Fix decoding of type of null string

decode_compile.go

@@ -106,7 +106,7 @@ func decodeCompileMapKey(typ *rtype, structName, fieldName string, structTypeToD
 		case *stringDecoder, *interfaceDecoder:
 			return dec, nil
 		case *boolDecoder, *intDecoder, *uintDecoder, *numberDecoder:
-			return newWrappedStringDecoder(dec, structName, fieldName), nil
+			return newWrappedStringDecoder(typ, dec, structName, fieldName), nil
 		case *ptrDecoder:
 			dec = t.dec
 		default:
@@ -393,7 +393,7 @@ func decodeCompileStruct(typ *rtype, structName, fieldName string, structTypeToD
 			}
 		} else {
 			if tag.isString {
-				dec = newWrappedStringDecoder(dec, structName, field.Name)
+				dec = newWrappedStringDecoder(type2rtype(field.Type), dec, structName, field.Name)
 			}
 			var key string
 			if tag.key != "" {

decode_string.go

@@ -233,7 +233,7 @@ func (d *stringDecoder) decodeStreamByte(s *stream) ([]byte, error) {
 			if err := nullBytes(s); err != nil {
 				return nil, err
 			}
-			return []byte{}, nil
+			return nil, nil
 		case nul:
 			if s.read() {
 				continue
@@ -322,7 +322,7 @@ func (d *stringDecoder) decodeByte(buf []byte, cursor int64) ([]byte, int64, err
 				return nil, 0, errInvalidCharacter(buf[cursor+3], "null", cursor)
 			}
 			cursor += 4
-			return []byte{}, cursor, nil
+			return nil, cursor, nil
 		default:
 			goto ERROR
 		}

decode_test.go

@@ -2052,7 +2052,6 @@ func TestEmptyString(t *testing.T) {
 	}
 }
 
-/*
 // Test that a null for ,string is not replaced with the previous quoted string (issue 7046).
 // It should also not be an error (issue 2540, issue 8587).
 func TestNullString(t *testing.T) {
@@ -2074,7 +2073,6 @@ func TestNullString(t *testing.T) {
 		t.Fatalf("after Unmarshal, s.B=%d, s.C=%p, want 1, nil", s.B, s.C)
 	}
 }
-*/
 
 func intp(x int) *int {
 	p := new(int)

decode_wrapped_string.go

@@ -1,22 +1,27 @@
 package json
 
 import (
+	"reflect"
 	"unsafe"
 )
 
 type wrappedStringDecoder struct {
+	typ           *rtype
 	dec           decoder
 	stringDecoder *stringDecoder
 	structName    string
 	fieldName     string
+	isPtrType     bool
 }
 
-func newWrappedStringDecoder(dec decoder, structName, fieldName string) *wrappedStringDecoder {
+func newWrappedStringDecoder(typ *rtype, dec decoder, structName, fieldName string) *wrappedStringDecoder {
 	return &wrappedStringDecoder{
+		typ:           typ,
 		dec:           dec,
 		stringDecoder: newStringDecoder(structName, fieldName),
 		structName:    structName,
 		fieldName:     fieldName,
+		isPtrType:     typ.Kind() == reflect.Ptr,
 	}
 }
 
@@ -25,6 +30,12 @@ func (d *wrappedStringDecoder) decodeStream(s *stream, p unsafe.Pointer) error {
 	if err != nil {
 		return err
 	}
+	if bytes == nil {
+		if d.isPtrType {
+			*(*unsafe.Pointer)(p) = nil
+		}
+		return nil
+	}
 	b := make([]byte, len(bytes)+1)
 	copy(b, bytes)
 	if _, err := d.dec.decode(b, 0, p); err != nil {
@@ -38,6 +49,12 @@ func (d *wrappedStringDecoder) decode(buf []byte, cursor int64, p unsafe.Pointer
 	if err != nil {
 		return 0, err
 	}
+	if bytes == nil {
+		if d.isPtrType {
+			*(*unsafe.Pointer)(p) = nil
+		}
+		return c, nil
+	}
 	bytes = append(bytes, nul)
 	if _, err := d.dec.decode(bytes, 0, p); err != nil {
 		return 0, err