Fix decoding of slice element

decode_slice.go

@@ -9,7 +9,6 @@ import (
 type sliceDecoder struct {
 	elemType          *rtype
 	isElemPointerType bool
-	isElemUnmarshalJSONType bool
 	valueDecoder      decoder
 	size              uintptr
 	arrayPool         sync.Pool
@@ -35,7 +34,6 @@ func newSliceDecoder(dec decoder, elemType *rtype, size uintptr, structName, fie
 		valueDecoder:      dec,
 		elemType:          elemType,
 		isElemPointerType: elemType.Kind() == reflect.Ptr || elemType.Kind() == reflect.Map,
-		isElemUnmarshalJSONType: rtype_ptrTo(elemType).Implements(unmarshalJSONType),
 		size:              size,
 		arrayPool: sync.Pool{
 			New: func() interface{} {
@@ -123,11 +121,11 @@ func (d *sliceDecoder) decodeStream(s *stream, depth int64, p unsafe.Pointer) er
 					copySlice(d.elemType, dst, src)
 				}
 				ep := unsafe.Pointer(uintptr(data) + uintptr(idx)*d.size)
-				if d.isElemUnmarshalJSONType {
+				if d.isElemPointerType {
+					**(**unsafe.Pointer)(unsafe.Pointer(&ep)) = nil // initialize elem pointer
+				} else {
 					// assign new element to the slice
 					typedmemmove(d.elemType, ep, unsafe_New(d.elemType))
-				} else if d.isElemPointerType {
-					**(**unsafe.Pointer)(unsafe.Pointer(&ep)) = nil // initialize elem pointer
 				}
 				if err := d.valueDecoder.decodeStream(s, depth, ep); err != nil {
 					return err
@@ -236,11 +234,11 @@ func (d *sliceDecoder) decode(buf []byte, cursor, depth int64, p unsafe.Pointer)
 					copySlice(d.elemType, dst, src)
 				}
 				ep := unsafe.Pointer(uintptr(data) + uintptr(idx)*d.size)
-				if d.isElemUnmarshalJSONType {
+				if d.isElemPointerType {
+					**(**unsafe.Pointer)(unsafe.Pointer(&ep)) = nil // initialize elem pointer
+				} else {
 					// assign new element to the slice
 					typedmemmove(d.elemType, ep, unsafe_New(d.elemType))
-				} else if d.isElemPointerType {
-					**(**unsafe.Pointer)(unsafe.Pointer(&ep)) = nil // initialize elem pointer
 				}
 				c, err := d.valueDecoder.decode(buf, cursor, depth, ep)
 				if err != nil {

decode_test.go

@@ -3508,3 +3508,64 @@ func TestDecodeBackSlash(t *testing.T) {
 		})
 	})
 }
+
+func TestIssue218(t *testing.T) {
+	type A struct {
+		X int
+	}
+	type B struct {
+		Y int
+	}
+	type S struct {
+		A *A `json:"a,omitempty"`
+		B *B `json:"b,omitempty"`
+	}
+	tests := []struct {
+		name     string
+		given    []S
+		expected []S
+	}{
+		{
+			name: "A should be correct",
+			given: []S{{
+				A: &A{
+					X: 1,
+				},
+			}},
+			expected: []S{{
+				A: &A{
+					X: 1,
+				},
+			}},
+		},
+		{
+			name: "B should be correct",
+			given: []S{{
+				B: &B{
+					Y: 2,
+				},
+			}},
+			expected: []S{{
+				B: &B{
+					Y: 2,
+				},
+			}},
+		},
+	}
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			if err := json.NewEncoder(&buf).Encode(test.given); err != nil {
+				t.Fatal(err)
+			}
+			var actual []S
+			if err := json.NewDecoder(bytes.NewReader(buf.Bytes())).Decode(&actual); err != nil {
+				t.Fatal(err)
+			}
+			if !reflect.DeepEqual(test.expected, actual) {
+				t.Fatalf("mismatch value: expected %v but got %v", test.expected, actual)
+			}
+		})
+	}
+}