[libc++] mark std::expected as nodiscard

libcxx/include/__cxx03/__expected/expected.h

@@ -58,7 +58,7 @@ _LIBCPP_PUSH_MACROS
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp, class _Err>
-class expected;
+class [[nodiscrad]] expected;
 
 template <class _Tp>
 struct __is_std_expected : false_type {};

libcxx/include/__expected/expected.h

@@ -60,7 +60,7 @@ _LIBCPP_PUSH_MACROS
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp, class _Err>
-class expected;
+class [[nodiscard]] expected;
 
 template <class _Tp>
 struct __is_std_expected : false_type {};

libcxx/test/libcxx/utilities/expected/expected.expected/nodiscard.verify.cpp

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <expected>
+
+// Test that std::expected generates [[nodiscard]] warnings
+
+#include <expected>
+
+std::expected<int, int> returns_expected() {
+  return std::expected<int, int>(5);
+}
+
+std::expected<void, int> returns_expected_void() {
+  return std::expected<void, int>();
+}
+
+void test() {
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  returns_expected();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  returns_expected_void();
+}