[llvm] [Support] Replace deprecated std::aligned_union, NFCI. (PR #127417)

[Jonas Hahnfeld via llvm-commits]

https://github.com/hahnjo created https://github.com/llvm/llvm-project/pull/127417

All `std::aligned_*` are deprecated in C++23. Implement the replacement suggested in [P1413R3](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1413r3.pdf) using `alignas` and `std::max`.

>From 6111896d93e8b8847d346ec4096d3ea65b8d34fa Mon Sep 17 00:00:00 2001
From: Jonas Hahnfeld <hahnjo at hahnjo.de>
Date: Sun, 16 Feb 2025 21:58:41 +0100
Subject: [PATCH] [Support] Replace deprecated std::aligned_union, NFCI.

All std::aligned_* are deprecated in C++23. Implement the replacement
suggested in P1413R3 using alignas and std::max.
---
 llvm/include/llvm/Support/AlignOf.h | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/llvm/include/llvm/Support/AlignOf.h b/llvm/include/llvm/Support/AlignOf.h
index f586d7f182aab..635bb8b101ef9 100644
--- a/llvm/include/llvm/Support/AlignOf.h
+++ b/llvm/include/llvm/Support/AlignOf.h
@@ -13,20 +13,14 @@
 #ifndef LLVM_SUPPORT_ALIGNOF_H
 #define LLVM_SUPPORT_ALIGNOF_H
 
-#include <type_traits>
+#include <algorithm>
 
 namespace llvm {
 
 /// A suitably aligned and sized character array member which can hold elements
 /// of any type.
-///
-/// This template is equivalent to std::aligned_union_t<1, ...>, but we cannot
-/// use it due to a bug in the MSVC x86 compiler:
-/// https://github.com/microsoft/STL/issues/1533
-/// Using `alignas` here works around the bug.
 template <typename T, typename... Ts> struct AlignedCharArrayUnion {
-  using AlignedUnion = std::aligned_union_t<1, T, Ts...>;
-  alignas(alignof(AlignedUnion)) char buffer[sizeof(AlignedUnion)];
+  alignas(T) alignas(Ts...) char buffer[std::max({sizeof(T), sizeof(Ts)...})];
 };
 
 } // end namespace llvm