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>

src/corelib/global/qtypetraits.h

@@ -506,6 +506,27 @@ Q_STATIC_ASSERT((!is_unsigned<qint64>::value));
 Q_STATIC_ASSERT((!is_signed<quint64>::value));
 Q_STATIC_ASSERT(( is_signed<qint64>::value));
 
+template<class T = void> struct is_default_constructible;
+
+template<> struct is_default_constructible<void>
+{
+protected:
+    template<bool> struct test { typedef char type; };
+public:
+    static bool const value = false;
+};
+template<> struct is_default_constructible<>::test<true> { typedef double type; };
+
+template<class T> struct is_default_constructible : is_default_constructible<>
+{
+private:
+    template<class U> static typename test<!!sizeof(::new U())>::type sfinae(U*);
+    template<class U> static char sfinae(...);
+public:
+    static bool const value = sizeof(sfinae<T>(0)) > 1;
+};
+
+
 } // namespace QtPrivate
 
 QT_END_NAMESPACE

src/corelib/kernel/qmetatype.h

@@ -1773,7 +1773,7 @@ template <typename T>
 struct QMetaTypeIdQObject<T, QMetaType::IsGadget>
 {
     enum {
-        Defined = 1
+        Defined = QtPrivate::is_default_constructible<T>::value
     };
 
     static int qt_metatype_id()

tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp

@@ -142,6 +142,11 @@ public:
 class CustomGadget {
     Q_GADGET
 };
+class CustomGadget_NonDefaultConstructible {
+    Q_GADGET
+public:
+    CustomGadget_NonDefaultConstructible(int) {};
+};
 
 class CustomNonQObject {};
 class GadgetDerived : public CustomGadget {};
@@ -159,7 +164,7 @@ void tst_QMetaType::defined()
     QVERIFY(!QMetaTypeId2<CustomQObject>::Defined);
     QVERIFY(!QMetaTypeId2<CustomNonQObject>::Defined);
     QVERIFY(!QMetaTypeId2<CustomNonQObject*>::Defined);
-    QVERIFY(!QMetaTypeId2<CustomGadget*>::Defined);
+    QVERIFY(!QMetaTypeId2<CustomGadget_NonDefaultConstructible>::Defined);
 }
 
 struct Bar

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