QmlCompiler: Do not generate lookups for bare meta enums

Such code is necessarily dead or uncompilable.

Pick-to: 6.5 6.2
Fixes: QTBUG-119090
Change-Id: I7319f7ceeb0b4994d5e974bbe8a9c3ba3bf72fc5
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 73f66c8f94)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/qmlcompiler/qqmljscodegenerator.cpp

@@ -782,9 +782,15 @@ void QQmlJSCodeGenerator::generateEnumLookup(int index)
 {
     const QString enumMember = m_state.accumulatorOut().enumMember();
 
-    // If we're referring to the type, there's nothing to do.
-    if (enumMember.isEmpty())
+    if (enumMember.isEmpty()) {
+        // If we're referring to the type, there's nothing to do.
+        // However, we should not get here since no one can ever use the enum metatype.
+        // The lookup is dead code and should be optimized away.
+        // ... unless you are actually trying to store the metatype itself in a property.
+        //     We cannot compile such code.
+        reject(u"Lookup of enum metatype"_s);
         return;
+    }
 
     // If the metaenum has the value, just use it and skip all the rest.
     const QQmlJSMetaEnum metaEnum = m_state.accumulatorOut().enumeration();

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

@@ -55,6 +55,7 @@ set(qml_files
     SelectionRectangle.qml
     ShadowedObjectName.qml
     ShadowedObjectNameDerived.qml
+    StoreMetaEnum.qml
     Test.qml
     TestCase.qml
     WindowDerived.qml

tests/auto/qml/qmlcppcodegen/data/StoreMetaEnum.qml

@@ -0,0 +1,12 @@
+import QtQml
+
+QtObject {
+    enum Foo {
+        Bar,
+        Baz
+    }
+
+    property var eF: StoreMetaEnum.Foo
+    property int bar: eF.Bar
+    property int baz: eF.Baz
+}

tests/auto/qml/qmlcppcodegen/tst_qmlcppcodegen.cpp

@@ -176,6 +176,7 @@ private slots:
     void signatureIgnored();
     void simpleBinding();
     void storeElementSideEffects();
+    void storeMetaEnum();
     void stringArg();
     void stringLength();
     void stringToByteArray();
@@ -3657,6 +3658,20 @@ void tst_QmlCppCodegen::storeElementSideEffects()
     QCOMPARE(prop.property(0).toInt(), 10);
 }
 
+void tst_QmlCppCodegen::storeMetaEnum()
+{
+    QQmlEngine engine;
+
+    QQmlComponent c(&engine, QUrl(u"qrc:/qt/qml/TestTypes/StoreMetaEnum.qml"_s));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+
+    QScopedPointer<QObject> o(c.create());
+    QVERIFY(o);
+
+    QCOMPARE(o->property("bar").toInt(), 0);
+    QCOMPARE(o->property("baz").toInt(), 1);
+}
+
 void tst_QmlCppCodegen::stringArg()
 {
     QQmlEngine engine;