Fix slice pointer

Author: lujjjh

src/encoding/json/encode.go

@@ -871,7 +871,9 @@ func (se sliceEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
 	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
 		// We're a large number of nested ptrEncoder.encode calls deep;
 		// start checking if we've run into a pointer cycle.
-		ptr := v.Pointer()
+		p := reflect.New(reflect.TypeOf(v))
+		p.Elem().Set(reflect.ValueOf(v))
+		ptr := p.Pointer()
 		if _, ok := e.ptrSeen[ptr]; ok {
 			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
 		}

src/encoding/json/encode_test.go

@@ -187,6 +187,7 @@ var (
 	pointerCycleIndirect = &PointerCycleIndirect{}
 	mapCycle             = make(map[string]interface{})
 	sliceCycle           = []interface{}{nil}
+	sliceNoCycle         = []interface{}{nil, nil}
 )
 
 func init() {
@@ -199,6 +200,10 @@ func init() {
 
 	mapCycle["x"] = mapCycle
 	sliceCycle[0] = sliceCycle
+	sliceNoCycle[1] = sliceNoCycle[:1]
+	for i := startDetectingCyclesAfter; i > 0; i-- {
+		sliceNoCycle = []interface{}{sliceNoCycle}
+	}
 }
 
 func TestSamePointerNoCycle(t *testing.T) {
@@ -207,6 +212,12 @@ func TestSamePointerNoCycle(t *testing.T) {
 	}
 }
 
+func TestSliceNoCycle(t *testing.T) {
+	if _, err := Marshal(sliceNoCycle); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 var unsupportedValues = []interface{}{
 	math.NaN(),
 	math.Inf(-1),