Add const overload for QLoggingCategory::operator()()

Change 85e57653 caused a compile error for code that does

  Q_DECLARE_LOGGING_CATEGORY(cat);
  //..
  qCDebug(cat()) << // ...

error: C3848: expression having type 'const QLoggingCategory' would lose
some const-volatile qualifiers in order to call 'QLoggingCategory
&QLoggingCategory::operator ()(void)'

This is a regression from Qt 5.2. Fix the error by adding a const version
of operator()().

Change-Id: I2fb04f2e155962adee0f98089fc5a159000bef56
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qloggingcategory.cpp

@@ -273,6 +273,14 @@ void QLoggingCategory::setEnabled(QtMsgType type, bool enable)
     \l qCWarning(), \l qCCritical() macros.
  */
 
+/*!
+    \fn const QLoggingCategory &QLoggingCategory::operator()() const
+
+    Returns the object itself. This allows both a QLoggingCategory variable, and
+    a factory method returning a QLoggingCategory, to be used in \l qCDebug(),
+    \l qCWarning(), \l qCCritical() macros.
+ */
+
 /*!
     Returns a pointer to the global category \c "default" that
     is used e.g. by qDebug(), qWarning(), qCritical(), qFatal().

src/corelib/io/qloggingcategory.h

@@ -65,6 +65,7 @@ public:
 
     // allows usage of both factory method and variable in qCX macros
     QLoggingCategory &operator()() { return *this; }
+    const QLoggingCategory &operator()() const { return *this; }
 
     static QLoggingCategory *defaultCategory();
 

tests/auto/corelib/io/qloggingcategory/tst_qloggingcategory.cpp

@@ -428,6 +428,8 @@ private slots:
         QCOMPARE(logMessage, QString());
         qCDebug(TST_LOG, "Check debug with no filter active");
         QCOMPARE(logMessage, QString());
+        qCDebug(TST_LOG(), "Check debug with no filter active");
+        QCOMPARE(logMessage, QString());
         buf = QStringLiteral("tst.log.warning: Check warning with no filter active");
         qCWarning(TST_LOG) << "Check warning with no filter active";
         QCOMPARE(logMessage, buf);