QSettings: overload value() instead of using a default argument

... of non-trivial type, which needs to be constructed every time the
function is called, no matter whether the user supplies one or not.

Effects on Linux AMD64 O2 C++20 tst_qsettings builds, as an example
heavy-duty QSettings::value() user:

GCC 11.2 libstdc++ (TEXT -= 4.6%):

     text    data     bss     dec     hex filename
   662317   10952    2824  676093   a50fd tst_qsettings-00-baseline
   631849   10960    2824  645633   9da01 tst_qsettings-01-qvariant

Clang 10.0.0 libc++ (TEXT -= 2.8%)

     text    data     bss     dec     hex filename
   808448   10616    2832  821896   c8a88 tst_qsettings-00-baseline
   785890   10624    2832  799346   c3272 tst_qsettings-01-qvariant

Pick-to: 6.3
Task-number: QTBUG-98117
Change-Id: I7366f063e3ec34c040fd35ffd274e8ed5a722132
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/io/qsettings.cpp

@@ -3254,6 +3254,9 @@ bool QSettings::event(QEvent *event)
 #endif
 
 /*!
+    \fn QSettings::value(const QString &key) const
+    \fn QSettings::value(const QString &key, const QVariant &defaultValue) const
+
     Returns the value for setting \a key. If the setting doesn't
     exist, returns \a defaultValue.
 
@@ -3271,14 +3274,29 @@ bool QSettings::event(QEvent *event)
 
     \sa setValue(), contains(), remove()
 */
+QVariant QSettings::value(const QString &key) const
+{
+    Q_D(const QSettings);
+    return d->value(key, nullptr);
+}
+
 QVariant QSettings::value(const QString &key, const QVariant &defaultValue) const
 {
     Q_D(const QSettings);
+    return d->value(key, &defaultValue);
+}
+
+QVariant QSettingsPrivate::value(const QString &key, const QVariant *defaultValue) const
+{
     if (key.isEmpty()) {
         qWarning("QSettings::value: Empty key passed");
         return QVariant();
     }
-    return d->get(d->actualKey(key)).value_or(defaultValue);
+    if (std::optional r = get(actualKey(key)))
+        return std::move(*r);
+    if (defaultValue)
+        return *defaultValue;
+    return QVariant();
 }
 
 /*!

src/corelib/io/qsettings.h

@@ -163,7 +163,8 @@ public:
     bool isWritable() const;
 
     void setValue(const QString &key, const QVariant &value);
-    QVariant value(const QString &key, const QVariant &defaultValue = QVariant()) const;
+    QVariant value(const QString &key, const QVariant &defaultValue) const;
+    QVariant value(const QString &key) const;
 
     void remove(const QString &key);
     bool contains(const QString &key) const;

src/corelib/io/qsettings_p.h

@@ -208,6 +208,7 @@ public:
     virtual bool isWritable() const = 0;
     virtual QString fileName() const = 0;
 
+    QVariant value(const QString &key, const QVariant *defaultValue) const;
     QString actualKey(const QString &key) const;
     void beginGroupOrArray(const QSettingsGroup &group);
     void setStatus(QSettings::Status status) const;