QCoreApplication: enforce non-null arguments when sending/posting events

Passing nullptr as receiver and/or as an event parameter to sendEvent,
postEvent, etc. is meaningless. It's also something that users can check
for. Therefore, it should not be allowed. Note that the current code
already relies on the arguments not to be null, albeit "indirectly"
(e.g. they get dereferenced without any null checks).

Hence: add asserts that check for non-null in all the relevant
codepaths, except for the ones in which there's currently just a
warning; for those, add a Qt 7 note.

Change-Id: Ia4c58551de88a5d1003f09efa448c1330b6cb122
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/kernel/qcoreapplication.cpp

@@ -1133,6 +1133,9 @@ bool QCoreApplication::forwardEvent(QObject *receiver, QEvent *event, QEvent *or
 
 bool QCoreApplication::notify(QObject *receiver, QEvent *event)
 {
+    Q_ASSERT(receiver);
+    Q_ASSERT(event);
+
     // no events are delivered after ~QCoreApplication() has started
     if (QCoreApplicationPrivate::is_app_closing)
         return true;
@@ -1141,6 +1144,9 @@ bool QCoreApplication::notify(QObject *receiver, QEvent *event)
 
 static bool doNotify(QObject *receiver, QEvent *event)
 {
+    Q_ASSERT(event);
+
+    // ### Qt 7: turn into an assert
     if (receiver == nullptr) {                        // serious error
         qWarning("QCoreApplication::notify: Unexpected null receiver");
         return true;
@@ -1464,10 +1470,12 @@ void QCoreApplication::exit(int returnCode)
 */
 bool QCoreApplication::sendEvent(QObject *receiver, QEvent *event)
 {
+    Q_ASSERT_X(receiver, "QCoreApplication::sendEvent", "Unexpected null receiver");
+    Q_ASSERT_X(event, "QCoreApplication::sendEvent", "Unexpected null event");
+
     Q_TRACE(QCoreApplication_sendEvent, receiver, event, event->type());
 
-    if (event)
+    event->m_spont = false;
-        event->m_spont = false;
     return notifyInternal2(receiver, event);
 }
 
@@ -1476,10 +1484,12 @@ bool QCoreApplication::sendEvent(QObject *receiver, QEvent *event)
 */
 bool QCoreApplication::sendSpontaneousEvent(QObject *receiver, QEvent *event)
 {
+    Q_ASSERT_X(receiver, "QCoreApplication::sendSpontaneousEvent", "Unexpected null receiver");
+    Q_ASSERT_X(event, "QCoreApplication::sendSpontaneousEvent", "Unexpected null event");
+
     Q_TRACE(QCoreApplication_sendSpontaneousEvent, receiver, event, event->type());
 
-    if (event)
+    event->m_spont = true;
-        event->m_spont = true;
     return notifyInternal2(receiver, event);
 }
 
@@ -1544,8 +1554,11 @@ QCoreApplicationPrivate::QPostEventListLocker QCoreApplicationPrivate::lockThrea
 */
 void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority)
 {
+    Q_ASSERT_X(event, "QCoreApplication::postEvent", "Unexpected null event");
+
     Q_TRACE_SCOPE(QCoreApplication_postEvent, receiver, event, event->type());
 
+    // ### Qt 7: turn into an assert
     if (receiver == nullptr) {
         qWarning("QCoreApplication::postEvent: Unexpected null receiver");
         delete event;
@@ -1615,11 +1628,11 @@ void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority)
 */
 bool QCoreApplication::compressEvent(QEvent *event, QObject *receiver, QPostEventList *postedEvents)
 {
-#ifdef Q_OS_WIN
     Q_ASSERT(event);
     Q_ASSERT(receiver);
     Q_ASSERT(postedEvents);
 
+#ifdef Q_OS_WIN
     // compress posted timers to this object.
     if (event->type() == QEvent::Timer && receiver->d_func()->postedEvents > 0) {
         int timerId = ((QTimerEvent *) event)->timerId();