QObject: make new-style-connects SFINAE-friendly

SFINAE out the new-style connect() overloads for const char*
functions.

Before, we detected the problem in the body of connect(), by failing
the static_assert for signal/slot argument compatibility, but that's
too late. We don't want to select that overload at all, because we
want that decltype(QObject::connect(QAction*, &QAction::triggered,
QObject*, SLOT(something())) SFINAEs instead of returning
QMetaObject::Connect, because then all the classes that provide
convenience new-style connect syntax by wrapping the
QObject::connect() call can just use that decltype() to accept
anything that will succeed in the wrapped connect() call instead of
manually trying (and failing) to provide proper overloads themselves.

[ChangeLog][QtCore][Potentially Source-Incompatible Changes]
QObject::connect(sender, pointer-to-member, [receiver, ], SLOT(~~~))
is now SFINAEd out instead of triggering a static_assert(). That means
it is now possible to use Expression SFINAE
(decltype(QObject::connect(<sender>, <signal>,
std::declval<Args>()...) to properly wrap new-style QObject::connect()
calls with a single wrapper function, greatly simplifying the
implementation, incl. passing Qt::ConnectionType arguments. For more
information on how to use this in your own code, see the sources of
QWidget::addAction().

Change-Id: I4052a49eacd63a432a9648a667adc435744a633d
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 8 insertions, 2 deletions

diff --git a/src/corelib/kernel/qobject.h b/src/corelib/kernel/qobject.h
index 6f5d574f70..6a8088ef12 100644
--- a/src/corelib/kernel/qobject.h
+++ b/src/corelib/kernel/qobject.h
@@ -285,7 +285,10 @@ public:
 
     //connect to a functor
     template <typename Func1, typename Func2>
-    static inline typename std::enable_if<QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1, QMetaObject::Connection>::type
+    static inline typename std::enable_if<
+        QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1 &&
+        !std::is_convertible_v<Func2, const char*>, // don't match old-style connect
+    QMetaObject::Connection>::type
             connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
     {
         return connect(sender, signal, sender, std::move(slot), Qt::DirectConnection);
@@ -293,7 +296,10 @@ public:
 
     //connect to a functor, with a "context" object defining in which event loop is going to be executed
     template <typename Func1, typename Func2>
-    static inline typename std::enable_if<QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1, QMetaObject::Connection>::type
+    static inline typename std::enable_if<
+        QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1 &&
+        !std::is_convertible_v<Func2, const char*>, // don't match old-style connect
+    QMetaObject::Connection>::type
             connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, const QObject *context, Func2 slot,
                     Qt::ConnectionType type = Qt::AutoConnection)
     {