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
Change-Id: I12e02bd97d2c410ea1a36efb0ce2389f21d50a30
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
(cherry picked from commit c2f01d4cfb1b144bcc2bf295399ef0d4ad70dae2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 15 insertions, 3 deletions

diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index e70f901a09..a641313c5b 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -246,7 +246,14 @@ using NonConstMetaTypeInterface = const QMetaTypeInterface;
 class QMetaTypeInterface
 {
 public:
-    ushort revision; // 0 in Qt 6.0. Can increase if new field are added
+
+    /* Revision: Can increase if new field are added, or if semantics changes
+       0: Initial Revision
+       1: the meaning of the NeedsDestruction flag changed
+    */
+    static inline constexpr ushort CurrentRevision = 1;
+
+    ushort revision;
     ushort alignment;
     uint size;
     uint flags;
@@ -2461,7 +2468,7 @@ struct QMetaTypeInterfaceWrapper
     using InterfaceType = std::conditional_t<IsConstMetaTypeInterface, const QMetaTypeInterface, NonConstMetaTypeInterface>;
 
     static inline InterfaceType metaType = {
-        /*.revision=*/ 0,
+        /*.revision=*/ QMetaTypeInterface::CurrentRevision,
         /*.alignment=*/ alignof(T),
         /*.size=*/ sizeof(T),
         /*.flags=*/ QMetaTypeForType<T>::Flags,
diff --git a/src/corelib/kernel/qmetatype_p.h b/src/corelib/kernel/qmetatype_p.h
index 3defbc70ef..e649394832 100644
--- a/src/corelib/kernel/qmetatype_p.h
+++ b/src/corelib/kernel/qmetatype_p.h
@@ -152,7 +152,12 @@ inline bool isMoveConstructible(const QtPrivate::QMetaTypeInterface *iface) noex
 
 inline bool isDestructible(const QtPrivate::QMetaTypeInterface *iface) noexcept
 {
-    return checkMetaTypeFlagOrPointer(iface, iface->dtor, QMetaType::NeedsDestruction);
+    /* For metatypes of revision 1, the NeedsDestruction was set even for trivially
+       destructible types, but their dtor pointer would be null.
+       For that reason, we need the additional check here.
+     */
+    return iface->revision < 1 ||
+           checkMetaTypeFlagOrPointer(iface, iface->dtor, QMetaType::NeedsDestruction);
 }
 
 inline void defaultConstruct(const QtPrivate::QMetaTypeInterface *iface, void *where)