P2198R1 Freestanding Feature-Test Macros and Implementation-Defined Extensions (Ben Craig)

P2198R2 Freestanding Feature-Test Macros and Implementation-Defined Extensions (Ben Craig)

P2198R3 Freestanding Feature-Test Macros and Implementation-Defined Extensions (Ben Craig)

2021-11-29 Library Evolution Telecon

P2198R3: Freestanding Feature-Test Macros and Implementation-Defined Extensions

2021-11-29 Library Evolution Telecon Minutes

Chair: Bryce Adelstein Lelbach

Champion: Ben Craig

Minute Taker: Inbal Levi

Start: 2021-11-29 11:56 Pacific

Does this proposal have:

• Examples?
  • Yes.
• Implementation experience?
  • No.
• Usage experience?
  • No.
• Deployment experience?
  • No.
• Discussion of prior art?
  • Yes.
• Changes Library Evolution previously requested?
  • Yes.
• Wording?
  • Yes.
• Feature test macro?
  • Yes (lol).

What's in this paper:

• Fixes old feature test macros
• Bumps old feature test macros
• Adds new per-header feature test macros
• Proposes feature test macro policy
• Allows impls to provide extra stuff
• Allows impls to =delete extra stuff

Open Questions:

• Do we want per-header freestanding feature test macros?
  • Why were we previously split on this?

If the C++20 feature macro covers things that are freestanding in C++20, then we don't need to bump the macros. If they were not freestanding in C++20, then they need to be bumped.

Instead of introducing new feature test macros, why not have a feature test macro that says "my feature test macros don't lie"? Then if that's true, you could use the regular feature test macros.

Topics to Polls:

• Add __cpp_lib_freestanding_* per-header feature test macros from P2198R3.
• Adopt the proposed feature test macro policy from P2198R3.
  • Has the Feature Test Macro study group (SG10) discussed this? Not yet, they were informed of this last year and haven't had a chance to look at it.

POLL: Adopt the proposed freestanding feature test macro policy from P2198R3 (to be confirmed with a Library Evolution electronic poll).

Attendance: 18

# of Authors: 1

Author Position: SF

Outcome: Weak consensus in favor.

Summary

While this is the first time Library Evolution has reviewed P2198, the paper is a continuation of P1642, and deals with the topic of feature testing macros for freestanding.

P2198 does the following:

• Fixes old feature test macros for freestanding.
• Bumps old feature test macros for freestanding.
• Adds new per-header freestanding feature test macros.
• Proposes freestanding feature test macro policy.
• Allows implementations to provide extra facilities in freestanding.
• Allows implementations to = delete extra stuff.

Much of our discussion revolved around the proposed bumps of old feature test macros. We determined that some of the proposed bumps were unnecessary, because they were for features that were already freestanding in C++20.

It was suggested that instead of bumping old feature test macros, we could add a new "does not lie in freestanding mode" feature test macro which, if true, indicates that the other feature test macros give the correct answers in freestanding mode.

We also discussed the proposed policy for freestanding feature test macros. We had consensus to adopt the policy, pending confirmation via electronic poll.

Outcome

Bring a revision of P2198R3 (Freestanding Feature-Test Macros and Implementation-Defined Extensions) that explores a "does not lie in freestanding mode" feature test macro to Library Evolution for further design review.

Adopt the proposed freestanding feature test macro policy from P2198R3 (to be confirmed with a Library Evolution electronic poll).

P2198R4 Freestanding Feature-Test Macros and Implementation-Defined Extensions (Ben Craig)

2021-12 Library Evolution Electronic Poll Outcomes

POLL: Adopt the freestanding feature test macro policy from [P2198R3] (Freestanding Feature-Test Macros And Implementation-Defined Extensions C++23).

Consensus in favor, forwarded to LWG

P2198R5 Freestanding Feature-Test Macros and Implementation-Defined Extensions (Ben Craig)

Author is not available to support updates and review in time remaining for c++23 -- re-targeting to c++26.

LWG reviewed P2198R7 today. https://wiki.edg.com/bin/view/Wg21telecons2022/P2198-20221214

Poll: Forward P2198R7 for C++26, at some future plenary.

P2198R6 Freestanding Feature-Test Macros and Implementation-Defined Extensions (Ben Craig)

P2198R7 Freestanding Feature-Test Macros and Implementation-Defined Extensions (Ben Craig)