QList: make QListSpecialMethods dtors protected

QList publicly inherits from QListSpecialMethods. Thus, any specialisation
of QListSpecialMethods should make their destructor protected, to avoid
deletion through a pointer to QListSpecialMethods invoking UB.

Change-Id: I7e317606f84826cc0faf1bfc05dee97da6eaf2eb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qbytearraylist.h

@@ -55,6 +55,10 @@ class QByteArrayList : public QList<QByteArray>
 template <> struct QListSpecialMethods<QByteArray>
 #endif
 {
+#ifndef Q_QDOC
+protected:
+    ~QListSpecialMethods() {}
+#endif
 public:
     inline QByteArray join() const
     { return QtPrivate::QByteArrayList_join(self(), 0, 0); }

src/corelib/tools/qlist.h

@@ -62,8 +62,13 @@ QT_BEGIN_NAMESPACE
 template <typename T> class QVector;
 template <typename T> class QSet;
 
-template <typename T> struct QListSpecialMethods { };
+template <typename T> struct QListSpecialMethods
+{
+protected:
+    ~QListSpecialMethods() {}
+};
 template <> struct QListSpecialMethods<QByteArray>;
+template <> struct QListSpecialMethods<QString>;
 
 struct Q_CORE_EXPORT QListData {
     // tags for tag-dispatching of QList implementations,

src/corelib/tools/qstringlist.h

@@ -57,6 +57,10 @@ class QStringList : public QList<QString>
 template <> struct QListSpecialMethods<QString>
 #endif
 {
+#ifndef Q_QDOC
+protected:
+    ~QListSpecialMethods() {}
+#endif
 public:
     inline void sort(Qt::CaseSensitivity cs = Qt::CaseSensitive);
     inline int removeDuplicates();