US257 23.02 Avoid double negatives for ranges opt-in variable templates P1871

[wg21bot]

23.02 and 24.2

Two of the ranges opt-in variable templates are negative and checked against, the other is positive and checked for. Double negatives are needlessly difficult to understand. Make all the opt-in variable templates enable_meow instead of disable_meow.

Proposed change:
Adopt P1871 cplusplus/papers#621

[tituswinters]

LEWG in Belfast: No consensus for the major part of this change.
However, a small part of P1871 was appealing: adding the "_for" suffix to the variable templates to improve readability. A revision of that paper has been produced along those lines, dropping the disable->enable toggle but adding "_for". LEWG consensus was to send that revision to LWG for C++20.

[JeffGarland]

Reviewed Wednesday by LWG small group - recommend adoption of D1871R1 (2nd revision) from LEWG wiki.

[JeffGarland]

Paper reviewed by LWG large group during Wed issue processing in Belfast

Poll to move in C++20 on Saturday:
No objections.

[jensmaurer]

Accepted.