json: unmarshal types that are byte slices.

The json package cheerfully would marshal

        type S struct {
                IP net.IP
        }

but would give an error when unmarshalling.  This change allows any
type whose concrete type is a byte slice to be unmarshalled from a
string.

Fixes #5086.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/11161044

src/pkg/encoding/json/decode.go

@@ -660,7 +660,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		default:
 			d.saveError(&UnmarshalTypeError{"string", v.Type()})
 		case reflect.Slice:
-			if v.Type() != byteSliceType {
+			if v.Type().Elem().Kind() != reflect.Uint8 {
 				d.saveError(&UnmarshalTypeError{"string", v.Type()})
 				break
 			}

src/pkg/encoding/json/decode_test.go

@@ -1191,3 +1191,32 @@ func TestSkipArrayObjects(t *testing.T) {
 		t.Errorf("got error %q, want nil", err)
 	}
 }
+
+// Test that types of byte slices (such as net.IP) both
+// marshal and unmarshal.
+func TestByteSliceType(t *testing.T) {
+	type A []byte
+	type S struct {
+		A A
+	}
+
+	for x, in := range []S{
+		S{},
+		S{A: []byte{'1'}},
+		S{A: []byte{'1', '2', '3', '4', '5'}},
+	} {
+		data, err := Marshal(&in)
+		if err != nil {
+			t.Errorf("#%d: got Marshal error %q, want nil", x, err)
+			continue
+		}
+		var out S
+		err = Unmarshal(data, &out)
+		if err != nil {
+			t.Fatalf("#%d: got Unmarshal error %q, want nil", x, err)
+		}
+		if !reflect.DeepEqual(&out, &in) {
+			t.Fatalf("#%d: got %v, want %v", x, &out, &in)
+		}
+	}
+}

src/pkg/encoding/json/encode.go

@@ -44,8 +44,8 @@ import (
 // The angle brackets "<" and ">" are escaped to "\u003c" and "\u003e"
 // to keep some browsers from misinterpreting JSON output as HTML.
 //
-// Array and slice values encode as JSON arrays, except that
-// []byte encodes as a base64-encoded string, and a nil slice
+// Array and slice values encode as JSON arrays, except that a slice of
+// bytes encodes as a base64-encoded string, and a nil slice
 // encodes as the null JSON object.
 //
 // Struct values encode as JSON objects. Each exported struct field