encoding/json: call (*T).MarshalJSON for addressable T values.

Fixes #2170.

R=golang-dev, cw, adg
CC=golang-dev
https://golang.org/cl/5618045

src/pkg/encoding/json/decode_test.go

@@ -598,3 +598,24 @@ var pallValueIndent = `{
 }`
 
 var pallValueCompact = strings.Map(noSpace, pallValueIndent)
+
+func TestRefUnmarshal(t *testing.T) {
+	type S struct {
+		// Ref is defined in encode_test.go.
+		R0 Ref
+		R1 *Ref
+	}
+	want := S{
+		R0: 12,
+		R1: new(Ref),
+	}
+	*want.R1 = 12
+
+	var got S
+	if err := Unmarshal([]byte(`{"R0":"ref","R1":"ref"}`), &got); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %+v, want %+v", got, want)
+	}
+}

src/pkg/encoding/json/encode.go

@@ -262,8 +262,18 @@ func (e *encodeState) reflectValueQuoted(v reflect.Value, quoted bool) {
 		return
 	}
 
-	if j, ok := v.Interface().(Marshaler); ok && (v.Kind() != reflect.Ptr || !v.IsNil()) {
-		b, err := j.MarshalJSON()
+	m, ok := v.Interface().(Marshaler)
+	if !ok {
+		// T doesn't match the interface. Check against *T too.
+		if v.Kind() != reflect.Ptr && v.CanAddr() {
+			m, ok = v.Addr().Interface().(Marshaler)
+			if ok {
+				v = v.Addr()
+			}
+		}
+	}
+	if ok && (v.Kind() != reflect.Ptr || !v.IsNil()) {
+		b, err := m.MarshalJSON()
 		if err == nil {
 			// copy JSON into buffer, checking validity.
 			err = Compact(&e.Buffer, b)

src/pkg/encoding/json/encode_test.go

@@ -126,3 +126,44 @@ func TestUnsupportedValues(t *testing.T) {
 		}
 	}
 }
+
+// Ref has Marshaler and Unmarshaler methods with pointer receiver.
+type Ref int
+
+func (*Ref) MarshalJSON() ([]byte, error) {
+	return []byte(`"ref"`), nil
+}
+
+func (r *Ref) UnmarshalJSON([]byte) error {
+	*r = 12
+	return nil
+}
+
+// Val has Marshaler methods with value receiver.
+type Val int
+
+func (Val) MarshalJSON() ([]byte, error) {
+	return []byte(`"val"`), nil
+}
+
+func TestRefValMarshal(t *testing.T) {
+	var s = struct {
+		R0 Ref
+		R1 *Ref
+		V0 Val
+		V1 *Val
+	}{
+		R0: 12,
+		R1: new(Ref),
+		V0: 13,
+		V1: new(Val),
+	}
+	const want = `{"R0":"ref","R1":"ref","V0":"val","V1":"val"}`
+	b, err := Marshal(&s)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if got := string(b); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}