FR-017-016 21.3 [type.traits] Allow efficient implementation of `std::conditional_t`

[wg21bot]

Please consider adopting P1715 which failed to be adopted for C++20, and C++23 and which has the potential of improving compile time significantly.

[brycelelbach]

The proposed resolution is to adopt P1715 https://github.com/cplusplus/papers/issues/481

[brycelelbach]

2023-02-06 13:00 to 15:15 Issaquah Library Evolution Meeting

P1715R1: Loosen restrictions on _t typedefs

2023-02-06 13:00 to 15:15 UTC-8 Issaquah Library Evolution Minutes

Champion: Jorg Brown (IP)

Chair: Bryce Adelstein Lelbach (IP) & Billy Baker (IP)

Minute Taker: Robert Leahy (IP)

Start: 2023-02-06 14:46 UTC-8

Does this paper have:

• Examples?
  • Yes.
• Field experience?
  • Some, but not of the entire proposal in a production Standard Library.
• Performance considerations?
  • Not in the paper - such performance data should be included.
• Discussion of prior art?
  • Yes.
• Changes Library Evolution previously requested?
  • Yes.
• Wording?
  • Yes.
• Breaking changes?
  • Yes. The impact hasn't been sufficiently evaluated.
• Feature test macro?
  • Not needed.
• Freestanding?
  • N/A.

__POLL: Send P1715R1 (Loosen restrictions on _t typedefs) to Library for C++23 as the resolution to C++23 National Body comment FR-017-016, classified as B2 - improvement.__

Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
6	10	5	4	1

Attendance: 23 (in person) + 14 (remote)

# of Authors: 1

Author Position: SF

Outcome: No consensus.

Break: 15:16

Resume: 15:31

__POLL: Send P1715R1 (Loosen restrictions on _t typedefs) to Library for C++23 as the resolution to C++23 National Body comment FR-017-016, classified as B2 - improvement.__

Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
6	11	2	7	2

Attendance: 25 (in person)

# of Authors: 1

Author Position: SF

Outcome: No consensus.

End: 15:58

Summary

We discussed this proposal to give implementations freedom to improve the compile time performance of the _t typedefs.

Some implementers said they'd be unable to use this freedom as it would break ABIs. Others said they hoped to use it in their unstable ABI modes. It is believed that some implementations may be able to make the change without breaking ABI. Also, new implementations could take advantage of the freedom.

There was a desire to learn more about the performance implications of this change. An addition to the paper with benchmark results would be helpful.

The proposed change could be source breaking in some cases. There was a general belief that the impact on real-world code would be small. More discussion and analysis of this in the paper may help.

Since there was an NB comment requesting this for C++23, we discussed whether we should try to squeeze it in. Some felt that there was no great urgency, and that adding it at this late stage might be risky. Ultimately, we did not have consensus to pursue it for C++23.

Next Steps

Revise P1715R1 (Loosen restrictions on _t typedefs) and return to Library Evolution for further design review for C++26.

Reject C++23 National Body comment FR-017-016 as we have no consensus for change.

[jensmaurer]

Rejected. There was no consensus for a change.