 1,