[libcxx-commits] [libcxx] [libc++] Applied `[[nodiscard]]` to Language Support (partially) (PR #169611)

[A. Jiang via libcxx-commits]

================
@@ -0,0 +1,91 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// Check that functions are marked [[nodiscard]]
+
+#include <compare>
+#include <coroutine>
+#include <functional>
+#include <initializer_list>
+
+#include "test_macros.h"
+
+void test() {
+#if TEST_STD_VER >= 20
+  { // <compare>
+    int x     = 94;
+    int y     = 82;
+    auto oRes = x <=> y;
+
+    std::is_eq(oRes);   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::is_neq(oRes);  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::is_lt(oRes);   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::is_lteq(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::is_gt(oRes);   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::is_gteq(oRes); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  }
+#endif
+
+#if TEST_STD_VER >= 20
+  { // <coroutine>
+    struct EmptyPromise {
+    } promise;
+
+    {
+      std::coroutine_handle<void> cr{};
+
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.address();
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      std::coroutine_handle<void>::from_address(&promise);
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.done();
+
+      std::hash<std::coroutine_handle<void>> hash;
+      hash(cr); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    }
+    {
+      std::coroutine_handle<EmptyPromise> cr;
+
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      std::coroutine_handle<EmptyPromise>::from_promise(promise);
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.address();
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      std::coroutine_handle<EmptyPromise>::from_address(&promise);
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.done();
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.promise();
+    }
+    {
+      std::coroutine_handle<std::noop_coroutine_promise> cr = std::noop_coroutine();
+
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.done();
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.promise();
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      cr.address();
+
+      // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+      std::noop_coroutine();
+    }
+  }
+#endif
+
+  { // <initializer_list>
+    std::initializer_list<int> il{94, 82, 49};
+
+    il.size();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    il.begin(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    il.end();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
----------------
frederick-vs-ja wrote:

Non-member `begin`/`end` were not changed. But IMO this is good, because they should be removed in all modes (by [P3016R6](https://wg21.link/p3016r6)) and we won't need to bother to delete lines here.

https://github.com/llvm/llvm-project/pull/169611