checker: unwrap for base type if ft.typ and inferred_typ are both opt (fix #25517) (#25728)

Author: dy-tea

vlib/v/checker/check_types.v

@@ -845,7 +845,12 @@ fn (mut c Checker) infer_struct_generic_types(typ ast.Type, node ast.StructInit)
 				if field_sym.name == gt_name {
 					for t in node.init_fields {
 						if ft.name == t.name && t.typ != 0 {
-							concrete_types << ast.mktyp(t.typ)
+							// unwrap to get base type for T if both are options
+							mut inferred_typ := ast.mktyp(t.typ)
+							if ft.typ.has_flag(.option) && inferred_typ.has_flag(.option) {
+								inferred_typ = inferred_typ.clear_flag(.option)
+							}
+							concrete_types << inferred_typ
 							continue gname
 						}
 					}

vlib/x/json2/tests/json_optional_both_type_field_test.v

@@ -0,0 +1,23 @@
+struct Response[T] {
+	code int
+	msg  string
+	data ?T
+}
+
+struct UserInfo {
+	name string
+	age  int
+}
+
+fn g[T](x T) string {
+	return 'abc'
+}
+
+fn response_to_string[T](response Response[T]) string {
+	return g(response)
+}
+
+fn test_main() {
+	res := response_to_string(Response{1, 'success', ?UserInfo{'Jay Chou', 46}})
+	assert res == 'abc'
+}