QmlCompiler: Fix comparison of null and undefined

Those are not stored. If we compare null to null or undefined to
undefined, we do not have to generate a comparison at all. the result is
statically known.

Pick-to: 6.4
Fixes: QTBUG-108634
Change-Id: I6a5323c2e0c023838609aec90d7ecc15b885dc08
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/qmlcompiler/qqmljscodegenerator.cpp

@@ -35,6 +35,15 @@ using namespace Qt::StringLiterals;
         m_body += u"// "_s + QStringLiteral(#function) + u'\n'; \
     }
 
+
+static bool isTypeStorable(const QQmlJSTypeResolver *resolver, const QQmlJSScope::ConstPtr &type)
+{
+    return !type.isNull()
+            && !resolver->equals(type, resolver->nullType())
+            && !resolver->equals(type, resolver->emptyListType())
+            && !resolver->equals(type, resolver->voidType());
+}
+
 QString QQmlJSCodeGenerator::castTargetName(const QQmlJSScope::ConstPtr &type) const
 {
     return type->augmentedInternalName();
@@ -75,14 +84,6 @@ QString QQmlJSCodeGenerator::metaObject(const QQmlJSScope::ConstPtr &objectType)
     return QString();
 }
 
-static bool isTypeStorable(const QQmlJSTypeResolver *resolver, const QQmlJSScope::ConstPtr &type)
-{
-    return !type.isNull()
-            && !resolver->equals(type, resolver->nullType())
-            && !resolver->equals(type, resolver->emptyListType())
-            && !resolver->equals(type, resolver->voidType());
-}
-
 QQmlJSAotFunction QQmlJSCodeGenerator::run(
         const Function *function, const InstructionAnnotations *annotations,
         QQmlJS::DiagnosticMessage *error)
@@ -2459,9 +2460,18 @@ void QQmlJSCodeGenerator::generateEqualityOperation(int lhs, const QString &func
 
     const auto primitive = m_typeResolver->jsPrimitiveType();
     if (m_typeResolver->equals(lhsType, rhsType) && !m_typeResolver->equals(lhsType, primitive)) {
-        m_body += conversion(m_typeResolver->boolType(), m_state.accumulatorOut().storedType(),
-                             registerVariable(lhs) + (invert ? u" != "_s : u" == "_s)
-                             + m_state.accumulatorVariableIn);
+        if (isTypeStorable(m_typeResolver, lhsType)) {
+            m_body += conversion(m_typeResolver->boolType(), m_state.accumulatorOut().storedType(),
+                                 registerVariable(lhs) + (invert ? u" != "_s : u" == "_s)
+                                 + m_state.accumulatorVariableIn);
+        } else if (m_typeResolver->equals(lhsType, m_typeResolver->emptyListType())) {
+            // We cannot compare two empty lists, because we don't know whether it's
+            // the same  instance or not. "[] === []" is false, but "var a = []; a === a" is true;
+            reject(u"comparison of two empty lists"_s);
+        } else {
+            // null === null and  undefined === undefined
+            m_body += invert ? u"false"_s : u"true"_s;
+        }
     } else {
         m_body += conversion(
                     m_typeResolver->boolType(), m_state.accumulatorOut().storedType(),

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

@@ -125,6 +125,7 @@ set(qml_files
     notEqualsInt.qml
     notNotString.qml
     nullAccess.qml
+    nullComparison.qml
     objectInVar.qml
     outOfBounds.qml
     overriddenMember.qml

tests/auto/qml/qmlcppcodegen/data/nullComparison.qml

@@ -0,0 +1,26 @@
+pragma Strict
+import QtQml
+
+QtObject {
+    property int v: 1
+    property int w: 1
+    property int x: 1
+    property int y: 1
+    Component.onCompleted: {
+        var g = null;
+        if (g !== null) {
+            v = 2;
+        }
+        if (g === null) {
+            w = 3;
+        }
+
+        var h = undefined;
+        if (h !== undefined) {
+            x = 4;
+        }
+        if (h === undefined) {
+            y = 5;
+        }
+    }
+}

tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp

@@ -145,6 +145,7 @@ private slots:
     void letAndConst();
     void signalIndexMismatch();
     void callWithSpread();
+    void nullComparison();
 };
 
 void tst_QmlCppCodegen::initTestCase()
@@ -2802,6 +2803,21 @@ void tst_QmlCppCodegen::callWithSpread()
     QTest::ignoreMessage(QtCriticalMsg, "That is great!");
     QScopedPointer<QObject> o(c.create());
     QVERIFY(!o.isNull());
+}
+
+void tst_QmlCppCodegen::nullComparison()
+{
+    QQmlEngine engine;
+
+    QQmlComponent c(&engine, QUrl(u"qrc:/qt/qml/TestTypes/nullComparison.qml"_s));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+    QScopedPointer<QObject> o(c.create());
+    QVERIFY(!o.isNull());
+
+    QCOMPARE(o->property("v").toInt(), 1);
+    QCOMPARE(o->property("w").toInt(), 3);
+    QCOMPARE(o->property("x").toInt(), 1);
+    QCOMPARE(o->property("y").toInt(), 5);
 };
 
 QTEST_MAIN(tst_QmlCppCodegen)