QMetaType: Fix compilation with non default constructible Q_GADGET

Do not try to automatically register the meta type for Q_GADGET that
are not default constructible.
This fixes a source incompatibility in the function pointer syntax
of QObject::connect when such types are used as an argument of a signal.

Task-number: QTBUG-45721
Change-Id: I3065f6d57bc1f37e16988d2dee99118de250ca56
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index 263cc5a07a..3ec84b5198 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
@@ -128,6 +128,7 @@ private slots:
     void connectWithReference();
     void connectManyArguments();
     void connectForwardDeclare();
+    void connectNoDefaultConstructorArg();
     void returnValue_data();
     void returnValue();
     void returnValue2_data();
@@ -5227,6 +5228,29 @@ void tst_QObject::connectForwardDeclare()
     QVERIFY(connect(&ob, &ForwardDeclareArguments::mySignal, &ob, &ForwardDeclareArguments::mySlot, Qt::QueuedConnection));
 }
 
+class NoDefaultConstructor
+{
+    Q_GADGET
+public:
+    NoDefaultConstructor(int) {}
+};
+
+class NoDefaultContructorArguments : public QObject
+{
+    Q_OBJECT
+signals:
+    void mySignal(const NoDefaultConstructor&);
+public slots:
+    void mySlot(const NoDefaultConstructor&) {}
+};
+
+void tst_QObject::connectNoDefaultConstructorArg()
+{
+    NoDefaultContructorArguments ob;
+    // it should compile
+    QVERIFY(connect(&ob, &NoDefaultContructorArguments::mySignal, &ob, &NoDefaultContructorArguments::mySlot, Qt::QueuedConnection));
+}
+
 class ReturnValue : public QObject {
 friend class tst_QObject;
 Q_OBJECT