[libcxx-commits] [libcxx] [libc++] P2641R4: Checking if a `union` alternative is active (`std::is_within_lifetime`) (PR #107450)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,119 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <type_traits>
+
+// template <class T>
+//   consteval bool is_within_lifetime(const T*) noexcept; // C++26
+
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+#ifndef __cpp_lib_is_within_lifetime
+
+// Check that it doesn't exist if the feature test macro isn't defined (via ADL)
+template <class T>
+constexpr decltype(static_cast<void>(is_within_lifetime(std::declval<T>())), bool{}) is_within_lifetime_exists(int) {
+  return true;
+}
+template <class T>
+constexpr bool is_within_lifetime_exists(long) {
+  return false;
+}
+
+static_assert(!is_within_lifetime_exists<const std::integral_constant<bool, false>*>(0), "");
+
+#elif TEST_STD_VER < 26
+#  error __cpp_lib_is_within_lifetime defined before C++26
+#else // defined(__cpp_lib_is_within_lifetime) && TEST_STD_VER >= 26
----------------
ldionne wrote:

We already have tests generated for the feature-test macro automatically. Also the test that we don't define `is_within_lifetime` when the FTM is undefined seems unnecessary. IMO I'd just remove this `#if #else`.

https://github.com/llvm/llvm-project/pull/107450