I expect it should work:
```
:) select CAST(CAST('{"x" : 1}', 'Object(\'json\')'), 'Object(Nullable(\'json\'))')

SELECT CAST(CAST('{"x" : 1}', 'Object(\'json\')'), 'Object(Nullable(\'json\'))')

Query id: fb057b45-b72a-4d22-9b38-a3136cdf2465


0 rows in set. Elapsed: 0.002 sec.

Received exception from server (version 22.5.1):
Code: 53. DB::Exception: Received from localhost:9000. DB::Exception: Cast to Object can be performed only from flatten named Tuple, Map or String. Got: Object('json'): While processing CAST(CAST('{"x" : 1}', 'Object(\'json\')'), 'Object(Nullable(\'json\'))'). (TYPE_MISMATCH)
```

@CurtizJ what do you think?
{
        const char * function_name = cast_name;
        return [function_name] (
            ColumnsWithTypeAndName & arguments, const DataTypePtr & res_type, const ColumnNullable * nullable_col, size_t )
        {
            const auto & first_col = arguments.front().column.get();
            const auto & result_type = typeid_cast<const EnumType &>(*res_type);
            const ColumnStringType * col = typeid_cast<const ColumnStringType *>(first_col);
            if (col && nullable_col && nullable_col->size() != col->size())
                throw Exception("ColumnNullable is not compatible with original", ErrorCodes::LOGICAL_ERROR);
            if (col)
            {
                const auto size = col->size();
                auto res = result_type.createColumn();
                auto & out_data = static_cast<typename EnumType::ColumnType &>(*res).getData();
                out_data.resize(size);
                auto default_enum_value = result_type.getValues().front().second;
                if (nullable_col)
                {
                    for (size_t i = 0; i < size; ++i)
                    {
                        if (!nullable_col->isNullAt(i))
                            out_data[i] = result_type.getValue(col->getDataAt(i));
                        else
                            out_data[i] = default_enum_value;
                    }