QmlDesigner: VariantProperty::value() return const &

- Use const reference to prevent rvalue errors
- Add -> operator to use "->" instead of ".value()."

Change-Id: I3ddf6afa8cd8448829f885a821ae01b86aba3c8f
Reviewed-by: Ali Kianian <ali.kianian@qt.io>

src/libs/qmlpuppetcommunication/types/enumeration.h

@@ -20,7 +20,7 @@ using EnumerationNameView = QByteArrayView;
 class Enumeration
 {
 public:
-    Enumeration() = default;
+    constexpr Enumeration() = default;
     Enumeration(EnumerationName enumerationName)
         : m_enumerationName{std::move(enumerationName)}
     {}

src/plugins/qmldesigner/libs/designercore/include/variantproperty.h

@@ -26,10 +26,14 @@ class QMLDESIGNERCORE_EXPORT VariantProperty final : public AbstractProperty
 
 public:
     void setValue(const QVariant &value);
-    QVariant value() const;
+    const QVariant &value() const;
+
+    const QVariant &operator->() const { return value(); }
+
+    const QVariant &operator*() const { return value(); }
 
     void setEnumeration(const EnumerationName &enumerationName);
-    Enumeration enumeration() const;
+    const Enumeration &enumeration() const;
     bool holdsEnumeration() const;
 
     void setDynamicTypeNameAndValue(const TypeName &type, const QVariant &value);

src/plugins/qmldesigner/libs/designercore/model/internalvariantproperty.cpp

@@ -11,7 +11,7 @@ InternalVariantProperty::InternalVariantProperty(PropertyNameView name, const In
 {
 }
 
-QVariant InternalVariantProperty::value() const
+const QVariant &InternalVariantProperty::value() const
 {
     return m_value;
 }

src/plugins/qmldesigner/libs/designercore/model/internalvariantproperty.h

@@ -18,7 +18,7 @@ public:
 
     bool isValid() const override;
 
-    QVariant value() const;
+    const QVariant &value() const;
     void setValue(const QVariant &value);
 
     void setDynamicValue(const TypeName &type, const QVariant &value);

src/plugins/qmldesigner/libs/designercore/model/variantproperty.cpp

@@ -47,7 +47,7 @@ void VariantProperty::setValue(const QVariant &value)
     privateModel()->setVariantProperty(internalNodeSharedPointer(), name(), value);
 }
 
-QVariant VariantProperty::value() const
+const QVariant &VariantProperty::value() const
 {
     if (isValid()) {
         auto property = internalNode()->variantProperty(name());
@@ -55,7 +55,9 @@ QVariant VariantProperty::value() const
             return property->value();
     }
 
-    return QVariant();
+    static const QVariant nullVariant;
+
+    return nullVariant;
 }
 
 void VariantProperty::setEnumeration(const EnumerationName &enumerationName)
@@ -63,9 +65,14 @@ void VariantProperty::setEnumeration(const EnumerationName &enumerationName)
     setValue(QVariant::fromValue(Enumeration(enumerationName)));
 }
 
-Enumeration VariantProperty::enumeration() const
+const Enumeration &VariantProperty::enumeration() const
 {
-    return value().value<Enumeration>();
+    if (auto enumeration = get_if<Enumeration>(&value()))
+        return *enumeration;
+
+    static constinit const Enumeration nullEnumeration;
+
+    return nullEnumeration;
 }
 
 bool VariantProperty::holdsEnumeration() const