Add [[nodiscard]] to std::async as part of P0600.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318889 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/thread/futures/futures.async/async.fail.cpp

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: c++98, c++03
+
+// <future>
+
+// template <class F, class... Args>
+//     future<typename result_of<F(Args...)>::type>
+//     async(F&& f, Args&&... args);
+
+// template <class F, class... Args>
+//     future<typename result_of<F(Args...)>::type>
+//     async(launch policy, F&& f, Args&&... args);
+
+
+#include <future>
+#include <atomic>
+#include <memory>
+#include <cassert>
+
+#include "test_macros.h"
+
+int foo (int x) { return x; }
+
+int main ()
+{
+	std::async(                    foo, 3); // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+	std::async(std::launch::async, foo, 3); // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+}