qDebug: add support for std::optional and std::nullopt_t

[ChangeLog][QtCore][QDebug] Added support for std::optional
and std::nullopt_t

Change-Id: I1e6196adb408401cae8776cd0c60af294a39a83f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/io/qdebug.cpp

@@ -1018,6 +1018,15 @@ QDebug &QDebug::resetFormat()
     \c T2 need to support streaming into QDebug.
 */
 
+/*!
+    \since 6.7
+    \fn template <class T> QDebug operator<<(QDebug debug, const std::optional<T> &opt)
+    \relates QDebug
+
+    Writes the contents of \a opt (or \c nullopt if not set) to \a debug.
+    \c T needs to support streaming into QDebug.
+*/
+
 /*!
     \fn template <typename T> QDebug operator<<(QDebug debug, const QContiguousCache<T> &cache)
     \relates QDebug
@@ -1050,6 +1059,13 @@ QDebug &QDebug::resetFormat()
   \internal
  */
 
+/*!
+    \since 6.7
+    \fn QDebug &QDebug::operator<<(std::nullopt_t)
+
+    Writes nullopt to the stream.
+*/
+
 /*!
     \class QDebugStateSaver
     \inmodule QtCore

src/corelib/io/qdebug.h

@@ -126,6 +126,7 @@ public:
     inline QDebug &operator<<(QByteArrayView t) { putByteArray(t.constData(), t.size(), ContainsBinary); return maybeSpace(); }
     inline QDebug &operator<<(const void * t) { stream->ts << t; return maybeSpace(); }
     inline QDebug &operator<<(std::nullptr_t) { stream->ts << "(nullptr)"; return maybeSpace(); }
+    inline QDebug &operator<<(std::nullopt_t) { stream->ts << "nullopt"; return maybeSpace(); }
     inline QDebug &operator<<(QTextStreamFunction f) {
         stream->ts << f;
         return *this;
@@ -361,6 +362,17 @@ inline QDebugIfHasDebugStreamContainer<QMultiHash<Key, T>, Key, T> operator<<(QD
     return QtPrivate::printAssociativeContainer(debug, "QMultiHash", hash);
 }
 
+template <class T>
+inline QDebugIfHasDebugStream<T> operator<<(QDebug debug, const std::optional<T> &opt)
+{
+    const QDebugStateSaver saver(debug);
+    if (!opt)
+        debug.nospace() << std::nullopt;
+    else
+        debug.nospace() << "std::optional(" << *opt << ')';
+    return debug;
+}
+
 template <class T1, class T2>
 inline QDebugIfHasDebugStream<T1, T2> operator<<(QDebug debug, const std::pair<T1, T2> &pair)
 {

tests/auto/corelib/io/qdebug/tst_qdebug.cpp

@@ -89,6 +89,7 @@ private slots:
     void qDebugQFlags() const;
     void qDebugStdChrono_data() const;
     void qDebugStdChrono() const;
+    void qDebugStdOptional() const;
     void textStreamModifiers() const;
     void resetFormat() const;
     void defaultMessagehandler() const;
@@ -1215,6 +1216,28 @@ void tst_QDebug::qDebugStdChrono() const
     QCOMPARE(fn(), expected);
 }
 
+void tst_QDebug::qDebugStdOptional() const
+{
+    QString file, function;
+    int line = 0;
+    MessageHandlerSetter mhs(myMessageHandler);
+    {
+        std::optional<QByteArray> notSet = std::nullopt;
+        std::optional<QByteArray> set("foo");
+        auto no = std::nullopt;
+        QDebug d = qDebug();
+        d << notSet << set << no;
+    }
+#ifndef QT_NO_MESSAGELOGCONTEXT
+    file = __FILE__; line = __LINE__ - 4; function = Q_FUNC_INFO;
+#endif
+    QCOMPARE(s_msgType, QtDebugMsg);
+    QCOMPARE(s_msg, QString::fromLatin1("nullopt std::optional(\"foo\") nullopt"));
+    QCOMPARE(QString::fromLatin1(s_file), file);
+    QCOMPARE(s_line, line);
+    QCOMPARE(QString::fromLatin1(s_function), function);
+}
+
 void tst_QDebug::textStreamModifiers() const
 {
     QString file, function;