QStringList: add op<< overload for QList<QString>

QStringList << QList<QString> already compiled, but was interpreted as
QStringList << QStringList(QList<QString>), which involves the QList
copy ctor. Adding the overload saves that copy.

Cannot use a using declaration here, since the return type is different.

Change-Id: I119cc98e7e2df24549a1abb158543b729edc30ef
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

src/corelib/tools/qstringlist.cpp

@@ -478,6 +478,16 @@ QString QtPrivate::QStringList_join(const QStringList *that, const QChar *sep, i
     the latter string list.
 */
 
+/*!
+    \fn QStringList &QStringList::operator<<(const QList<QString> &other)
+    \since 5.4
+
+    \overload
+
+    Appends the \a other string list to the string list and returns a reference to
+    the latter string list.
+*/
+
 #ifndef QT_NO_DATASTREAM
 /*!
     \fn QDataStream &operator>>(QDataStream &in, QStringList &list)

src/corelib/tools/qstringlist.h

@@ -85,6 +85,8 @@ public:
     { append(str); return *this; }
     inline QStringList &operator<<(const QStringList &l)
     { *this += l; return *this; }
+    inline QStringList &operator<<(const QList<QString> &l)
+    { *this += l; return *this; }
 
 #ifndef QT_NO_REGEXP
     inline QStringList filter(const QRegExp &rx) const;

tests/auto/corelib/tools/qstringlist/tst_qstringlist.cpp

@@ -321,7 +321,12 @@ void tst_QStringList::streamingOperator()
     list << "hei";
     list << list << "hopp" << list;
 
+    QList<QString> slist = list;
+    list << slist;
+
     QCOMPARE(list, QStringList()
+            << "hei" << "hei" << "hopp"
+            << "hei" << "hei" << "hopp"
             << "hei" << "hei" << "hopp"
             << "hei" << "hei" << "hopp");