Restore compatibility with custom QVariantList/QVariantMap converters

QVariant::toValue<QVariantList> does not honor explicit custom converters
for a sequentially iterable container to a QVariantList. This breaks
compatibility with Qt 5.12.3 and before, where the old code path using
QVariant::toList() used the custom converter. Do the same for
QVariantMap.

This patch restores the compatibility with old code. The real fix will
target the QVariantList converter code in QtCore - this is just a hotfix
within Qt WebChannel for now.

Task-number: QTBUG-80751
Change-Id: Ic70c6a353aad43ddbaefbc6626a0af87bd0d024f
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

3 files changed, 49 insertions, 2 deletions

diff --git a/src/webchannel/qmetaobjectpublisher.cpp b/src/webchannel/qmetaobjectpublisher.cpp
index e23921d..ea7ddc9 100644
--- a/src/webchannel/qmetaobjectpublisher.cpp
+++ b/src/webchannel/qmetaobjectpublisher.cpp
@@ -788,10 +788,19 @@ QJsonValue QMetaObjectPublisher::wrapResult(const QVariant &result, QWebChannelA
         // *don't* use result.toList() as that *only* works for QVariantList and QStringList!
         // Also, don't use QSequentialIterable (yet), since that seems to trigger QTBUG-42016
         // in certain cases.
-        return wrapList(result.value<QVariantList>(), transport);
+        // additionally, when there's a direct converter to QVariantList, use that one via convert
+        // but recover when conversion fails and fall back to the .value<QVariantList> conversion
+        // see also: https://bugreports.qt.io/browse/QTBUG-80751
+        auto list = result;
+        if (!list.convert(qMetaTypeId<QVariantList>()))
+            list = result;
+        return wrapList(list.value<QVariantList>(), transport);
     } else if (result.canConvert<QVariantMap>()) {
         // recurse and potentially wrap contents of the map
-        return wrapMap(result.toMap(), transport);
+        auto map = result;
+        if (!map.convert(qMetaTypeId<QVariantMap>()))
+            map = result;
+        return wrapMap(map.value<QVariantMap>(), transport);
     }
 
     return QJsonValue::fromVariant(result);
diff --git a/tests/auto/webchannel/tst_webchannel.cpp b/tests/auto/webchannel/tst_webchannel.cpp
index c47cd6b..f9719cf 100644
--- a/tests/auto/webchannel/tst_webchannel.cpp
+++ b/tests/auto/webchannel/tst_webchannel.cpp
@@ -183,6 +183,20 @@ void TestJSEngine::initWebChannelJS()
 
 #endif // WEBCHANNEL_TESTS_CAN_USE_JS_ENGINE
 
+namespace {
+QVariantList convert_to_js(const TestStructVector &list)
+{
+    QVariantList ret;
+    ret.reserve(list.size());
+    std::transform(list.begin(), list.end(), std::back_inserter(ret), [](const TestStruct &value) -> QVariant {
+        QVariantMap map;
+        map["foo"] = value.foo;
+        map["bar"] = value.bar;
+        return map;
+    });
+    return ret;
+}
+}
 
 TestWebChannel::TestWebChannel(QObject *parent)
     : QObject(parent)
@@ -191,6 +205,9 @@ TestWebChannel::TestWebChannel(QObject *parent)
     , m_lastBool(false)
     , m_lastDouble(0)
 {
+    qRegisterMetaType<TestStruct>();
+    qRegisterMetaType<TestStructVector>();
+    QMetaType::registerConverter<TestStructVector, QVariantList>(convert_to_js);
 }
 
 TestWebChannel::~TestWebChannel()
@@ -864,6 +881,14 @@ void TestWebChannel::testWrapValues()
         QVERIFY(value.isArray());
         QCOMPARE(value.toArray(), QJsonArray({1, 2, 3}));
     }
+    {
+        TestStructVector vec{{1, 2}, {3, 4}};
+        QVariant variant = QVariant::fromValue(vec);
+        QJsonValue value = channel.d_func()->publisher->wrapResult(variant, m_dummyTransport);
+        QVERIFY(value.isArray());
+        QCOMPARE(value.toArray(), QJsonArray({QJsonObject{{"foo", 1}, {"bar", 2}},
+                                             QJsonObject{{"foo", 3}, {"bar", 4}}}));
+    }
 }
 
 void TestWebChannel::testWrapObjectWithMultipleTransports()
diff --git a/tests/auto/webchannel/tst_webchannel.h b/tests/auto/webchannel/tst_webchannel.h
index 7cfce06..eae21f4 100644
--- a/tests/auto/webchannel/tst_webchannel.h
+++ b/tests/auto/webchannel/tst_webchannel.h
@@ -39,6 +39,19 @@
 
 #include <QtWebChannel/QWebChannelAbstractTransport>
 
+struct TestStruct
+{
+    TestStruct(int foo = 0, int bar = 0)
+        : foo(foo)
+        , bar(bar)
+    {}
+    int foo;
+    int bar;
+};
+Q_DECLARE_METATYPE(TestStruct)
+using TestStructVector = std::vector<TestStruct>;
+Q_DECLARE_METATYPE(TestStructVector)
+
 QT_BEGIN_NAMESPACE
 
 class DummyTransport : public QWebChannelAbstractTransport