QMetaType: Store QMetaObject for pointer to const QObject, too

Before this change, the QMetaType for T const* where T is derived from
QObject would not store the static QMetaObject of T. This commit changes
this. As a consequence, the metatype system can now convert between
const and non-const pointers to QObject. Note that this allows casting
const away; but so does C++ with const_cast.
In addition, a new flag, QMetaType::IsImmutable is introduced, and used
to tag the metatypes of pointer to const types. This allows code to
discern between pointers to mutable and const QObjects, which is
relevant for the QML engine.

Task-number: QTBUG-82354
Change-Id: I3e4e4f39f565bd99a65e161528ce5304df73d6d6
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 14 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype2.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype2.cpp
index 3c097ad0d8..856b56941a 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype2.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype2.cpp
@@ -278,6 +278,20 @@ void tst_QMetaType::convertCustomType()
     QCOMPARE(v.value<CustomConvertibleType2>().m_foo, testCustom.m_foo);
 }
 
+void tst_QMetaType::convertConstNonConst()
+{
+    auto mtConstObj = QMetaType::fromType<QObject const*>();
+    auto mtObj = QMetaType::fromType<QObject *>();
+    auto mtConstDerived = QMetaType::fromType<Derived const*>();
+    auto mtDerived = QMetaType::fromType<Derived *>();
+
+    QVERIFY(QMetaType::canConvert(mtConstObj, mtObj));
+    QVERIFY(QMetaType::canConvert(mtObj, mtConstObj)); // casting const away is allowed (but can lead to UB)
+    QVERIFY(QMetaType::canConvert(mtConstDerived, mtObj));
+    QVERIFY(QMetaType::canConvert(mtDerived, mtConstObj));
+    QVERIFY(QMetaType::canConvert(mtObj, mtConstDerived));
+}
+
 void tst_QMetaType::compareCustomEqualOnlyType()
 {
     QMetaType type = QMetaType::fromType<CustomEqualsOnlyType>();