[PATCH] D93355: Document that AlignedCharArrayUnion exists to work around an MSVC bug

[Reid Kleckner via Phabricator via llvm-commits]

rnk created this revision.
rnk added reviewers: dexonsmith, jfb.
rnk requested review of this revision.
Herald added a project: LLVM.

Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D93355

Files:
  llvm/include/llvm/Support/AlignOf.h


Index: llvm/include/llvm/Support/AlignOf.h
===================================================================
--- llvm/include/llvm/Support/AlignOf.h
+++ llvm/include/llvm/Support/AlignOf.h
@@ -20,9 +20,10 @@
 /// A suitably aligned and sized character array member which can hold elements
 /// of any type.
 ///
-/// These types may be arrays, structs, or any other types. This exposes a
-/// `buffer` member which can be used as suitable storage for a placement new of
-/// any of these types.
+/// 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)];


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