encoding/json: use reflect.SetBytes when decoding bytes

This allows slices of custom types with byte as underlying type to be
decoded, fixing a regression introduced in CL 9371.

Fixes #12921.

Change-Id: I62a715eaeaaa912b6bc599e94f9981a9ba5cb242
Reviewed-on: https://go-review.googlesource.com/16303Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/decode.go

@@ -757,7 +757,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 				d.saveError(err)
 				break
 			}
-			v.Set(reflect.ValueOf(b[0:n]))
+			v.SetBytes(b[:n])
 		case reflect.String:
 			v.SetString(string(s))
 		case reflect.Interface:

src/encoding/json/decode_test.go

@@ -1253,6 +1253,27 @@ func TestByteKind(t *testing.T) {
 	}
 }
 
+// The fix for issue 8962 introduced a regression.
+// Issue 12921.
+func TestSliceOfCustomByte(t *testing.T) {
+	type Uint8 uint8
+
+	a := []Uint8("hello")
+
+	data, err := Marshal(a)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b []Uint8
+	err = Unmarshal(data, &b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(a, b) {
+		t.Fatal("expected %v == %v", a, b)
+	}
+}
+
 var decodeTypeErrorTests = []struct {
 	dest interface{}
 	src  string