QQmlInfo: print ancestor of object if it has no QML engine

This results in the following message for objects without a QML engine:

QML AttachedObject (parent or ancestor of Attached): Binding loop
detected for property "a"

for this QML file, named AttachedObject.qml:

import QtQuick 2.0
import org.qtproject.Test 1.0

Item {
    Attached.a: Attached.a
}

This, in turn, allows the warning to be emitted via the
QQmlEngine::warnings signal, since QQmlEnginePrivate::warning() is now
passed a valid engine. This solves the awkward situation where a
binding loop warning can not be detected at all by auto tests involving
attached C++ objects (as message handlers do not receive these messages
either).

Change-Id: I07589974207bd5448d22a6086a52b9230d23e298
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 31c892118ce822ca2e7ded99ff261187ce4cf597)

1 files changed, 52 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmlinfo/attached.cpp b/tests/auto/qml/qqmlinfo/attached.cpp
new file mode 100644
index 0000000000..f4b866a547
--- /dev/null
+++ b/tests/auto/qml/qqmlinfo/attached.cpp
@@ -0,0 +1,52 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "attached.h"
+
+Attached::Attached(QObject *parent) :
+    QObject(parent)
+{
+}
+
+int Attached::a() const
+{
+    return mA;
+}
+
+void Attached::setA(int a)
+{
+    // Intentionally omit the early return in order to force a binding loop.
+
+    mA = a;
+    emit aChanged();
+}
+
+Attached *Attached::qmlAttachedProperties(QObject *object)
+{
+    return new Attached(object);
+}