runAsync: Fix build issue with MSVC2015

MSVC2015 has a bug that reference_wrappers have a call operator even if
the wrapped object isn't callable.

Change-Id: I02d96712418f55470034098caab5a61f687194ac
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/libs/utils/runextensions.h

@@ -143,7 +143,7 @@ struct resultTypeHasCallOperator<Function, false>
 
 template <typename Callable>
 struct resultTypeHasCallOperator<Callable, true>
-        : public resultTypeTakesArguments<decltype(&Callable::operator()), 1, (functionTraits<decltype(&Callable::operator())>::arity > 1)>
+        : public resultTypeTakesArguments<Callable, 0, (functionTraits<Callable>::arity > 0)>
 {
 };
 
@@ -168,6 +168,17 @@ struct resultType<Function &&> : public resultType<Function>
 {
 };
 
+// work around bug in MSVC 2015 where a reference_wrapper has a call operator even if the wrapped
+// object doesn't
+template <typename Function>
+struct resultType<std::reference_wrapper<Function>> : public resultType<Function>
+{
+};
+template <typename Function>
+struct resultType<std::reference_wrapper<const Function>> : public resultType<Function>
+{
+};
+
 /*
    Callable object that wraps a member function pointer with the object it
    will be called on.