Fix source incompatibility while connecting signals with forward declared arguments

QObject::connect tries to determine if the arguments are registered
metatypes. This used to work even for arguments that were forward
declared. But now, the metatype system tries to call
QtPrivate::IsQEnumHelper<T>::Value to know if it is registered.
That fails on gcc if T is forward declared.

Apparently gcc needs to know the full type of T to pass it in the
ellipsis function, even within a sizeof expression.

So change the ellipsis expression to a template one.

Task-number: QTBUG-44496
Change-Id: I7fa07bd3cde470b134c2ec53b0d581333d16a6f1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index 2c2dbeef9d..55f8fc9b2c 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -1364,15 +1364,15 @@ namespace QtPrivate
         enum { Value = sizeof(checkType(static_cast<T*>(0))) == sizeof(void*)  };
     };
 
-    char qt_getEnumMetaObject(...);
-    char qt_getEnumMetaObject(); // Workaround bugs in MSVC.
+    template<typename T> char qt_getEnumMetaObject(const T&);
 
     template<typename T>
     struct IsQEnumHelper {
         static const T &declval();
-        // If the type was declared with Q_ENUM, the friend qt_getEnumMetaObject(T) declared in the
+        // If the type was declared with Q_ENUM, the friend qt_getEnumMetaObject() declared in the
         // Q_ENUM macro will be chosen by ADL, and the return type will be QMetaObject*.
-        // Otherwise the chosen overload will be qt_getEnumMetaObject(...) which returne 'char'
+        // Otherwise the chosen overload will be the catch all template function
+        // qt_getEnumMetaObject(T) which returns 'char'
         enum { Value = sizeof(qt_getEnumMetaObject(declval())) == sizeof(QMetaObject*) };
     };