[libcxx-commits] [libcxx] [libc++] Simplify std::launder (PR #147985)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne updated https://github.com/llvm/llvm-project/pull/147985

>From 6f3806ae738c6cd7bba4116c90d4d587a60b0787 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Thu, 10 Jul 2025 17:29:52 +0200
Subject: [PATCH 1/2] [libc++] Simplify std::launder

---
 libcxx/include/__new/launder.h                           | 7 ++-----
 .../support.dynamic/ptr.launder/launder.types.verify.cpp | 9 ++++-----
 2 files changed, 6 insertions(+), 10 deletions(-)

diff --git a/libcxx/include/__new/launder.h b/libcxx/include/__new/launder.h
index 83d80015913d9..df9adbff149df 100644
--- a/libcxx/include/__new/launder.h
+++ b/libcxx/include/__new/launder.h
@@ -10,8 +10,6 @@
 #define _LIBCPP___NEW_LAUNDER_H
 
 #include <__config>
-#include <__type_traits/is_function.h>
-#include <__type_traits/is_void.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -20,15 +18,14 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Tp>
 [[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Tp* __launder(_Tp* __p) _NOEXCEPT {
-  static_assert(!(is_function<_Tp>::value), "can't launder functions");
-  static_assert(!is_void<_Tp>::value, "can't launder cv-void");
+  // The compiler diagnoses misuses of __builtin_launder, so we don't need to add any static_asserts.
   return __builtin_launder(__p);
 }
 
 #if _LIBCPP_STD_VER >= 17
 template <class _Tp>
 [[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI constexpr _Tp* launder(_Tp* __p) noexcept {
-  return std::__launder(__p);
+  return __builtin_launder(__p);
 }
 #endif
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/std/language.support/support.dynamic/ptr.launder/launder.types.verify.cpp b/libcxx/test/std/language.support/support.dynamic/ptr.launder/launder.types.verify.cpp
index 4f2b627b8601a..5f6e53f98ad34 100644
--- a/libcxx/test/std/language.support/support.dynamic/ptr.launder/launder.types.verify.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/ptr.launder/launder.types.verify.cpp
@@ -25,12 +25,11 @@ int main(int, char**) {
   (void)std::launder((void*)nullptr);
   (void)std::launder((const void*)nullptr);
   (void)std::launder((volatile void*)nullptr);
-  (void)std::launder(
-      (const volatile void*)nullptr); // expected-error-re@*:* 4 {{static assertion failed{{.*}}can't launder cv-void}}
-  // expected-error@*:* 0-4 {{void pointer argument to '__builtin_launder' is not allowed}}
+  (void)std::launder((const volatile void*)nullptr);
+  // expected-error@*:* 4 {{void pointer argument to '__builtin_launder' is not allowed}}
 
-  (void)std::launder(foo); // expected-error-re@*:* 1 {{static assertion failed{{.*}}can't launder functions}}
-  // expected-error@*:* 0-1 {{function pointer argument to '__builtin_launder' is not allowed}}
+  (void)std::launder(foo);
+  // expected-error@*:* 1 {{function pointer argument to '__builtin_launder' is not allowed}}
 
   return 0;
 }

>From 2f16bf5e1b53f0704d7f0041d158f3d5faaef710 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 15 Jul 2025 13:12:54 -0400
Subject: [PATCH 2/2] Update libcxx/include/__new/launder.h

---
 libcxx/include/__new/launder.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libcxx/include/__new/launder.h b/libcxx/include/__new/launder.h
index df9adbff149df..886f614eed2e7 100644
--- a/libcxx/include/__new/launder.h
+++ b/libcxx/include/__new/launder.h
@@ -18,7 +18,8 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Tp>
 [[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Tp* __launder(_Tp* __p) _NOEXCEPT {
-  // The compiler diagnoses misuses of __builtin_launder, so we don't need to add any static_asserts.
+  // The compiler diagnoses misuses of __builtin_launder, so we don't need to add any static_asserts
+  // to implement the Mandates.
   return __builtin_launder(__p);
 }