fix: decode struct with pointer must be nil (#473)

decode_test.go

@@ -197,6 +197,13 @@ func Test_Decoder(t *testing.T) {
 			assertEq(t, "interface{}", v.F, nil)
 			assertEq(t, "nilfunc", true, v.G == nil)
 		})
+		t.Run("struct.pointer must be nil", func(t *testing.T) {
+			var v struct {
+				A *int
+			}
+			json.Unmarshal([]byte(`{"a": "alpha"}`), &v)
+			assertEq(t, "struct.A", v.A, (*int)(nil))
+		})
 	})
 	t.Run("interface", func(t *testing.T) {
 		t.Run("number", func(t *testing.T) {

internal/decoder/ptr.go

@@ -85,6 +85,7 @@ func (d *ptrDecoder) Decode(ctx *RuntimeContext, cursor, depth int64, p unsafe.P
 	}
 	c, err := d.dec.Decode(ctx, cursor, depth, newptr)
 	if err != nil {
+		*(*unsafe.Pointer)(p) = nil
 		return 0, err
 	}
 	cursor = c