Add missed test in r276090.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@276091 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/func.wrap/func.wrap.func/derive_from.pass.cpp

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+// See https://llvm.org/bugs/show_bug.cgi?id=20002
+
+#include <functional>
+#include <type_traits>
+
+using Fn = std::function<void()>;
+struct S : Fn { using function::function; };
+
+int main() {
+    S f1( Fn{} );
+    S f2(std::allocator_arg, std::allocator<void>{}, Fn{});
+}