fix issue 28 (#29)

Author: Achille

json/codec.go

@@ -156,7 +156,7 @@ func constructCodec(t reflect.Type, seen map[reflect.Type]*structType, canAddr b
 		c = codec{encode: encoder.encodeString, decode: decoder.decodeString}
 
 	case reflect.Interface:
-		c = codec{encode: encoder.encodeInterface, decode: constructMaybeEmptyInterfaceDecoderFunc(t)}
+		c = constructInterfaceCodec(t)
 
 	case reflect.Array:
 		c = constructArrayCodec(t, seen, canAddr)
@@ -711,6 +711,19 @@ func constructPointerDecodeFunc(t reflect.Type, decode decodeFunc) decodeFunc {
 	}
 }
 
+func constructInterfaceCodec(t reflect.Type) codec {
+	return codec{
+		encode: constructMaybeEmptyInterfaceEncoderFunc(t),
+		decode: constructMaybeEmptyInterfaceDecoderFunc(t),
+	}
+}
+
+func constructMaybeEmptyInterfaceEncoderFunc(t reflect.Type) encodeFunc {
+	return func(e encoder, b []byte, p unsafe.Pointer) ([]byte, error) {
+		return e.encodeMaybeEmptyInterface(b, p, t)
+	}
+}
+
 func constructMaybeEmptyInterfaceDecoderFunc(t reflect.Type) decodeFunc {
 	return func(d decoder, b []byte, p unsafe.Pointer) ([]byte, error) {
 		return d.decodeMaybeEmptyInterface(b, p, t)

json/encode.go

@@ -592,6 +592,10 @@ func (e encoder) encodeInterface(b []byte, p unsafe.Pointer) ([]byte, error) {
 	return Append(b, *(*interface{})(p), e.flags)
 }
 
+func (e encoder) encodeMaybeEmptyInterface(b []byte, p unsafe.Pointer, t reflect.Type) ([]byte, error) {
+	return Append(b, reflect.NewAt(t, p).Elem().Interface(), e.flags)
+}
+
 func (e encoder) encodeUnsupportedTypeError(b []byte, p unsafe.Pointer, t reflect.Type) ([]byte, error) {
 	return b, &UnsupportedTypeError{Type: t}
 }

json/json_test.go

@@ -5,6 +5,7 @@ import (
 	"compress/gzip"
 	"encoding"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -1475,3 +1476,16 @@ func TestGithubIssue26(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestGithubIssue28(t *testing.T) {
+	type A struct {
+		Err error `json:"err"`
+	}
+
+	if b, err := Marshal(&A{Err: errors.New("ABC")}); err != nil {
+		t.Error(err)
+	} else if string(b) != `{"err":{}}` {
+		t.Error(string(b))
+	}
+
+}