P3074R4 trivial unions (was std::uninitialized<T>)

[wg21bot]

P3074R0 constexpr union lifetime (Barry Revzin)

[hanickadot]

EWG telecon 31th January:

P3074R1: We prefer language based solution over library based solution

SF/F/N/A/SA
5/4/4/2/1

Consensus.

[wg21bot]

P3074R1 std::uninitialized<T> (Barry Revzin)

[wg21bot]

P3074R2 std::uninitialized<T> (Barry Revzin)

[hanickadot]

EWG discussed P3074R2 in Tokyo on Thursday.

P3074 std::uninitialized: EWG is interested in solving presented problem (lazy initialization).

SF	F	N	A	SA
9	11	1	1	0

Consensus.

P3074 std::uninitialized: EWG prefers language solution to presented problem over std::uninitialized library type.

SF	F	N	A	SA
6	7	3	4	2

Not consensus.

P3074 std::uninitialized: Require to have language feature to appear only in limited context.

SF	F	N	A	SA
3	5	9	1	1

Not consensus.

[wg21bot]

P3074R3 trivial union (was std::uninitialized<T>) (Barry Revzin)

[cor3ntin]

As of R3, this no longer impacts library.

[jfbastien]

Seen in EWG in St Louis:

Poll: P3074R3 — trivial union (was std::uninitialized), we are interested in pursuing solution “trivial union”.

SF	F	N	A	SA
0	3	13	4	1

Poll: P3074R3 — trivial union (was std::uninitialized), we are interested in pursuing solution “just make it work”.

SF	F	N	A	SA
4	14	3	2	0

Winner!!!

Poll: P3074R3 — trivial union (was std::uninitialized), we would rather have a library solution.

SF	F	N	A	SA
4	3	6	10	0

[wg21bot]

P3074R4 trivial unions (was std::uninitialized<T>) (Barry Revzin)

[jfbastien]

Seen in EWG on Thursday:

Poll: P3074r4 — trivial unions (was std::uninitialized): forward to LEWG and CWG for inclusion in C++26.

SF	F	N	A	SA
8	14	0	0	0

Result: consensus in favor

[bstamour]

LWG looked at this in Wroclaw: Poll: LWG is happy with the library wording in P3074r4 for C++26?

|F|A|N|
|8|0|0|

[jensmaurer]

CWG 2024-11-22 in Wroclaw: Reviewed D3074R5.

This adds situations with undefined behavior that were previously ill-formed.

union U { std::string s; };
std::string f() {
  U u;  // previously ill-formed; now OK
  return u.s;  // undefined behavior
}

Whether the added undefined behavior in this low-level facility is worth the benefits of the proposed facility is a judgment call that EWG should make explicitly.