Add two new macros: _LIBCPP_NODISCARD_AFTER_CXX17 and _LIBCPP_CONSTEXPR_AFTER_CXX17, along with a way to turn off the NODISCARD one: _LIBCPP_DISABLE_NODISCARD_AFTER_CXX17. No one is using these yet, but we will be ... soon

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318208 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/diagnostics/nodiscard.fail.cpp

@@ -0,0 +1,24 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// Test that _LIBCPP_NODISCARD_AFTER_CXX17 works
+//	#define _LIBCPP_NODISCARD_AFTER_CXX17 [[nodiscard]]
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+// UNSUPPORTED: clang-3.3, clang-3.4, clang-3.5, clang-3.6, clang-3.7, clang-3.8
+
+#include <__config>
+
+_LIBCPP_NODISCARD_AFTER_CXX17 int foo() { return 6; }
+
+int main ()
+{
+	foo();	// expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

test/libcxx/diagnostics/nodiscard.pass.cpp

@@ -0,0 +1,24 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// Test that _LIBCPP_NODISCARD_AFTER_CXX17 works
+//	#define _LIBCPP_NODISCARD_AFTER_CXX17 [[nodiscard]]
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+#define _LIBCPP_DISABLE_AFTER_CXX17_NODISCARD
+#include <__config>
+
+_LIBCPP_NODISCARD_AFTER_CXX17 int foo() { return 6; }
+
+int main ()
+{
+	foo();	// no error here!
+}