Let's make a slight modification to the example at QTBUG-139463:

import QtQuick
import ValueTypeStudy

Window {
    width: 640
    height: 480
    visible: true

    property myStructuredType t: ({
        number: 42,
        truth: true
    })

    Component.onCompleted: {
        console.log("=== Component Complete ===")
        console.log(t.toString())
    }
}

There are no more compiler warnings now. However, other issues stand out.

Outcomes

Constructing: 0 false
Constructing: 0 false
Setting number: 42
Setting truth: true
qml: === Component Complete ===
Constructing: 0 false
qml: myStructuredType(42, true)

• At initialization, the default constructor is called twice.
  • Note: It is only called once if AOT compilation is suppressed, e.g. by adding NO_CACHEGEN or by adding an invalid property (as QTBUG-139463 did)
• At completion, the default constructor is called when referencing an existing instance: console.log(t.toString())