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

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/147985

Both Clang and GCC diagnose invalid calls to `__builtin_launder`, which causes duplicate diagnostics when using `std::launder` in an invalid way. While the diagnostic message for the builtin isn't perferct, it's definitely good enough to understand the problem and adding our own diagnostic doesn't really make things any clearer. Because of that, this patch simply removes the `static_assert`s and lets the compiler handle diagnosing incorrect arguments instead. This not only simplifies our implementation, but also improves compile times a bit, since we avoid instantiating some type traits.


>From f1bcc045a35790e0d15c9c7d5915904b532f16fd 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] [libc++] Simplify std::launder

---
 libcxx/include/__new/launder.h                           | 6 +-----
 .../support.dynamic/ptr.launder/launder.types.verify.cpp | 9 ++++-----
 2 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/libcxx/include/__new/launder.h b/libcxx/include/__new/launder.h
index 83d80015913d9..78a7a415625e0 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 +18,13 @@
 _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");
   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;
 }