GB-081 17.14.4 [stdalign.h.syn] Deprecate contents of `<stdalign.h>`

[wg21bot]

C2x defines alignas as a keyword, so <stdalign.h> is empty in C2x. C++23 should deprecate the __alignas_is_defined macro now, rather than wait until a future C++ standard is based on C2x. That gives users longer to prepare for the removal of the macro.

Deprecate __alignas_is_defined and move it to Annex D. Maybe keep a note in [stdalign.h.syn]
that the macro is present but deprecated.

[jfbastien]

Related: #453
This deprecation would obviate the need for #502

[billy-baker]

2022-11-08 13:00 to 15:00 UTC-10 Kona Library Evolution Meeting

GB-081: Deprecate contents of <stdalign.h>

2022-11-08 13:00 to 15:00 UTC-10 Kona Library Evolution Minutes

Champion: Jonathan Wakely (remote)

Chair: Billy Baker & Nevin Liber

Minute Taker: Mark Zeren

POLL: Accept GB-81 to deprecate __alignas_is_defined and move it to Annex D.

Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
9	8	2	0	0

Attendance: 17 (in person) + 6 (remote)

# of Authors: 1

Author Position: SF

Outcome: Strong consensus to accept GB-81.

Next Steps

Take a Library Evolution to send C++23 NB comment GB-081 (Deprecate contents of <stdalign.h>) to Library for C++23, classified as B2 - improvement.

[jwakely]

This will be addressed by LWG 3827