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

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+
+// Type of `std::not_fn<NTTP>()` is always empty.
+
+#include <functional>
+#include <type_traits>
+
+struct NonEmptyFunctionObject {
+  bool val = true;
+  bool operator()() const; // not defined
+};
+
+void test() {
+  using ResultWithEmptyFuncObject = decltype(std::not_fn<std::false_type{}>());
+  static_assert(std::is_empty_v<ResultWithEmptyFuncObject>);
+
+  using ResultWithNotEmptyFuncObject = decltype(std::not_fn<NonEmptyFunctionObject{}>());
+  static_assert(std::is_empty_v<ResultWithNotEmptyFuncObject>);
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ldionne wrote:

This doesn't need to be in a function, it can be at file scope!

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