json: treat renamed byte slices the same as []byte

Fixes #2163.

R=rsc
CC=golang-dev
https://golang.org/cl/5488068

src/pkg/encoding/json/encode.go

@@ -339,13 +339,10 @@ func (e *encodeState) reflectValueQuoted(v reflect.Value, quoted bool) {
 			e.WriteString("null")
 			break
 		}
-		// Slices can be marshalled as nil, but otherwise are handled
-		// as arrays.
-		fallthrough
-	case reflect.Array:
-		if v.Type() == byteSliceType {
+		if v.Type().Elem().Kind() == reflect.Uint8 {
+			// Byte slices get special treatment; arrays don't.
+			s := v.Bytes()
 			e.WriteByte('"')
-			s := v.Interface().([]byte)
 			if len(s) < 1024 {
 				// for small buffers, using Encode directly is much faster.
 				dst := make([]byte, base64.StdEncoding.EncodedLen(len(s)))
@@ -361,6 +358,10 @@ func (e *encodeState) reflectValueQuoted(v reflect.Value, quoted bool) {
 			e.WriteByte('"')
 			break
 		}
+		// Slices can be marshalled as nil, but otherwise are handled
+		// as arrays.
+		fallthrough
+	case reflect.Array:
 		e.WriteByte('[')
 		n := v.Len()
 		for i := 0; i < n; i++ {

src/pkg/encoding/json/encode_test.go

@@ -82,3 +82,28 @@ func TestStringTag(t *testing.T) {
 		t.Fatalf("decode didn't match.\nsource: %#v\nEncoded as:\n%s\ndecode: %#v", s, string(got), s2)
 	}
 }
+
+// byte slices are special even if they're renamed types.
+type renamedByte byte
+type renamedByteSlice []byte
+type renamedRenamedByteSlice []renamedByte
+
+func TestEncodeRenamedByteSlice(t *testing.T) {
+	s := renamedByteSlice("abc")
+	result, err := Marshal(s)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expect := `"YWJj"`
+	if string(result) != expect {
+		t.Errorf(" got %s want %s", result, expect)
+	}
+	r := renamedRenamedByteSlice("abc")
+	result, err = Marshal(r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(result) != expect {
+		t.Errorf(" got %s want %s", result, expect)
+	}
+}