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>

2 files changed, 22 insertions, 1 deletions

diff --git a/src/corelib/global/qtypetraits.h b/src/corelib/global/qtypetraits.h
index 3a305713e6..488e257e0f 100644
--- a/src/corelib/global/qtypetraits.h
+++ b/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
diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index 9ad8702e79..1b214e9f74 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/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()