std::aligned_storage deprecated in C++23

[Colin MacLean]

C++23 deprecated std::aligned_storage: http://wg21.link/p1413. We make use of this in several places and these should be replaced with equivalent code that won’t trigger warnings once compilers implement p1413.

[Dan Bonachea]

As of release 2022.9.0 std::aligned_storage has been removed from our training materials, and in many use cases replaced with std::optional.

std::aligned_storage is still used in a few tests (for deprecated or undocumented serialization interfaces), and internally by a several library headers/modules (notably inside detail::raw_storage). We should eventually remove the remaining references (at least in C++23 mode) to avoid warnings and comply with the capricious whims of the C++ committee.

[Dan Bonachea]

Deferred until we see warnings with stakeholder interest

[Paul Hargrove]

Issue #606 was marked as a duplicate of this issue.

[Paul Hargrove]

Extracted from issue 606, which I've closed as a dup of this one:

Testing of g++ 13.1.0 with -std=c++23 yields the following warnings (truncated for brevity) when compiling even a trivial application such as test/hello_upcxx.cpp:

include/upcxx/future/core.hpp:388:43: warning: 'template<long unsigned int _Len, long unsigned int _Align> struct std::aligned_storage' is deprecated [-Wdeprecated-declarations]
include/upcxx/serialization.hpp:1274:23: warning: 'template<long unsigned int _Len, long unsigned int _Align> struct std::aligned_storage' is deprecated [-Wdeprecated-declarations]
include/upcxx/serialization.hpp:1767:23: warning: 'template<long unsigned int _Len, long unsigned int _Align> struct std::aligned_storage' is deprecated [-Wdeprecated-declarations]
include/upcxx/utility.hpp:329:19: warning: 'template<long unsigned int _Len, long unsigned int _Align> struct std::aligned_storage' is deprecated [-Wdeprecated-declarations]
include/upcxx/utility.hpp:379:19: warning: 'template<long unsigned int _Len, long unsigned int _Align> struct std::aligned_storage' is deprecated [-Wdeprecated-declarations]