QVariant: Fix support for metatypes created by Qt < 6.5

In Qt >= 6.1, < 6.5, a trivially constructible type would have the
NeedsDestruction flag set, but it's dtor pointer would have been null.

In Qt 6.5, the meaning of the NeedsDestruction flag was changed to be
more aligned with what the name suggests, and thus would only be set for
non-trivially destructible types. For QMetaType this was fine, but
QVariant has a check for acceptable metatypes which attempts to verify
whether a QMetaType is usable for QVariant. The check assumes the
semantics of Qt 6.5, and thus fails for metatypes created by older Qt
versions.

To fix this issue, we increment the QMetaType revision field, and only
check the metatype's destruction support if the revision is high enough.

In theory, that allows passing unsuitable metatypes from older Qt
versions to QVariant; however, such code would have been broken in prior
Qt releases already (which didn't attempt the check), and no code that
used to work in any released Qt version will break (as we simply skip a
check that was passing before).

Fixes: QTBUG-113227
Pick-to: 6.5
Change-Id: I12e02bd97d2c410ea1a36efb0ce2389f21d50a30
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 28 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
index 698d117f70..4fa42a1d52 100644
--- a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
+++ b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
@@ -376,6 +376,7 @@ private slots:
 
     void constructFromIncompatibleMetaType_data();
     void constructFromIncompatibleMetaType();
+    void constructFromQtLT65MetaType();
     void copyNonDefaultConstructible();
 
     void inplaceConstruct();
@@ -5702,6 +5703,33 @@ void tst_QVariant::constructFromIncompatibleMetaType()
    QVERIFY(!QVariant(regular).convert(type));
 }
 
+void tst_QVariant::constructFromQtLT65MetaType()
+{
+   auto qsizeIface = QtPrivate::qMetaTypeInterfaceForType<QSize>();
+
+   QtPrivate::QMetaTypeInterface qsize64Iface = {
+       /*revision*/0,
+       8,
+       8,
+       QMetaType::NeedsConstruction | QMetaType::NeedsDestruction,
+       0,
+       qsizeIface->metaObjectFn,
+       "FakeQSize",
+       qsizeIface->defaultCtr,
+       qsizeIface->copyCtr,
+       qsizeIface->moveCtr,
+       /*dtor =*/  nullptr,
+       qsizeIface->equals,
+       qsizeIface->lessThan,
+       qsizeIface->debugStream,
+       qsizeIface->dataStreamOut,
+       qsizeIface->dataStreamIn,
+       /*legacyregop =*/ nullptr
+   };
+   QVariant var{ QMetaType(&qsize64Iface) };
+   QVERIFY(var.isValid());
+}
+
 void tst_QVariant::copyNonDefaultConstructible()
 {
     NonDefaultConstructible ndc(42);