[libcxx] Fix PR 22468 - std::function<void()> does not accept non-void-returning functions

Summary:
The bug can be found here: http://llvm.org/bugs/show_bug.cgi?id=22468

`__invoke_void_return_wrapper` is needed to properly handle calling a function that returns a value but where the std::function return type is void. Without this '-Wsystem-headers' will cause `function::operator()(...)` to not compile. 

Reviewers: eugenis, K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7444

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@228705 91177308-0d34-0410-b5e6-96231b3b80d8

include/__functional_03

@@ -369,7 +369,8 @@ template<class _Fp, class _Alloc, class _Rp>
 _Rp
 __func<_Fp, _Alloc, _Rp()>::operator()()
 {
-    return __invoke(__f_.first());
+    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
+    return _Invoker::__call(__f_.first());
 }
 
 #ifndef _LIBCPP_NO_RTTI
@@ -452,7 +453,8 @@ template<class _Fp, class _Alloc, class _Rp, class _A0>
 _Rp
 __func<_Fp, _Alloc, _Rp(_A0)>::operator()(_A0 __a0)
 {
-    return __invoke(__f_.first(), __a0);
+    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
+    return _Invoker::__call(__f_.first(), __a0);
 }
 
 #ifndef _LIBCPP_NO_RTTI
@@ -535,7 +537,8 @@ template<class _Fp, class _Alloc, class _Rp, class _A0, class _A1>
 _Rp
 __func<_Fp, _Alloc, _Rp(_A0, _A1)>::operator()(_A0 __a0, _A1 __a1)
 {
-    return __invoke(__f_.first(), __a0, __a1);
+    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
+    return _Invoker::__call(__f_.first(), __a0, __a1);
 }
 
 #ifndef _LIBCPP_NO_RTTI
@@ -618,7 +621,8 @@ template<class _Fp, class _Alloc, class _Rp, class _A0, class _A1, class _A2>
 _Rp
 __func<_Fp, _Alloc, _Rp(_A0, _A1, _A2)>::operator()(_A0 __a0, _A1 __a1, _A2 __a2)
 {
-    return __invoke(__f_.first(), __a0, __a1, __a2);
+    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
+    return _Invoker::__call(__f_.first(), __a0, __a1, __a2);
 }
 
 #ifndef _LIBCPP_NO_RTTI

include/__functional_base

@@ -419,6 +419,26 @@ struct __invoke_return
     typedef decltype(__invoke(_VSTD::declval<_Tp>(), _VSTD::declval<_Args>()...)) type;
 };
 
+template <class _Ret>
+struct __invoke_void_return_wrapper
+{
+    template <class ..._Args>
+    static _Ret __call(_Args&&... __args)
+    {
+        return __invoke(_VSTD::forward<_Args>(__args)...);
+    }
+};
+
+template <>
+struct __invoke_void_return_wrapper<void>
+{
+    template <class ..._Args>
+    static void __call(_Args&&... __args)
+    {
+        __invoke(_VSTD::forward<_Args>(__args)...);
+    }
+};
+
 template <class _Tp>
 class _LIBCPP_TYPE_VIS_ONLY reference_wrapper
     : public __weak_result_type<_Tp>

include/__functional_base_03

@@ -995,6 +995,63 @@ struct __invoke_return2
                                                     _VSTD::declval<_A2>())) type;
 };
 
+template <class _Ret>
+struct __invoke_void_return_wrapper
+{
+    template <class _Fn>
+    static _Ret __call(_Fn __f)
+    {
+        return __invoke(__f);
+    }
+
+    template <class _Fn, class _A0>
+    static _Ret __call(_Fn __f, _A0& __a0)
+    {
+        return __invoke(__f, __a0);
+    }
+
+    template <class _Fn, class _A0, class _A1>
+    static _Ret __call(_Fn __f, _A0& __a0, _A1& __a1)
+    {
+        return __invoke(__f, __a0, __a1);
+    }
+
+    template <class _Fn, class _A0, class _A1, class _A2>
+    static _Ret __call(_Fn __f, _A0& __a0, _A1& __a1, _A2& __a2)
+    {
+        return __invoke(__f, __a0, __a1, __a2);
+    }
+};
+
+
+template <>
+struct __invoke_void_return_wrapper<void>
+{
+    template <class _Fn>
+    static void __call(_Fn __f)
+    {
+        __invoke(__f);
+    }
+
+    template <class _Fn, class _A0>
+    static void __call(_Fn __f, _A0& __a0)
+    {
+        __invoke(__f, __a0);
+    }
+
+    template <class _Fn, class _A0, class _A1>
+    static void __call(_Fn __f, _A0& __a0, _A1& __a1)
+    {
+        __invoke(__f, __a0, __a1);
+    }
+
+    template <class _Fn, class _A0, class _A1, class _A2>
+    static void __call(_Fn __f, _A0& __a0, _A1& __a1, _A2& __a2)
+    {
+        __invoke(__f, __a0, __a1, __a2);
+    }
+};
+
 template <class _Tp>
 class _LIBCPP_TYPE_VIS_ONLY reference_wrapper
     : public __weak_result_type<_Tp>

include/functional

@@ -1367,7 +1367,8 @@ template<class _Fp, class _Alloc, class _Rp, class ..._ArgTypes>
 _Rp
 __func<_Fp, _Alloc, _Rp(_ArgTypes...)>::operator()(_ArgTypes&& ... __arg)
 {
-    return __invoke(__f_.first(), _VSTD::forward<_ArgTypes>(__arg)...);
+    typedef __invoke_void_return_wrapper<_Rp> _Invoker;
+    return _Invoker::__call(__f_.first(), _VSTD::forward<_ArgTypes>(__arg)...);
 }
 
 #ifndef _LIBCPP_NO_RTTI
@@ -1429,7 +1430,7 @@ class _LIBCPP_TYPE_VIS_ONLY function<_Rp(_ArgTypes...)>
     template <class _Fp>
         struct __callable<_Fp, true>
         {
-            static const bool value =
+            static const bool value = is_same<void, _Rp>::value ||
                 is_convertible<typename __invoke_of<_Fp&, _ArgTypes...>::type,
                                _Rp>::value;
         };