Pass correct default button in QMessageBox::showNewMessageBox()

showNewMessageBox() shows an "old" message box, if a default button
argument was passed and the buttons argument doesn't contain a default
button. It passed the int value of defaultButton to showOldMessageBox,
where it was interpreted as a normal button.

The StandardButton::Default flag was not set on the default button.
This relied on the QDialogButtonBox owned by QMessageBox to show the
expected default button by co-incidence. As this was not always the
case, tst_QMessageBox::staticSourceCompat() even tested wrong expected
results.

=> Add the Default flag to the default button, before passing it as an
int value.
=> As a drive-by,
- replace c-style casting with static casting.
- add braces to multi-line if clause.

Task-number: QTBUG-118489
Pick-to: 6.7 6.6 6.5
Change-Id: I9cf93c8f93d6ab80e7be5ab25e56bc59d3d6209c
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/dialogs/qmessagebox.cpp

@@ -1729,11 +1729,14 @@ static QMessageBox::StandardButton showNewMessageBox(QWidget *parent,
 {
     // necessary for source compatibility with Qt 4.0 and 4.1
     // handles (Yes, No) and (Yes|Default, No)
-    if (defaultButton && !(buttons & defaultButton))
-        return (QMessageBox::StandardButton)
-                    QMessageBoxPrivate::showOldMessageBox(parent, icon, title,
-                                                            text, int(buttons),
-                                                            int(defaultButton), 0);
+    if (defaultButton && !(buttons & defaultButton)) {
+        const int defaultButtons = defaultButton | QMessageBox::Default;
+        const int otherButtons = static_cast<int>(buttons);
+        const int ret = QMessageBoxPrivate::showOldMessageBox(parent, icon, title,
+                                                              text, otherButtons,
+                                                              defaultButtons, 0);
+        return static_cast<QMessageBox::StandardButton>(ret);
+    }
 
     QMessageBox msgBox(icon, title, text, QMessageBox::NoButton, parent);
     QDialogButtonBox *buttonBox = msgBox.findChild<QDialogButtonBox*>();