[libcxx-commits] [libcxx] [libc++] P2641R4: Checking if a `union` alternative is active (`std::is_within_lifetime`) (PR #165243)

[Nikolas Klauser via libcxx-commits]

================
@@ -0,0 +1,148 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+// UNSUPPORTED: gcc-15, apple-clang-17
+
+// <type_traits>
+
+// template <class T>
+//   consteval bool is_within_lifetime(const T*) noexcept; // C++26
+
+#include <cassert>
+#include <type_traits>
+#include <utility>
+
+#include "test_macros.h"
+
+ASSERT_SAME_TYPE(decltype(std::is_within_lifetime(std::declval<int*>())), bool);
+ASSERT_SAME_TYPE(decltype(std::is_within_lifetime(std::declval<const int*>())), bool);
+ASSERT_SAME_TYPE(decltype(std::is_within_lifetime(std::declval<void*>())), bool);
+ASSERT_SAME_TYPE(decltype(std::is_within_lifetime(std::declval<const void*>())), bool);
+
+ASSERT_NOEXCEPT(std::is_within_lifetime(std::declval<int*>()));
+ASSERT_NOEXCEPT(std::is_within_lifetime(std::declval<const int*>()));
+ASSERT_NOEXCEPT(std::is_within_lifetime(std::declval<void*>()));
+ASSERT_NOEXCEPT(std::is_within_lifetime(std::declval<const void*>()));
+
+template <class T>
+concept is_within_lifetime_exists = requires(T t) { std::is_within_lifetime(t); };
+
+struct S {};
+
+static_assert(is_within_lifetime_exists<int*>);
+static_assert(is_within_lifetime_exists<const int*>);
+static_assert(is_within_lifetime_exists<void*>);
+static_assert(is_within_lifetime_exists<const void*>);
+static_assert(!is_within_lifetime_exists<int>);               // Not a pointer
+static_assert(!is_within_lifetime_exists<decltype(nullptr)>); // Not a pointer
+static_assert(!is_within_lifetime_exists<void() const>);      // Not a pointer
+static_assert(!is_within_lifetime_exists<int S::*>);          // Doesn't accept pointer-to-data-member
+static_assert(!is_within_lifetime_exists<void (S::*)()>);     // Doesn't accept pointer-to-member-function
+static_assert(!is_within_lifetime_exists<void (*)()>);        // Doesn't match `const T*`
+
+consteval bool f() {
+  // Test that it works with global variables whose lifetime is in a
+  // different constant expression
+  {
+    static constexpr int i = 0;
+    static_assert(std::is_within_lifetime(&i));
+    // (Even when cast to a different type)
+    static_assert(std::is_within_lifetime(const_cast<int*>(&i)));
+    static_assert(std::is_within_lifetime(static_cast<const void*>(&i)));
+    static_assert(std::is_within_lifetime(static_cast<void*>(const_cast<int*>(&i))));
+    static_assert(std::is_within_lifetime<const int>(&i));
+    static_assert(std::is_within_lifetime<int>(const_cast<int*>(&i)));
+    static_assert(std::is_within_lifetime<const void>(static_cast<const void*>(&i)));
+    static_assert(std::is_within_lifetime<void>(static_cast<void*>(const_cast<int*>(&i))));
+  }
+
+  {
+    static constexpr union {
+      int member1;
+      int member2;
+    } u{.member2 = 1};
+    static_assert(!std::is_within_lifetime(&u.member1) && std::is_within_lifetime(&u.member2));
+  }
+
+  // Test that it works for varibles inside the same constant expression
+  {
+    int i = 0;
+    assert(std::is_within_lifetime(&i));
+    // (Even when cast to a different type)
+    assert(std::is_within_lifetime(const_cast<int*>(&i)));
+    assert(std::is_within_lifetime(static_cast<const void*>(&i)));
+    assert(std::is_within_lifetime(static_cast<void*>(const_cast<int*>(&i))));
+    assert(std::is_within_lifetime<const int>(&i));
+    assert(std::is_within_lifetime<int>(const_cast<int*>(&i)));
+    assert(std::is_within_lifetime<const void>(static_cast<const void*>(&i)));
+    assert(std::is_within_lifetime<void>(static_cast<void*>(const_cast<int*>(&i))));
+  }
+  // Anonymous union
+  {
+    union {
+      int member1;
+      int member2;
+    };
+    assert(!std::is_within_lifetime(&member1) && !std::is_within_lifetime(&member2));
+    member1 = 1;
+    assert(std::is_within_lifetime(&member1) && !std::is_within_lifetime(&member2));
+    member2 = 1;
+    assert(!std::is_within_lifetime(&member1) && std::is_within_lifetime(&member2));
+  }
+  // Variant members
+  {
+    struct X {
+      union {
+        int member1;
+        int member2;
+      };
+    } x;
+    assert(!std::is_within_lifetime(&x.member1) && !std::is_within_lifetime(&x.member2));
+    x.member1 = 1;
+    assert(std::is_within_lifetime(&x.member1) && !std::is_within_lifetime(&x.member2));
+    x.member2 = 1;
+    assert(!std::is_within_lifetime(&x.member1) && std::is_within_lifetime(&x.member2));
+  }
+  // Unions
+  {
+    union X {
+      int member1;
+      int member2;
+    } x;
+    assert(!std::is_within_lifetime(&x.member1) && !std::is_within_lifetime(&x.member2));
+    x.member1 = 1;
+    assert(std::is_within_lifetime(&x.member1) && !std::is_within_lifetime(&x.member2));
+    x.member2 = 1;
+    assert(!std::is_within_lifetime(&x.member1) && std::is_within_lifetime(&x.member2));
+  }
----------------
philnik777 wrote:

I don't think that's necessary. The different ways to stop and start lifetimes seems like something the compiler should make sure work. This is more of a compiler than a library feature after all.

https://github.com/llvm/llvm-project/pull/165243