expected ',' or '...' before 'struct' error

Good day,

I have the first time this kind of error.

// another.h
struct Item {
  QString name = QString();
  QString description = QString();
  QVariant value = QVariant();
  struct Interface {
    uint id = 0;
    uint pos = 0;
  } mkio, uks;
  struct Element {
    float weight = 0;
    struct Range {
      uint from = 0;
      uint to = 0;
    } range;
  int index = 0;
} element;
};

I would like to store this nested struct in a stream. So,

// another.h
QDataStream &operator<<(QDataStream &out, const Item::Interface &interface)
{
  return out << interface.id
             << interface.pos;
}
// and another overload operator>> and operator<<...
// Another fields of the `Item` struct are compiling without any error.

1) error: expected ',' or '...' before 'struct' QDataStream &operator<<(QDataStream &out, const Item::Interface &interface)

2) another.h:34:17: error: expected primary-expression before 'struct'
return out << interface.id
^
another.h:34:17: error: expected ';' before 'struct'
another.h:34:17: error: expected primary-expression before 'struct'