Binding: Reevaluate when before the target changes

...and do not warn about missing properties if when is disabled. Besides
avoiding spurious warnings, this also avoids modifying a property only
to restore its binding/value directly afterwards.

Note that when the binding gets re-enabled, we still trigger the
warning.

Fixes: QTBUG-112860
Pick-to: 6.5
Change-Id: I5ddd32f2de2dec9da372b08ab4bb5bdb88873e51
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/qml/types/qqmlbind.cpp

@@ -463,10 +463,26 @@ void QQmlBind::setObject(QObject *obj)
         eval();
         d->when = true;
     }
+    /* if "when" and "target" depend on the same property, we might
+       end up here before we could have updated "when". So reevaluate
+       when manually here.
+     */
+    const QQmlProperty whenProp(this, QLatin1StringView("when"));
+    const auto potentialWhenBinding = QQmlAnyBinding::ofProperty(whenProp);
+    if (auto abstractBinding = potentialWhenBinding.asAbstractBinding()) {
+        QQmlBinding *binding = static_cast<QQmlBinding *>(abstractBinding);
+        if (binding->hasValidContext()) {
+            const auto boolType = QMetaType::fromType<bool>();
+            bool when;
+            binding->evaluate(&when, boolType);
+            d->when = when;
+        }
+    }
     d->obj = obj;
     if (d->componentComplete) {
         setTarget(QQmlProperty(d->obj, d->propName, qmlContext(this)));
-        d->validate(this);
+        if (d->when)
+            d->validate(this);
     }
     eval();
 }
@@ -520,7 +536,8 @@ void QQmlBind::setProperty(const QString &p)
     d->propName = p;
     if (d->componentComplete) {
         setTarget(QQmlProperty(d->obj, d->propName, qmlContext(this)));
-        d->validate(this);
+        if (d->when)
+            d->validate(this);
     }
     eval();
 }

tests/auto/qml/qqmlbinding/data/whenEvaluatedEarlyEnough.qml

@@ -0,0 +1,23 @@
+import QtQuick
+
+Item {
+    id: root
+    property bool toggle: true
+    property bool forceEnable: false
+
+    Item {
+        id: item1
+        property int i
+    }
+
+    Item {
+        id: item2
+    }
+
+    Binding {
+        target: root.toggle ? item1 : item2 
+        when:  root.forceEnable || (root.toggle ? item1 : item2).hasOwnProperty("i")
+        property: "i"
+        value: 42
+    }
+}

tests/auto/qml/qqmlbinding/tst_qqmlbinding.cpp

@@ -38,6 +38,7 @@ private slots:
     void intOverflow();
     void generalizedGroupedProperties();
     void localSignalHandler();
+    void whenEvaluatedEarlyEnough();
 
 private:
     QQmlEngine engine;
@@ -596,6 +597,19 @@ void tst_qqmlbinding::localSignalHandler()
     QCOMPARE(o->property("output").toString(), QStringLiteral("abc"));
 }
 
+void tst_qqmlbinding::whenEvaluatedEarlyEnough()
+{
+    QQmlEngine e;
+    QQmlComponent c(&e, testFileUrl("whenEvaluatedEarlyEnough.qml"));
+    QTest::failOnWarning(QRegularExpression(".*"));
+    std::unique_ptr<QObject> root { c.create() };
+    root->setProperty("toggle", false); // should not cause warnings
+    // until "when" is actually true
+    QTest::ignoreMessage(QtMsgType::QtWarningMsg,
+                         QRegularExpression(".*QML Binding: Property 'i' does not exist on Item.*"));
+    root->setProperty("forceEnable", true);
+}
+
 QTEST_MAIN(tst_qqmlbinding)
 
 #include "tst_qqmlbinding.moc"