[struct_json]fix for deserialize optional string (#845)

include/ylt/standalone/iguana/json_reader.hpp

@@ -450,8 +450,6 @@ IGUANA_INLINE void from_json_impl(U &value, It &&it, It &&end) {
 template <typename U, typename It, std::enable_if_t<optional_v<U>, int> = 0>
 IGUANA_INLINE void from_json_impl(U &value, It &&it, It &&end) {
   skip_ws(it, end);
-  if (it < end && *it == '"')
-    IGUANA_LIKELY { ++it; }
   using T = std::remove_reference_t<U>;
   if (it == end)
     IGUANA_UNLIKELY { throw std::runtime_error("Unexexpected eof"); }
@@ -469,6 +467,8 @@ IGUANA_INLINE void from_json_impl(U &value, It &&it, It &&end) {
     using value_type = typename T::value_type;
     value_type t;
     if constexpr (string_v<value_type> || string_view_v<value_type>) {
+      if (it < end && *it == '"')
+        IGUANA_LIKELY { ++it; }
       from_json_impl<true>(t, it, end);
     }
     else {

src/struct_json/examples/main.cpp

@@ -92,6 +92,22 @@ void test_escape_serialize() {
 #endif
 }
 
+struct test_optstr_reader_null {
+  std::optional<std::string> name;
+};
+YLT_REFL(test_optstr_reader_null, name);
+
+void test_optional() {
+  test_optstr_reader_null v;
+  v.name = "name";  // optional<string> begin with 'n'
+  std::string json;
+  iguana::to_json(v, json);
+
+  test_optstr_reader_null v1;
+  iguana::from_json(v1, json);
+  assert(v.name == v1.name);
+}
+
 int main() {
   person p{"tom", 20};
   std::string str;
@@ -115,4 +131,5 @@ int main() {
   use_smart_pointer();
   test_escape_serialize();
   test_user_defined_struct();
+  test_optional();
 }