Allow connect to functors with less parameters than the signal

Before, the functor slot (or lambda expression) had to have the same amount
of arguments as the signal.
This shown to be a big problem to be able to connect to signals that had
a QPrivateSlot.

This implementation use the type of the operator() of the functor to
know how many arguments we have.
As a bonus, we also can check the arguments in a static assert.

The test comes from https://codereview.qt-project.org/#change,38703
But this patch also works without variadic template

If the compiler does not support decltype, we workaround the lack of it
by using another level of indirection.

Change-Id: I9850b43e8caf77356a2ec3f4c0b0ed532d96029e
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

1 files changed, 36 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index f48f86cceb..c340d00361 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
@@ -140,6 +140,7 @@ private slots:
     void returnValue2_data();
     void returnValue2();
     void connectVirtualSlots();
+    void connectFunctorArgDifference();
 };
 
 class SenderObject : public QObject
@@ -5484,5 +5485,40 @@ void tst_QObject::connectVirtualSlots()
     */
 }
 
+struct SlotFunctor
+{
+    void operator()() {}
+};
+
+struct SlotFunctorString
+{
+    void operator()(const QString &) {}
+};
+
+void tst_QObject::connectFunctorArgDifference()
+{
+    QTimer timer;
+    // Compile-time tests that the connection is successful.
+    connect(&timer, &QTimer::timeout, SlotFunctor());
+    connect(&timer, &QTimer::objectNameChanged, SlotFunctorString());
+    connect(qApp, &QCoreApplication::aboutToQuit, SlotFunctor());
+
+    connect(&timer, &QTimer::objectNameChanged, SlotFunctor());
+    QStringListModel model;
+    connect(&model, &QStringListModel::rowsInserted, SlotFunctor());
+
+#if defined(Q_COMPILER_LAMBDA)
+    connect(&timer, &QTimer::timeout, [=](){});
+    connect(&timer, &QTimer::objectNameChanged, [=](const QString &){});
+    connect(qApp, &QCoreApplication::aboutToQuit, [=](){});
+
+    connect(&timer, &QTimer::objectNameChanged, [=](){});
+    connect(&model, &QStringListModel::rowsInserted, [=](){});
+    connect(&model, &QStringListModel::rowsInserted, [=](const QModelIndex &){});
+#endif
+
+    QVERIFY(true);
+}
+
 QTEST_MAIN(tst_QObject)
 #include "tst_qobject.moc"