QmlCompiler: Fix conversion from/to void

We can convert everything to void, but we can only convert from void if
the result is either void or the invalid type.

Pick-to: 6.6
Fixes: QTBUG-116088
Change-Id: I532055405865c5b1581f79cc5d76c253bce6138d
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/qmlcompiler/qqmljscodegenerator.cpp

@@ -3338,10 +3338,14 @@ QString QQmlJSCodeGenerator::convertStored(
             return u"std::numeric_limits<double>::quiet_NaN()"_s;
         if (m_typeResolver->equals(to, m_typeResolver->stringType()))
             return QQmlJSUtils::toLiteral(u"undefined"_s);
+        if (m_typeResolver->equals(to, m_typeResolver->varType()))
+            return u"QVariant()"_s;
+        if (m_typeResolver->equals(to, m_typeResolver->jsValueType()))
+            return u"QJSValue();"_s;
+        if (m_typeResolver->equals(to, m_typeResolver->jsPrimitiveType()))
+            return u"QJSPrimitiveValue()"_s;
         if (m_typeResolver->equals(from, to))
             return QString();
-        // Anything else is just the default constructed type.
-        return to->augmentedInternalName() + u"()"_s;
     }
 
     if (m_typeResolver->equals(from, m_typeResolver->nullType())) {

src/qmlcompiler/qqmljstypepropagator.cpp

@@ -83,9 +83,10 @@ void QQmlJSTypePropagator::generate_Ret()
 
     if (m_function->isSignalHandler) {
         // Signal handlers cannot return anything.
-    } else if (!m_returnType.isValid() && m_state.accumulatorIn().isValid()
-               && !m_typeResolver->registerContains(
-                   m_state.accumulatorIn(), m_typeResolver->voidType())) {
+    } else if (m_typeResolver->registerContains(
+                       m_state.accumulatorIn(), m_typeResolver->voidType())) {
+        // You can always return undefined.
+    } else if (!m_returnType.isValid() && m_state.accumulatorIn().isValid()) {
         setError(u"function without return type annotation returns %1"_s
                          .arg(m_state.accumulatorIn().descriptiveName()));
 

src/qmlcompiler/qqmljstyperesolver.cpp

@@ -1192,11 +1192,11 @@ bool QQmlJSTypeResolver::canPrimitivelyConvertFromTo(
         return true;
     }
 
-    if (equals(from, m_voidType) || equals(to, m_voidType))
+    if (equals(to, m_voidType))
         return true;
 
     if (to.isNull())
-        return false;
+        return equals(from, m_voidType);
 
     const auto types = { m_dateTimeType, m_dateType, m_timeType, m_stringType };
     for (const auto &originType : types) {

tests/auto/qml/qmlcppcodegen/data/CMakeLists.txt

@@ -245,6 +245,7 @@ set(qml_files
     variantReturn.qml
     variantlist.qml
     versionmismatch.qml
+    voidConversion.qml
     voidfunction.qml
     dummy_imports.qml
 )

tests/auto/qml/qmlcppcodegen/data/voidConversion.qml

@@ -0,0 +1,10 @@
+import QtQml
+
+QtObject {
+    id: item
+    property point p: Qt.point(20, 10)
+
+    Component.onCompleted: {
+        item.p = undefined
+    }
+}

tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp

@@ -206,6 +206,7 @@ private slots:
     void variantMapLookup();
     void variantReturn();
     void variantlist();
+    void voidConversion();
     void voidFunction();
 };
 
@@ -4189,6 +4190,23 @@ void tst_QmlCppCodegen::variantlist()
     QCOMPARE(things[1].toInt(), 30);
 }
 
+void tst_QmlCppCodegen::voidConversion()
+{
+    QQmlEngine engine;
+    const QUrl url(u"qrc:/qt/qml/TestTypes/voidConversion.qml"_s);
+    QQmlComponent c(&engine, url);
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+
+    QTest::ignoreMessage(
+            QtWarningMsg,
+            qPrintable(url.toString() + u":8: Error: Cannot assign [undefined] to QPointF"_s));
+
+    QScopedPointer<QObject> o(c.create());
+    QVERIFY(o);
+
+    QCOMPARE(o->property("p"), QPointF(20, 10));
+}
+
 void tst_QmlCppCodegen::voidFunction()
 {
     QQmlEngine engine;

tests/auto/qml/qmllint/data/pluginQuick_anchorsUndefined.qml

@@ -5,5 +5,6 @@ Item {
         anchors.horizontalCenter: undefined
         anchors.verticalCenter: undefined
         anchors.baseline: undefined
+        Component.onCompleted: anchors.bottom = undefined
     }
 }