[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

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.con/F.pass.cpp

@@ -81,4 +81,10 @@ int main()
     assert(globalMemCounter.checkOutstandingNewEq(0));
     assert(f.target<int (A::*)(int) const>() != 0);
     }
+    {
+      std::function<void(int)> f(&g);
+      assert(f);
+      assert(f.target<int(*)(int)>() != 0);
+      f(1);
+    }
 }

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.con/F_assign.pass.cpp

@@ -88,4 +88,11 @@ int main()
     assert(globalMemCounter.checkOutstandingNewEq(0));
     assert(f.target<int (A::*)(int) const>() != 0);
     }
+    {
+    std::function<void(int)> f;
+    f = &g;
+    assert(f);
+    assert(f.target<int(*)(int)>() != 0);
+    f(1);
+    }
 }

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.con/alloc_F.pass.cpp

@@ -89,4 +89,12 @@ int main()
     fun(10);
     }
 #endif
+    {
+    std::function<void(int)> fun(std::allocator_arg,
+                                 test_allocator<int(*)(int)>(),
+                                 &g);
+    assert(fun);
+    assert(fun.target<int(*)(int)>() != 0);
+    fun(10);
+    }
 }

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.inv/invoke_no_variadics.pass.cpp

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <functional>
+
+// class function<R()>
+
+// Test that we properly return both values and void for all non-variadic
+// overloads of function::operator()(...)
+
+#define _LIBCPP_HAS_NO_VARIADICS
+#include <functional>
+#include <cassert>
+
+int foo0() { return 42; }
+int foo1(int) { return 42; }
+int foo2(int, int) { return 42; }
+int foo3(int, int, int) { return 42; }
+
+int main()
+{
+    {
+        std::function<int()> f(&foo0);
+        assert(f() == 42);
+    }
+    {
+        std::function<int(int)> f(&foo1);
+        assert(f(1) == 42);
+    }
+    {
+        std::function<int(int, int)> f(&foo2);
+        assert(f(1, 1) == 42);
+    }
+    {
+        std::function<int(int, int, int)> f(&foo3);
+        assert(f(1, 1, 1) == 42);
+    }
+    {
+        std::function<void()> f(&foo0);
+        f();
+    }
+    {
+        std::function<void(int)> f(&foo1);
+        f(1);
+    }
+    {
+        std::function<void(int, int)> f(&foo2);
+        f(1, 1);
+    }
+    {
+        std::function<void(int, int, int)> f(&foo3);
+        f(1, 1, 1);
+    }
+}