QtQml: Document and uphold precondition of metaTypeFromJS()

The value needs to be a default-constructed instance. Otherwise a number
of branches in this method produce unwanted effects, such as appending
to an already existing array rather than creating a new one.

Amends commit 1b89c1edcae68351632c2755e5408410c2ff98e3.

Pick-to: 6.5
Fixes: QTBUG-125429
Change-Id: If175a02b3a794573abc03df206fbddd41f2855b4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit c16a3e5adb59f6da5e39e51ca15a4d5324d68d1c)
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>

5 files changed, 92 insertions, 6 deletions

diff --git a/src/qml/jsruntime/qv4engine.cpp b/src/qml/jsruntime/qv4engine.cpp
index 3a45823734..dd70e84dcc 100644
--- a/src/qml/jsruntime/qv4engine.cpp
+++ b/src/qml/jsruntime/qv4engine.cpp
@@ -2440,18 +2440,22 @@ bool convertToIterable(QMetaType metaType, void *data, Source *sequence)
         return false;
 
     const QMetaType elementMetaType = iterable.valueMetaType();
-    QVariant element(elementMetaType);
     for (qsizetype i = 0, end = sequence->getLength(); i < end; ++i) {
-        if (!ExecutionEngine::metaTypeFromJS(sequence->get(i), elementMetaType, element.data()))
-            element = QVariant(elementMetaType);
+        QVariant element(elementMetaType);
+        ExecutionEngine::metaTypeFromJS(sequence->get(i), elementMetaType, element.data());
         iterable.addValue(element, QSequentialIterable::AtEnd);
     }
     return true;
 }
 
-// Converts a JS value to a meta-type.
-// data must point to a place that can store a value of the given type.
-// Returns true if conversion succeeded, false otherwise.
+/*!
+ * \internal
+ *
+ * Converts a JS value to a meta-type.
+ * \a data must point to a default-constructed instance of \a metaType.
+ * Returns \c true if conversion succeeded, \c false otherwise. In the latter case,
+ * \a data is not modified.
+ */
 bool ExecutionEngine::metaTypeFromJS(const Value &value, QMetaType metaType, void *data)
 {
     // check if it's one of the types we know
diff --git a/tests/auto/qml/qqmllanguage/data/nestedVectors.qml b/tests/auto/qml/qqmllanguage/data/nestedVectors.qml
new file mode 100644
index 0000000000..0bcea52133
--- /dev/null
+++ b/tests/auto/qml/qqmllanguage/data/nestedVectors.qml
@@ -0,0 +1,27 @@
+import Test
+import QtQml
+
+NestedVectors {
+    id: self
+
+    property var list1
+
+    Component.onCompleted: {
+        list1 = self.getList()
+
+        let list2 = []
+        let data1 = []
+        data1.push(2)
+        data1.push(3)
+        data1.push(4)
+
+        let data2 = []
+        data2.push(5)
+        data2.push(6)
+
+        list2.push(data1)
+        list2.push(data2)
+
+        self.setList(list2)
+    }
+}
diff --git a/tests/auto/qml/qqmllanguage/testtypes.cpp b/tests/auto/qml/qqmllanguage/testtypes.cpp
index ffff0a6979..2fce6567be 100644
--- a/tests/auto/qml/qqmllanguage/testtypes.cpp
+++ b/tests/auto/qml/qqmllanguage/testtypes.cpp
@@ -174,6 +174,7 @@ void registerTypes()
     qmlRegisterTypesAndRevisions<NonSingleton>("EnumScopeTest", 1);
     qmlRegisterTypesAndRevisions<EnumProviderSingletonQml>("EnumScopeTest", 1);
 
+    qmlRegisterTypesAndRevisions<NestedVectors>("Test", 1);
 }
 
 QVariant myCustomVariantTypeConverter(const QString &data)
diff --git a/tests/auto/qml/qqmllanguage/testtypes.h b/tests/auto/qml/qqmllanguage/testtypes.h
index bcf02c1cf9..6d5c9b5b38 100644
--- a/tests/auto/qml/qqmllanguage/testtypes.h
+++ b/tests/auto/qml/qqmllanguage/testtypes.h
@@ -2942,4 +2942,36 @@ public:
     }
 };
 
+class NestedVectors : public QObject
+{
+    Q_OBJECT
+    QML_ELEMENT
+public:
+    NestedVectors(QObject *parent = nullptr) : QObject(parent)
+    {
+        std::vector<int> data;
+        data.push_back(1);
+        data.push_back(2);
+        data.push_back(3);
+        m_list.push_back(data);
+        data.clear();
+        data.push_back(4);
+        data.push_back(5);
+        m_list.push_back(data);
+    }
+
+    Q_INVOKABLE std::vector<std::vector<int>> getList()
+    {
+        return m_list;
+    }
+
+    Q_INVOKABLE void setList(std::vector<std::vector<int>> list)
+    {
+        m_list = list;
+    }
+
+private:
+    std::vector<std::vector<int>> m_list;
+};
+
 #endif // TESTTYPES_H
diff --git a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
index 7231b977ce..1e4de3449c 100644
--- a/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
+++ b/tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp
@@ -454,6 +454,8 @@ private slots:
 
     void typedObjectList();
 
+    void nestedVectors();
+
 private:
     QQmlEngine engine;
     QStringList defaultImportPathList;
@@ -8655,6 +8657,26 @@ void tst_qqmllanguage::typedObjectList()
     QVERIFY(list.at(&list, 0) != nullptr);
 }
 
+void tst_qqmllanguage::nestedVectors()
+{
+    QQmlEngine e;
+    QQmlComponent c(&e, testFileUrl("nestedVectors.qml"));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+    QScopedPointer<QObject> o(c.create());
+    QVERIFY(!o.isNull());
+
+    NestedVectors *n = qobject_cast<NestedVectors *>(o.data());
+    QVERIFY(n);
+
+    const std::vector<std::vector<int>> expected1 { { 1, 2, 3 }, { 4, 5 } };
+    const QVariant list1 = n->property("list1");
+    QCOMPARE(list1.metaType(), QMetaType::fromType<std::vector<std::vector<int>>>());
+    QCOMPARE(list1.value<std::vector<std::vector<int>>>(), expected1);
+
+    const std::vector<std::vector<int>> expected2 { { 2, 3, 4 }, { 5, 6 } };
+    QCOMPARE(n->getList(), expected2);
+}
+
 QTEST_MAIN(tst_qqmllanguage)
 
 #include "tst_qqmllanguage.moc"