US235,US238 22.02.7 [unord.reg] p11,17 Heterogenous lookup using Hash::transparent_key_equal is problematic P1690

[wg21bot]

C++20 design fix: the use of Hash::transparent_key_equal to enable heterogeneous lookup for unordered associative containers deviates from prior art, does not address the incompatibility concerns raised in the original LEWG review, and adds more subtle and confusing corner cases and will likely surprise and confuse the user.

For details on the problem, see https://isocpp.org/files/papers/P1690R1.html#design-minimize-confusion

P1690R0 proposed a fix that was reviewed by LEWG in Cologne, which was forwarded to LWG for C++20 (poll results below) with a suggestion to prioritize as it would be a break to do later; unfortunately LWG ran out of time before getting to it.
SF F N A SA
5 6 7 0 0

Proposed change:
See: P1690r1
For details on the problem, see https://isocpp.org/files/papers/P1690R1.html#design-minimize-confusion

[tituswinters]

LEWG in Belfast: We would still like this for C++20.
(Prioritization on this vs. other similar papers will be shared with LWG chair when LEWG finishes NB comment processing.)

[dietmarkuehl]

LWG small group: recommend we look at this

[JeffGarland]

Reviewed by LWG full group during Wed issue processing in Belfast. Unanimous consent to adopt.

[jensmaurer]

Accepted.