x.json2.decoder2: fix option decode (fix #24861) (#24881)

vlib/x/json2/decoder2/decode.v

@@ -797,7 +797,9 @@ fn (mut decoder Decoder) decode_value[T](mut val T) ! {
 											}
 										} else {
 											$if field.typ is $option {
-												mut unwrapped_val := create_value_from_optional(val.$(field.name))
+												mut unwrapped_val := create_value_from_optional(val.$(field.name)) or {
+													return
+												}
 												decoder.decode_value(mut unwrapped_val)!
 												val.$(field.name) = unwrapped_val
 											} $else {
@@ -961,7 +963,7 @@ fn get_value_kind(value u8) ValueKind {
 	return .unknown
 }
 
-fn create_value_from_optional[T](val ?T) T {
+fn create_value_from_optional[T](val ?T) ?T {
 	return T{}
 }
 

vlib/x/json2/tests/decode_option_test.v

@@ -0,0 +1,21 @@
+import x.json2.decoder2
+
+struct F1 {
+	f ?struct {
+		a int
+	}
+}
+
+fn test_main() {
+	j1 := decoder2.decode[F1]('{"f":{"a":1}}')!
+	assert '${j1}' == 'F1{
+    f: Option(struct {
+        a: 1
+    })
+}'
+
+	j2 := decoder2.decode[F1]('{}')!
+	assert '${j2}' == 'F1{
+    f: Option(none)
+}'
+}