Fix QMetaObject::invokeMethod for free functions and std::bind

Amends 3bf5b5f8944dd417530b09dd6f1cd568717c8cc1, after which free
functions and std::bind could no longer be used as callables in
QMetaMethod::invokeMethod.

For free functions to work we need to decay to function pointers when
choosing what type QtPrivate::Callable aliases.

And std::bind has operator() overloads and the return type cannot be
deduced. So simplify the definition of the ZeroArgFunctor - we know
the function prototype if we know the return type.

Add testcase for calling std::bind and free function, and remove the
now unneeded helpers for functor argument and return type deduction.

Change-Id: I54aac5cb6d660267e6b2f5ab05d583e8826cdf9a
Reviewed-by: Zoltan Gera <zoltan.gera@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/kernel/qobjectdefs.h

@@ -419,6 +419,8 @@ struct Q_CORE_EXPORT QMetaObject
                  Qt::ConnectionType type = Qt::AutoConnection,
                  typename QtPrivate::Callable<Func>::ReturnType *ret = nullptr)
     {
+        static_assert(QtPrivate::Callable<Func>::ArgumentCount <= 0,
+                      "QMetaObject::invokeMethod cannot call functions with arguments!");
         using Prototype = typename QtPrivate::Callable<Func>::Function;
         return invokeMethodImpl(object, QtPrivate::makeCallableObject<Prototype>(std::forward<Func>(function)), type, ret);
     }

src/corelib/kernel/qobjectdefs_impl.h

@@ -330,20 +330,6 @@ namespace QtPrivate {
         typedef decltype(std::declval<Functor>().operator()((std::declval<ArgList>())...)) Value;
     };
 
-    // Get the function prototype for a functor. There can only be one call operator
-    // in a functor, otherwise we get errors from ambiguity. But that's good enough.
-    template <typename Ret, typename... Args>
-    using FunctionTypeForTypes = Ret(*)(Args...);
-
-    template <typename Ret, typename Obj, typename... Args>
-    FunctionTypeForTypes<Ret, Args...> FunctorPrototype(Ret(Obj::*)(Args...) const) { return nullptr; }
-    template <typename Ret, typename Obj, typename... Args>
-    FunctionTypeForTypes<Ret, Args...> FunctorPrototype(Ret(Obj::*)(Args...)) { return nullptr; }
-    template <typename Ret, typename Obj, typename... Args>
-    FunctionTypeForTypes<Ret, Args...> FunctorPrototype(Ret(Obj::*)(Args...) const noexcept) { return nullptr; }
-    template <typename Ret, typename Obj, typename... Args>
-    FunctionTypeForTypes<Ret, Args...> FunctorPrototype(Ret(Obj::*)(Args...) noexcept) { return nullptr; }
-
     template<typename Function, int N> struct Functor
     {
         template <typename SignalArgs, typename R>
@@ -355,16 +341,16 @@ namespace QtPrivate {
     template<typename Func>
     struct ZeroArgFunctor : Functor<Func, 0>
     {
-        using Function = decltype(FunctorPrototype(&std::decay_t<Func>::operator()));
+        using ReturnType = decltype(std::declval<Func>()());
+        using Function = ReturnType(*)();
         enum {ArgumentCount = 0};
         using Arguments = QtPrivate::List<>;
-        using ReturnType = typename FunctionPointer<Function>::ReturnType;
     };
 
     template<typename Func>
-    using Callable = std::conditional_t<FunctionPointer<Func>::ArgumentCount == -1,
-        ZeroArgFunctor<Func>,
-        FunctionPointer<Func>
+    using Callable = std::conditional_t<FunctionPointer<std::decay_t<Func>>::ArgumentCount == -1,
+        ZeroArgFunctor<std::decay_t<Func>>,
+        FunctionPointer<std::decay_t<Func>>
     >;
 
     /*

tests/auto/corelib/kernel/qmetaobject/tst_qmetaobject.cpp

@@ -302,6 +302,8 @@ private slots:
     void invokeTypedefTypes();
     void invokeException();
     void invokeQueuedAutoRegister();
+    void invokeFreeFunction();
+    void invokeBind();
     void qtMetaObjectInheritance();
     void normalizedSignature_data();
     void normalizedSignature();
@@ -1997,6 +1999,49 @@ void tst_QMetaObject::invokeQueuedAutoRegister()
              QString("slotWithRegistrableArgument:myShared-myShared-myShared-myShared-00"));
 }
 
+namespace FunctionTest {
+    static void function0() {}
+    static int functionNoExcept() noexcept
+    {
+        return 42;
+    }
+}
+
+void tst_QMetaObject::invokeFreeFunction()
+{
+    using namespace FunctionTest;
+    QtTestObject obj;
+    QMetaObject::invokeMethod(&obj, function0);
+    int retInt = -1;
+    QMetaObject::invokeMethod(&obj, functionNoExcept, &retInt);
+    QCOMPARE(retInt, functionNoExcept());
+}
+
+void tst_QMetaObject::invokeBind()
+{
+    QtTestObject obj;
+
+    struct {
+        int number;
+        QString string;
+    } results;
+
+    const auto function = [&results](int number, const QString &string) -> bool {
+        results.number = number;
+        results.string = string;
+        return true;
+    };
+
+    const int number = 42;
+    const QString string("Test");
+    const auto binding = std::bind(function, number, string);
+    bool ret = false;
+    QMetaObject::invokeMethod(&obj, binding, &ret);
+    QVERIFY(ret);
+    QCOMPARE(results.number, number);
+    QCOMPARE(results.string, string);
+}
+
 void tst_QMetaObject::normalizedSignature_data()
 {
     QTest::addColumn<QString>("signature");