"Invalid nesting" error when trying to use an optional vector in ABI

[michael-drygola]

I've got an EOS smart contract which uses std::optional<std::vector<eosio::name>> as a type of one of its fields. When compiled with AntelopeIO CDT v4.0.1, it produces an ABI which seems to be valid and works fine when deployed to blockchain. However, the abieos lib fails to parse the ABI and throws an "Invalid nesting" error.

Here is a minimum reproducible code to illustrate the problem:

#include <abieos.h>
#include <iostream>

int main( int argc, char** argv ) {
    const char *bad_abi = R"(
    {
      "version": "eosio::abi/1.2",
      "types": [{
          "new_type_name": "B_vector_name_E",
          "type": "name[]"
        }
      ],
      "structs": [{
          "name": "actionname",
          "base": "",
          "fields": [{
              "name": "opt_list",
              "type": "B_vector_name_E?"
            }
          ]
        }
      ],
      "actions": [{
          "name": "actionname",
          "type": "actionname",
          "ricardian_contract": ""
        }
      ],
      "ricardian_clauses": [],
      "error_messages": [],
      "abi_extensions": [],
      "variants": [],
      "action_results": []
    }
    )";

    auto context = abieos_create ();
    if (abieos_set_abi (context, 1, bad_abi)) {
        std::cout << "OK";
    } else {
        std::cout << abieos_get_error (context); // getting "Invalid nesting" here
    }

    return 0;
}

Considering this ABI is valid, would be nice to add support of such nested structures to the lib :)