Q_GADGET's property setters aren't called if assigned to, if that gadget is returned from a read only Q_PROPERTY.

struct A {
  Q_GADGET
  Q_PROPERTY(QString v READ v WRITE setV FINAL)

  QString m_v;
public:
  QString v() const { return m_v; }
  void setV(QString v) {
    qDebug() << __FUNCTION__ << v;
    m_v = v;
  }
};

class B : public QObject
{
  Q_OBJECT
  QML_ELEMENT
  Q_PROPERTY(A a READ getA CONSTANT FINAL);
  Q_PROPERTY(A a2 READ getA WRITE setA NOTIFY aChanged FINAL);
public:
  Q_INVOKABLE A getA() const { return {}; }
  void setA(const A &a) const { qDebug() << __FUNCTION__ << a.v(); };
signals:
  void aChanged();
};

In the following QML code, setV() is not called if A was retrieved using the b.a property. If the a2 property is used, it is called, but the result is not observed.

Window {
  visible: true
  B { id: b }
  Component.onCompleted: {
    let x = b.a // broken
    //let x = b.a2 // almost works
    //let x = b.getA() // works
    x.v = "foo"
    console.info(x.v) // expected output: "foo"
  }
}

However, if the gadget is returned from a Q_INVOKABLE, it works as expected.

There is an argument to be made that Q_GADGETs are really constant, but as of Qt 6.7.1 this behavior is poorly documented at best

I would like to have at least my setters called, as they can have side effects - I use Q_GADGETs as proxy objects that only contain a handle to another C++ object, so workaround b.a2 is good enough for me now - I assume that in that case the engine creates a copy of A when assigning to v - which is then promptly discarded.