tst_QPointer: add checks involving multiple inheritance

Add a check that we can compare QObjects implementing interfaces to
said interface.

We can.

Pick-to: 6.5
Task-number: QTBUG-135626
Change-Id: I0e1164b43d9112e051add2c034dea50ab5192b2d
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 154d175c9b7b990e5f27bd4779ae2525ec02a1e2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 4f325cb6d51652fa7866a20b1f6ea38981bde3fa)
(cherry picked from commit f3218e1f603c34f2502d8ebbbb3c8086a6ded377)

tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp

@@ -16,6 +16,23 @@
 
 using namespace std::chrono_literals;
 
+class Interface
+{
+public:
+    virtual ~Interface() = default;
+    virtual void meep() const = 0;
+};
+
+class ObjectImplementingInterface : public QObject, public Interface
+{
+    Q_OBJECT
+public:
+    using QObject::QObject;
+
+public Q_SLOTS:
+    void meep() const override {}
+};
+
 class tst_QPointer : public QObject
 {
     Q_OBJECT
@@ -31,6 +48,7 @@ private slots:
     void assignment_operators();
     void compareCompiles();
     void equality_operators();
+    void equality_operators_interface();
     void swap();
     void isNull();
     void dereference_operators();
@@ -218,6 +236,7 @@ void tst_QPointer::compareCompiles()
     QTestPrivate::testEqualityOperatorsCompile<QPointer<QObject>, QWidget*>();
     QTestPrivate::testEqualityOperatorsCompile<QPointer<QObject>, QPointer<QWidget>>();
     QTestPrivate::testEqualityOperatorsCompile<QPointer<QObject>, std::nullptr_t>();
+    QTestPrivate::testEqualityOperatorsCompile<QPointer<ObjectImplementingInterface>, Interface*>();
 }
 
 void tst_QPointer::equality_operators()
@@ -259,6 +278,25 @@ void tst_QPointer::equality_operators()
 #endif
 }
 
+void tst_QPointer::equality_operators_interface()
+{
+    QObject reaper;
+    QPointer<ObjectImplementingInterface> p(new ObjectImplementingInterface(&reaper));
+    Interface *i = p.get();
+
+    ObjectImplementingInterface otherP;
+    Interface *otherI = &otherP;
+
+    // things that are equal
+    QT_TEST_EQUALITY_OPS(p, p, true);
+    QT_TEST_EQUALITY_OPS(p, i, true);
+
+    // things that are not equal
+    QT_TEST_EQUALITY_OPS(p, nullptr, false);
+    QT_TEST_EQUALITY_OPS(p, &otherP, false);
+    QT_TEST_EQUALITY_OPS(p, otherI, false);
+}
+
 void tst_QPointer::swap()
 {
     QPointer<QObject> c1, c2;