The tests for these are just copy-pasted from the tests for std::{strong,weak,partial}_order,
and then I added an extra clause in each (test_2()) to test the stuff that's not just the same
as std::*_order.
This also includes the fix for https://wg21.link/LWG3465 (which falls naturally out of the
"you must write it three times" style, but I've added test cases for it also).
There is an action item here to go back and give good diagnostics for SFINAE failures
in these CPOs. I've filed this as https://github.com/llvm/llvm-project/issues/53456 .
Differential Revision: https://reviews.llvm.org/D111514
Add issue tracking and assignment for the implementation of P1614R2: The Mothership has Landed.
Reviewed By: cjdb, #libc, Mordante, Quuxplusone
Differential Revision: https://reviews.llvm.org/D107877
Arthur O'Dwyer