QML: Add a pragma for value type behavior

Unfortunately value types behave differently when compiled to C++.
Document the difference and introduce a pragma to make them behave one
way or the other.

Pick-to: 6.5
Fixes: QTBUG-109221
Change-Id: Ib2685153c0b4ae209bafbea7a01229377fdb47dd
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

4 files changed, 105 insertions, 0 deletions

diff --git a/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt b/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt
index e66af303ca..be74e062c0 100644
--- a/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt
+++ b/tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt
@@ -190,8 +190,10 @@ set(qml_files
     unusedAttached.qml
     urlString.qml
     usingCxxTypesFromFileImports.qml
+    valueTypeCopy.qml
     valueTypeLists.qml
     valueTypeProperty.qml
+    valueTypeReference.qml
     variantlist.qml
     versionmismatch.qml
     voidfunction.qml
diff --git a/tests/auto/qml/qmlcppcodegen/data/valueTypeCopy.qml b/tests/auto/qml/qmlcppcodegen/data/valueTypeCopy.qml
new file mode 100644
index 0000000000..cca634753d
--- /dev/null
+++ b/tests/auto/qml/qmlcppcodegen/data/valueTypeCopy.qml
@@ -0,0 +1,34 @@
+pragma ValueTypeBehavior: Copy
+import QtQml
+
+QtObject {
+    id: root
+
+    property list<double> numbers: {
+        var result = [];
+        for (var i = 0; i < 10; ++i)
+            result[i] = i;
+        return result;
+    }
+
+    property rect r: ({x: 1, y: 2, width: 3, height: 4})
+
+    function evil() : double {
+        var numbers = root.numbers;
+        root.numbers = [];
+        var a = 0;
+        for (var j = 0; j < 10; ++j) {
+            a += numbers[j];
+        }
+        return a;
+    }
+
+    function fvil() : double {
+        var r = root.r;
+        root.r = {x: 5, y: 6, width: 7, height: 8};
+        return r.x;
+    }
+
+    property double e: evil()
+    property double f: fvil()
+}
diff --git a/tests/auto/qml/qmlcppcodegen/data/valueTypeReference.qml b/tests/auto/qml/qmlcppcodegen/data/valueTypeReference.qml
new file mode 100644
index 0000000000..568f39820c
--- /dev/null
+++ b/tests/auto/qml/qmlcppcodegen/data/valueTypeReference.qml
@@ -0,0 +1,34 @@
+pragma ValueTypeBehavior: Reference
+import QtQml
+
+QtObject {
+    id: root
+
+    property list<double> numbers: {
+        var result = [];
+        for (var i = 0; i < 10; ++i)
+            result[i] = i;
+        return result;
+    }
+
+    property rect r: ({x: 1, y: 2, width: 3, height: 4})
+
+    function evil() : double {
+        var numbers = root.numbers;
+        root.numbers = [];
+        var a = 0;
+        for (var j = 0; j < 10; ++j) {
+            a += numbers[j];
+        }
+        return a;
+    }
+
+    function fvil() : double {
+        var r = root.r;
+        root.r = {x: 5, y: 6, width: 7, height: 8};
+        return r.x;
+    }
+
+    property double e: evil()
+    property double f: fvil()
+}
diff --git a/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp b/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp
index 1e88ecd7b7..405e65853a 100644
--- a/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp
+++ b/tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp
@@ -162,6 +162,7 @@ private slots:
     void equalityVarAndNonStorable();
     void equalityQObjects();
     void dateConversions();
+    void valueTypeBehavior();
 };
 
 void tst_QmlCppCodegen::initTestCase()
@@ -3125,6 +3126,40 @@ void tst_QmlCppCodegen::dateConversions()
     QCOMPARE(ref->myTime(), (engine.coerceValue<QDate, QTime>(date)));
 }
 
+static QRegularExpression bindingLoopMessage(const QUrl &url, char var)
+{
+    // The actual string depends on how many times QObject* was registered with what parameters.
+    return QRegularExpression(
+                "%1:4:1: QML [^:]+: Binding loop detected for property \"%2\""_L1
+                .arg(url.toString()).arg(QLatin1Char(var)));
+}
+
+void tst_QmlCppCodegen::valueTypeBehavior()
+{
+    QQmlEngine engine;
+
+    const QUrl copy(u"qrc:/qt/qml/TestTypes/valueTypeCopy.qml"_s);
+
+    QQmlComponent c1(&engine, copy);
+    QVERIFY2(c1.isReady(), qPrintable(c1.errorString()));
+    QTest::ignoreMessage(QtWarningMsg, bindingLoopMessage(copy, 'e'));
+    QTest::ignoreMessage(QtWarningMsg, bindingLoopMessage(copy, 'f'));
+    QScopedPointer<QObject> o1(c1.create());
+    QVERIFY(!o1.isNull());
+    QCOMPARE(o1->property("e").toDouble(), 45.0);
+    QCOMPARE(o1->property("f").toDouble(), 1.0);
+
+    const QUrl reference(u"qrc:/qt/qml/TestTypes/valueTypeReference.qml"_s);
+    QQmlComponent c2(&engine, reference);
+    QVERIFY2(c2.isReady(), qPrintable(c2.errorString()));
+    QTest::ignoreMessage(QtWarningMsg, bindingLoopMessage(reference, 'e'));
+    QTest::ignoreMessage(QtWarningMsg, bindingLoopMessage(reference, 'f'));
+    QScopedPointer<QObject> o2(c2.create());
+    QVERIFY(!o2.isNull());
+    QVERIFY(qIsNaN(o2->property("e").toDouble()));
+    QCOMPARE(o2->property("f").toDouble(), 5.0);
+}
+
 QTEST_MAIN(tst_QmlCppCodegen)
 
 #include "tst_qmlcppcodegen.moc"