P2231 Missing `constexpr` in `std::optional` and `std::variant`

[wg21bot]

P2231R0 Missing constexpr in std::optional and std::variant (Barry Revzin)

[brycelelbach]

2021-01-25 Library Evolution Telecon

P2231R0: Missing constexpr in std::optional and std::variant

2021-01-25 Library Evolution Telecon Minutes

Chair: Ben Craig

Champion: Barry Revzin

Minute Taker: Mark Hoemmen

SUMMARY:

In C++17, you can't change the active member of a union in a constexpr context. In C++20, you can. This paper takes advantage of that change to make optional and variant more constexpr than they were to begin with. optional was one of the motivating reasons for allowing constexpr union changes in the first place.

The next revision of the paper needs to bump some feature test macros.

POLL: Send the next revision of P2231 (P2231R1) (Missing constexpr in std::optional and std::variant) to the upcoming Winter electronic ballot for LWG for C++23, with priority bucket 2 (Improvement), with the addition of feature test macro bumps for __cpp_lib_optional and __cpp_lib_variant. __

Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
8	6	1	0	0

Attendance: 25

# of Authors: 1

Author Position: SF

Outcome: Consensus, forwarding to the electronic ballot.

OUTCOME:

Modify P2231R0 (Missing constexpr in optional and variant) by adding bumps of the feature test macros __cpp_lib_optional and __cpp_lib_variant, and then send the revised paper to electronic balloting to be forwarded to LWG for C++23, classified as an improvement of an existing feature (P0592R4 bucket 2 item).