[libc++] P2641R4: Checking if a union alternative is active (std::is_within_lifetime)

MitalAshok · philnik777 · commit babe9fca6481 · 2025-10-28T17:48:33.000+01:00
Address comments

Address comments
diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
@@ -474,7 +474,7 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_is_virtual_base_of``                           ``202406L``
     ---------------------------------------------------------- -----------------
-    ``__cpp_lib_is_within_lifetime``                           *unimplemented*
+    ``__cpp_lib_is_within_lifetime``                           ``202306L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_linalg``                                       *unimplemented*
     ---------------------------------------------------------- -----------------
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
@@ -877,6 +877,7 @@ set(files
   __type_traits/is_valid_expansion.h
   __type_traits/is_void.h
   __type_traits/is_volatile.h
+  __type_traits/is_within_lifetime.h
   __type_traits/lazy.h
   __type_traits/make_32_64_or_128_bit.h
   __type_traits/make_const_lvalue_ref.h
diff --git a/libcxx/include/__type_traits/is_within_lifetime.h b/libcxx/include/__type_traits/is_within_lifetime.h
@@ -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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_WITHIN_LIFETIME_H
+#define _LIBCPP___TYPE_TRAITS_IS_WITHIN_LIFETIME_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26 && __has_builtin(__builtin_is_within_lifetime)
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI consteval bool is_within_lifetime(const _Tp* __p) noexcept {
+  return __builtin_is_within_lifetime(__p);
+}
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_WITHIN_LIFETIME_H
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
@@ -350,6 +350,7 @@ module std_core [system] {
       header "__type_traits/is_volatile.h"
       export std_core.type_traits.integral_constant
     }
+    module is_within_lifetime                         { header "__type_traits/is_within_lifetime.h" }
     module lazy                                       { header "__type_traits/lazy.h" }
     module make_32_64_or_128_bit                      { header "__type_traits/make_32_64_or_128_bit.h" }
     module make_const_lvalue_ref                      { header "__type_traits/make_const_lvalue_ref.h" }
diff --git a/libcxx/include/type_traits b/libcxx/include/type_traits
@@ -454,6 +454,10 @@ namespace std
       template<class B> inline constexpr bool negation_v
         = negation<B>::value;                                   // since C++17
 
+      // [meta.const.eval], constant evaluation context
+      constexpr bool is_constant_evaluated() noexcept;                   // C++20
+      template<class T>
+        consteval bool is_within_lifetime(const T*) noexcept;            // C++26
 }
 
 */
@@ -559,6 +563,10 @@ namespace std
 #    include <__type_traits/reference_converts_from_temporary.h>
 #  endif
 
+#  if _LIBCPP_STD_VER >= 26
+#    include <__type_traits/is_within_lifetime.h>
+#  endif
+
 #  include <version>
 
 #  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
diff --git a/libcxx/include/version b/libcxx/include/version
@@ -582,7 +582,9 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # if __has_builtin(__builtin_is_virtual_base_of)
 #   define __cpp_lib_is_virtual_base_of                 202406L
 # endif
-// # define __cpp_lib_is_within_lifetime                   202306L
+# if __has_builtin(__builtin_is_within_lifetime)
+#   define __cpp_lib_is_within_lifetime                 202306L
+# endif
 // # define __cpp_lib_linalg                               202311L
 # undef  __cpp_lib_mdspan
 # define __cpp_lib_mdspan                               202406L
diff --git a/libcxx/modules/std/type_traits.inc b/libcxx/modules/std/type_traits.inc
@@ -330,6 +330,9 @@ export namespace std {
 
   // [meta.const.eval], constant evaluation context
   using std::is_constant_evaluated;
+#if _LIBCPP_STD_VER >= 26 && __has_builtin(__builtin_is_within_lifetime)
+  using std::is_within_lifetime;
+#endif
 
   // [depr.meta.types]
   using std::aligned_storage;
diff --git a/libcxx/test/libcxx/utilities/meta/is_within_lifetime.verify.cpp b/libcxx/test/libcxx/utilities/meta/is_within_lifetime.verify.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: clang-18, clang-19, gcc-14, apple-clang-16
+
+// <type_traits>
+
+// LWG4138 <https://cplusplus.github.io/LWG/issue4138>
+// std::is_within_lifetime shouldn't work when a function type is
+// explicitly specified, even if it isn't evaluated
+
+#include <type_traits>
+
+template <class T>
+consteval bool checked_is_within_lifetime(T* p) {
+  return p ? std::is_within_lifetime<T>(p) : false;
+}
+static_assert(!checked_is_within_lifetime<int>(nullptr));
+static_assert(!checked_is_within_lifetime<void()>(nullptr));
+// expected-error@*:* {{function pointer argument to '__builtin_is_within_lifetime' is not allowed}}
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/type_traits.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/type_traits.version.compile.pass.cpp
@@ -918,7 +918,7 @@
 #    endif
 #  endif
 
-#  if !defined(_LIBCPP_VERSION)
+#  if __has_builtin(__builtin_is_within_lifetime)
 #    ifndef __cpp_lib_is_within_lifetime
 #      error "__cpp_lib_is_within_lifetime should be defined in c++26"
 #    endif
@@ -927,7 +927,7 @@
 #    endif
 #  else
 #    ifdef __cpp_lib_is_within_lifetime
-#      error "__cpp_lib_is_within_lifetime should not be defined because it is unimplemented in libc++!"
+#      error "__cpp_lib_is_within_lifetime should not be defined when the requirement '__has_builtin(__builtin_is_within_lifetime)' is not met!"
 #    endif
 #  endif
 
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
@@ -7256,7 +7256,7 @@
 #    endif
 #  endif
 
-#  if !defined(_LIBCPP_VERSION)
+#  if __has_builtin(__builtin_is_within_lifetime)
 #    ifndef __cpp_lib_is_within_lifetime
 #      error "__cpp_lib_is_within_lifetime should be defined in c++26"
 #    endif
@@ -7265,7 +7265,7 @@
 #    endif
 #  else
 #    ifdef __cpp_lib_is_within_lifetime
-#      error "__cpp_lib_is_within_lifetime should not be defined because it is unimplemented in libc++!"
+#      error "__cpp_lib_is_within_lifetime should not be defined when the requirement '__has_builtin(__builtin_is_within_lifetime)' is not met!"
 #    endif
 #  endif
 
diff --git a/libcxx/test/std/utilities/meta/meta.const.eval/is_within_lifetime.compile.pass.cpp b/libcxx/test/std/utilities/meta/meta.const.eval/is_within_lifetime.compile.pass.cpp
@@ -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
+
+// <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));
+  }
+  {
+    S s; // uninitialised
+    assert(std::is_within_lifetime(&s));
+  }
+
+  return true;
+}
+static_assert(f());
+
+// Check that it is a consteval (and consteval-propagating) function
+// (i.e., taking the address of below will fail because it will be an immediate function)
+template <typename T>
+constexpr void does_escalate(T p) {
+  std::is_within_lifetime(p);
+}
+template <typename T, void (*)(T) = &does_escalate<T>>
+constexpr bool check_escalated(int) {
+  return false;
+}
+template <typename T>
+constexpr bool check_escalated(long) {
+  return true;
+}
+static_assert(check_escalated<int*>(0), "");
+static_assert(check_escalated<void*>(0), "");
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
@@ -863,7 +863,8 @@ def add_version_header(tc):
                 "c++26": 202306  # P2641R4 Checking if a union alternative is active
             },
             "headers": ["type_traits"],
-            "unimplemented": True,
+            "test_suite_guard": "__has_builtin(__builtin_is_within_lifetime)",
+            "libcxx_guard": "__has_builtin(__builtin_is_within_lifetime)",
         },
         {
             "name": "__cpp_lib_jthread",

Original file line number	Diff line number	Diff line change
`@@ -350,6 +350,7 @@ module std_core [system] {`
`350`	`350`	`header "__type_traits/is_volatile.h"`
`351`	`351`	`export std_core.type_traits.integral_constant`
`352`	`352`	`}`
	`353`	`+ module is_within_lifetime { header "__type_traits/is_within_lifetime.h" }`
`353`	`354`	`module lazy { header "__type_traits/lazy.h" }`
`354`	`355`	`module make_32_64_or_128_bit { header "__type_traits/make_32_64_or_128_bit.h" }`
`355`	`356`	`module make_const_lvalue_ref { header "__type_traits/make_const_lvalue_ref.h" }`