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Document that AlignedCharArrayUnion exists to work around an MSVC bug
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Authored by rnk on Dec 15 2020, 4:00 PM.

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rnk requested review of this revision.Dec 15 2020, 4:00 PM

rnk created this revision.

LGTM, thanks!

This revision is now accepted and ready to land.Dec 15 2020, 4:01 PM

This revision was landed with ongoing or failed builds.Dec 15 2020, 4:03 PM

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	#include <type_traits>			#include <type_traits>

	namespace llvm {			namespace llvm {

	/// A suitably aligned and sized character array member which can hold elements			/// A suitably aligned and sized character array member which can hold elements
	/// of any type.			/// of any type.
	///			///
	/// These types may be arrays, structs, or any other types. This exposes a			/// This template is equivalent to std::aligned_union_t<1, ...>, but we cannot
	/// `buffer` member which can be used as suitable storage for a placement new of			/// use it due to a bug in the MSVC x86 compiler:
	/// any of these types.			/// https://github.com/microsoft/STL/issues/1533
				/// Using `alignas` here works around the bug.
	template <typename T, typename... Ts> struct AlignedCharArrayUnion {			template <typename T, typename... Ts> struct AlignedCharArrayUnion {
	using AlignedUnion = std::aligned_union_t<1, T, Ts...>;			using AlignedUnion = std::aligned_union_t<1, T, Ts...>;
	alignas(alignof(AlignedUnion)) char buffer[sizeof(AlignedUnion)];			alignas(alignof(AlignedUnion)) char buffer[sizeof(AlignedUnion)];
	};			};

	} // end namespace llvm			} // end namespace llvm

	#endif // LLVM_SUPPORT_ALIGNOF_H			#endif // LLVM_SUPPORT_ALIGNOF_H