[libcxx-commits] [libcxx] [libc++][functional] Implement `not_fn<NTTP>` (PR #86133)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++20, c++23
+
+// <functional>
+
+// Test the libc++ extension that std::not_fn<NTTP> is marked as [[nodiscard]].
+
+#include <functional>
+#include <type_traits>
+
+void test() {
+  using F = std::true_type;
+  std::not_fn<F{}>(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
----------------
ldionne wrote:

We should also be testing that actually calling the `not_fn` is nodiscard, since you marked it as such.

```
auto negated = std::not_fn<F{}>();
negated(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
```

https://github.com/llvm/llvm-project/pull/86133