US236 22.02.7 [unord.req] Novel heterogenous lookup is problematic P1690

[wg21bot]

The working paper has an implementation of heterogeneous lookup that differs substantially from existing practice. Once we ship the design currently in the CD, we will have a difficult time retrofitting the design in P1690.
LEWG reviewed and approved P1690 for C++20 in Cologne, but limited LWG review time prevented this from being moved.

Proposed change:
Merge P1690 into the working paper

[wg21bot]

Duplicate of US235 #232.

[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: duplicate of US235 #232.

[JeffGarland]

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