functionTraits: Add reusable "functionTakesArgument" type

Instead of doing the dispatch "function has enough arguments" +
"argument is of correct type" for runAsync specifically, extract a
reusable functionTakesArgument<Function, unsigned index, Type>, which is
either of type std::true_type or std::false_type.

Change-Id: Iecfa867a0230d29fd96ae500423a5bdd1d475106
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/libs/utils/functiontraits.h

@@ -29,6 +29,26 @@
 
 namespace Utils {
 
+/*
+    struct functionTraits<Function>
+    {
+        using ResultType; // Return type of Function
+        struct argument<unsigned index>
+        {
+            using type; // type of Function argument at index (starting with 0)
+        }
+    }
+
+    struct functionTakesArgument<Function, unsigned index, ArgumentType>;
+
+    Is derived from std::true_type if Function takes an argument of type ArgumentType at index.
+    Otherwise derived from std::false_type.
+*/
+
+////////////////////
+// functionTraits
+////////////////////
+
 // for callables. defined below.
 template <typename Callable>
 struct functionTraits;
@@ -135,4 +155,34 @@ struct functionTraits<Callable&&> : public functionTraits<Callable>
 {
 };
 
+////////////////////
+// functionTakesArgument
+////////////////////
+namespace Internal {
+
+template <typename HasArity/*true_type or false_type*/,
+          typename Function, unsigned index, typename T>
+struct functionTakesArgumentArityDispatch;
+
+template <typename Function, unsigned index, typename T>
+struct functionTakesArgumentArityDispatch<std::false_type, Function, index, T>
+        : public std::false_type
+{
+};
+
+template <typename Function, unsigned index, typename T>
+struct functionTakesArgumentArityDispatch<std::true_type, Function, index, T>
+        : public std::is_same<T, typename functionTraits<Function>::template argument<index>::type>
+{
+};
+
+} // Internal
+
+template <typename Function, unsigned index, typename T>
+struct functionTakesArgument : public Internal::functionTakesArgumentArityDispatch<
+        std::integral_constant<bool, (functionTraits<Function>::arity > index)>,
+        Function, index, T>
+{
+};
+
 } // Utils

src/libs/utils/runextensions.h

@@ -260,23 +260,6 @@ void runAsyncQFutureInterfaceDispatch(std::false_type, QFutureInterface<ResultTy
                                futureInterface, std::forward<Function>(function), std::forward<Args>(args)...);
 }
 
-// function that takes at least one argument which could be QFutureInterface
-template <typename ResultType, typename Function, typename... Args>
-void runAsyncArityDispatch(std::true_type, QFutureInterface<ResultType> futureInterface, Function &&function, Args&&... args)
-{
-    runAsyncQFutureInterfaceDispatch(std::is_same<QFutureInterface<ResultType>&,
-                                                  typename functionTraits<Function>::template argument<0>::type>(),
-                                     futureInterface, std::forward<Function>(function), std::forward<Args>(args)...);
-}
-
-// function that does not take an argument, so it does not take a QFutureInterface
-template <typename ResultType, typename Function, typename... Args>
-void runAsyncArityDispatch(std::false_type, QFutureInterface<ResultType> futureInterface, Function &&function, Args&&... args)
-{
-    runAsyncQFutureInterfaceDispatch(std::false_type(),
-                                     futureInterface, std::forward<Function>(function), std::forward<Args>(args)...);
-}
-
 // function, function pointer, or other callable object that is no member pointer
 template <typename ResultType, typename Function, typename... Args,
           typename = typename std::enable_if<
@@ -284,8 +267,8 @@ template <typename ResultType, typename Function, typename... Args,
               >::type>
 void runAsyncMemberDispatch(QFutureInterface<ResultType> futureInterface, Function &&function, Args&&... args)
 {
-    runAsyncArityDispatch(std::integral_constant<bool, (functionTraits<Function>::arity > 0)>(),
-                          futureInterface, std::forward<Function>(function), std::forward<Args>(args)...);
+    runAsyncQFutureInterfaceDispatch(functionTakesArgument<Function, 0, QFutureInterface<ResultType>&>(),
+                                     futureInterface, std::forward<Function>(function), std::forward<Args>(args)...);
 }
 
 // Function = member function