std::aligned_storage is deprecated in C++23

[StephanTLavavej]

I work on MSVC's C++ Standard Library, where we've recently implemented a C++23 paper that affects your project. (We regularly build many open-source projects to prevent compiler/library regressions, and this also allows us to notify projects of breaking changes that will affect them.)

The C++23 paper is https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1413r3.pdf "Deprecate std::aligned_storage and std::aligned_union". We've merged a PR (microsoft/STL#2583 ) that makes aligned_storage, aligned_storage_t, aligned_union, and aligned_union_t emit deprecation warnings when compiled in C++23 mode (/std:c++latest). This will be available in VS 2022 17.3 Preview 3. Although your project might not use /std:c++latest right now, we run such test coverage to identify issues that you might encounter in the future.

Examples of affected lines:

asio/asio/include/asio/detail/type_traits.hpp

Line 55 in f0a1e1c

using std::aligned_storage;

asio/asio/include/asio/execution/any_executor.hpp

Lines 1169 to 1172 in f0a1e1c

	typedef aligned_storage<
	sizeof(asio::detail::shared_ptr<void>) + sizeof(void*),
	alignment_of<asio::detail::shared_ptr<void> >::value
	>::type object_type;

(These emitted warnings when used by the cpprestsdk project.)

In general, the recommended replacement is to use the alignas keyword. We do provide an "escape hatch" macro to silence the deprecation warning, if necessary.