fix(option): fix Option. UnmarshalJSON

Change-Id: I991fb08dfa29fa2cd841c58ffc509a6e20dce981

option.go

@@ -3,6 +3,7 @@ package gust
 import (
 	"encoding/json"
 	"fmt"
+	"unsafe"
 )
 
 // BoolOpt wraps a value as an Option.
@@ -404,14 +405,20 @@ func (o *Option[T]) Replace(some T) (old Option[T]) {
 	return old
 }
 
+const null = "null"
+
 func (o Option[T]) MarshalJSON() ([]byte, error) {
 	if o.IsNone() {
-		return []byte("null"), nil
+		return []byte(null), nil
 	}
 	return json.Marshal(o.value)
 }
 
 func (o *Option[T]) UnmarshalJSON(b []byte) error {
+	o.value = nil
+	if *(*string)(unsafe.Pointer(&b)) == null {
+		return nil
+	}
 	var value = new(T)
 	err := json.Unmarshal(b, value)
 	if err == nil {

option_test.go

@@ -32,3 +32,35 @@ func TestOptionJSON(t *testing.T) {
 	assert.True(t, r4.IsNone())
 	assert.Equal(t, "json: cannot unmarshal number into Go value of type gust_test.T", err4.Error())
 }
+
+func TestOptionJSON2(t *testing.T) {
+	type T struct {
+		Name gust.Option[string]
+	}
+	var r = T{Name: gust.Some("andeya")}
+	var b, err = json.Marshal(r)
+	assert.NoError(t, err)
+	assert.Equal(t, `{"Name":"andeya"}`, string(b))
+	var r2 T
+	err2 := json.Unmarshal(b, &r2)
+	assert.NoError(t, err2)
+	assert.Equal(t, r, r2)
+
+	var r3 = T{Name: gust.Some("")}
+	var b3, err3 = json.Marshal(r3)
+	assert.NoError(t, err3)
+	assert.Equal(t, `{"Name":""}`, string(b3))
+	var r4 T
+	err4 := json.Unmarshal(b3, &r4)
+	assert.NoError(t, err4)
+	assert.Equal(t, r3, r4)
+
+	var r5 = T{Name: gust.None[string]()}
+	var b5, err5 = json.Marshal(r5)
+	assert.NoError(t, err5)
+	assert.Equal(t, `{"Name":null}`, string(b5))
+	var r6 T
+	err6 := json.Unmarshal(b5, &r6)
+	assert.NoError(t, err6)
+	assert.Equal(t, r5, r6)
+}