Avoid new warning C5267 for deprecated implicit copy ctor/assign

[StephanTLavavej]

@JonCavesMSFT just implemented a new off-by-default warning for VS 2022 17.7 Preview 1 (internal MSVC-PR-452406) that some internal teams are very interested in using. This warns about the deprecated case in WG21-N4928 [class.copy.ctor]/6 (emphasis mine):

If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defaulted (9.5). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor (D.8).

and [class.copy.assign]/2 (emphasis mine):

If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defaulted (9.5). The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor (D.8).

The warning (which uses the same number for copy ctors and copy assigns) looks like:

warning C5267: definition of implicit copy constructor for 'X' is deprecated because it has a user-provided destructor
warning C5267: definition of implicit assignment operator for 'X' is deprecated because it has a user-provided copy constructor

We can't enable the warning until 17.7 Preview 1 ships, but we can clean up the product and test code now. (When we enable the warning, the proper places to do so will likely be tests/std/tests/prefix.lst and tests/tr1/prefix.lst, unless libcxx receives numerous cleanups, in which case tests/universal_prefix.lst will be the place.)

I have filed VSO-1753270 "/clr C++20 alignas with defaulted SMFs emits fatal error C1193: an error expected in ...\yyaction.cpp(2976) not reached", which prevents one type from being cleaned up here (aligned_type in P0220R1_any).

Notes

• bitset
  • This is the only affected product code that I found.
  • Copy ctors are ABI-relevant (did you ever hear the tragedy of Darth tuple<> The Wise?). However, this should be ABI-safe because the copy ctor is being defaulted and therefore remains trivial.
  • I marked this as _CONSTEXPR23 to match the WP. (It is implicitly noexcept.)
  • One unrelated cleanup affecting nearby but different code: move reference::flip to match the WP's order.
• P0220R1_any
  • I've chosen to user-provide copy ctors that increment count, to keep it balanced with the destructor. They must be marked noexcept so that these types remain eligible for any's Small Object Optimization:

    STL/stl/inc/any

    Lines 51 to 53 in 86acfb1

    	template <class _Ty>
    	inline constexpr bool _Any_is_small = alignof(_Ty) <= alignof(max_align_t)
    	&& is_nothrow_move_constructible_v<_Ty> && sizeof(_Ty) <= _Any_small_space_size;

• P0220R1_optional
  • The last X also needs a defaulted default ctor (otherwise the defaulted copy ctor will suppress it).
• P0608R3_improved_variant_converting_constructor
  • convertible_bool didn't need a defaulted destructor at all, so I have simply dropped it.
• P0674R1_make_shared_for_arrays
  • Similarly, WeirdDeleter needs a defaulted default ctor.
• P0784R7_library_support_for_more_constexpr_containers
  • I've chosen to mark these defaulted functions as constexpr and noexcept for local consistency, although I could be persuaded otherwise.
• P0896R4_views_join
  • I noticed that non_trivially_destructible_input_iterator didn't have any way to construct a fresh object (it intentionally deletes its default ctor below). I've seen compilers warn about that, so it seems prudent to introduce a hypothetical ctor, which I've commented.

[CaseyCarter]

FWIW, I've submitted https://reviews.llvm.org/D144694 to mirror changes to portions of the any and optional tests that come from libc++. (Including my suggestions.)

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.