First part of P0600 - '[[nodiscard] in the standard library'. Mark the 'empty()' methods of all the containers as nodiscard. If you're calling empty() w/o looking at the result, you probably meanto to call 'clear()'. c++2a only

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318269 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/containers/sequences/array/empty.fail.cpp

@@ -0,0 +1,28 @@
+// -*- 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.
+//
+//===----------------------------------------------------------------------===//
+
+// <array>
+
+// class array
+
+// bool empty() const noexcept;
+
+// 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 <array>
+
+#include "test_macros.h"
+
+int main ()
+{
+    std::array<int, 1> c;
+    c.empty();  // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+}