P1871 Concept traits should be named after concepts

[wg21bot]

P1871R0 Should concepts be enabled or disabled? (Barry Revzin)

[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.

[wg21bot]

P1871R1 Concept traits should be named after concepts (Barry Revzin)