QMetaType: add a missing check for null d_ptr

Bug introduced in 6.0. This is the only unprotected d_ptr I could find.

[ChangeLog][QtCore][QMetaType] Fixed a bug that would cause
QMetaType::compare() and QVariant::compare() to crash on invalid meta
types and variants.

Pick-to: 6.2 6.3
Fixes: QTBUG-99960
Change-Id: I0e5f6bec596a4a78bd3bfffd16cb1f7b2d146688
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qmetatype.cpp

@@ -750,7 +750,7 @@ QPartialOrdering QMetaType::compare(const void *lhs, const void *rhs) const
 {
     if (!lhs || !rhs)
         return QPartialOrdering::Unordered;
-    if (d_ptr->flags & QMetaType::IsPointer)
+    if (d_ptr && d_ptr->flags & QMetaType::IsPointer)
         return threeWayCompare(*reinterpret_cast<const void * const *>(lhs),
                                *reinterpret_cast<const void * const *>(rhs));
     if (d_ptr && d_ptr->lessThan) {

tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp

@@ -39,6 +39,7 @@
 #include <vector>
 
 Q_DECLARE_METATYPE(QMetaType::Type)
+Q_DECLARE_METATYPE(QPartialOrdering)
 
 namespace CheckTypeTraits
 {
@@ -1346,6 +1347,58 @@ FOR_EACH_CORE_METATYPE(RETURN_CONSTRUCT_COPY_FUNCTION)
     TypeTestFunctionGetter::get(type)();
 }
 
+void tst_QMetaType::selfCompare_data()
+{
+    qRegisterMetaType<QPartialOrdering>();
+    QTest::addColumn<int>("type");
+    QTest::addColumn<QPartialOrdering>("order");
+
+    auto orderingFor = [](QMetaType::Type t) {
+        if (t == QMetaType::UnknownType || t == QMetaType::Void)
+            return QPartialOrdering::Unordered;
+        return QPartialOrdering::Equivalent;
+    };
+
+    QTest::newRow("unknown-type") << int(QMetaType::UnknownType) << orderingFor(QMetaType::UnknownType);
+
+#define ADD_METATYPE_TEST_ROW(MetaTypeName, MetaTypeId, RealType) \
+    QTest::newRow(QMetaType::typeName(QMetaType::MetaTypeName)) << int(QMetaType::MetaTypeName) << orderingFor(QMetaType::MetaTypeName);
+FOR_EACH_CORE_METATYPE(ADD_METATYPE_TEST_ROW)
+#undef ADD_METATYPE_TEST_ROW
+}
+
+void tst_QMetaType::selfCompare()
+{
+    QFETCH(int, type);
+    QFETCH(QPartialOrdering, order);
+
+    QMetaType t(type);
+    void *v1 = t.create(nullptr);
+    void *v2 = t.create(nullptr);
+    auto scope = qScopeGuard([=] {
+        t.destroy(v1);
+        t.destroy(v2);
+    });
+
+    // all these types have an equality comparator
+    QCOMPARE(t.equals(v1, v2), order == QPartialOrdering::Equivalent);
+
+    if (t.iface() && t.iface()->lessThan)
+        QCOMPARE(t.compare(v1, v2), order);
+
+    // for the primitive types, do a memcmp() too
+    switch (type) {
+    default:
+        break;
+
+#define ADD_METATYPE_CASE(MetaTypeName, MetaTypeId, RealType) \
+    case QMetaType::MetaTypeName:
+FOR_EACH_PRIMITIVE_METATYPE(ADD_METATYPE_CASE)
+#undef ADD_METATYPE_CASE
+        QCOMPARE(memcmp(v1, v2, t.sizeOf()), 0);
+    }
+}
+
 typedef QString CustomString;
 Q_DECLARE_METATYPE(CustomString) //this line is useless
 

tests/auto/corelib/kernel/qmetatype/tst_qmetatype.h

@@ -104,6 +104,8 @@ private slots:
     void typedConstruct();
     void constructCopy_data();
     void constructCopy();
+    void selfCompare_data();
+    void selfCompare();
     void typedefs();
     void registerType();
     void isRegistered_data();