Tighten template on new QRunnable create method

Only instantiate with object/methods that are invokable with void and
returns void.

Change-Id: Iab2e43bb8e061e3875a6cca8e06ebbfbfa9e6fe8
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/thread/qrunnable.cpp

@@ -78,13 +78,16 @@ QRunnable::~QRunnable()
 */
 
 /*!
-    \fn template<typename Callable> QRunnable *QRunnable::create(Callable &&callableToRun);
+    \fn template<typename Callable, if_callable<Callable>> QRunnable *QRunnable::create(Callable &&callableToRun);
     \since 5.15
 
     Creates a QRunnable that calls \a callableToRun in run().
 
     Auto-deletion is enabled by default.
 
+    \note This function participates in overload resolution only if \c Callable
+    is a function or function object which can be called with zero arguments.
+
     \note In Qt versions prior to 6.6, this method took copyable functions only.
 
     \sa run(), autoDelete()

src/corelib/thread/qrunnable.h

@@ -24,6 +24,9 @@ public:
     static QRunnable *create(std::function<void()> functionToRun);
 #endif
     template <typename Callable>
+    using if_callable = std::enable_if_t<std::is_invocable_r_v<void, Callable>, bool>;
+
+    template <typename Callable, if_callable<Callable> = true>
     static QRunnable *create(Callable &&functionToRun);
     bool autoDelete() const { return m_autoDelete; }
     void setAutoDelete(bool autoDelete) { m_autoDelete = autoDelete; }
@@ -76,7 +79,7 @@ public:
     }
 };
 
-template <typename Callable>
+template <typename Callable, QRunnable::if_callable<Callable>>
 QRunnable *QRunnable::create(Callable &&functionToRun)
 {
     return new QGenericRunnable(