The note beside the close's wake claimed the narrowing to `notify_one` was
equivalent, and gave the weaker half of the reason. The equivalence needs two
premises together: `closing` is set permanently under the gate before the notify
and appears in every waiter's predicate, so after a close begins no poller can
block at all and the set of waiters is fixed; and each exiting call's own scope
destructor takes the gate, decrements and wakes everyone, so one wake starts a
chain that reaches all of them. Neither carries it alone. Both are now written
where the claim is made, so it can be checked rather than believed.
Narrowing both wakes also survives, and that is not equivalent by any argument:
a wake consumed by the closer, waiting on the same variable for the count to
reach zero, leaves the chain stopped and the rest of the pollers sitting out
their timeouts. It passes on wait-queue ordering — the pollers are queued before
the closer, so a FIFO head is never the closer until it is the only waiter left.
Recorded as unreached, with the reason, and deliberately not as unreachable:
that claim has been wrong twice in this campaign. A scenario cannot force it
while the first premise holds, since the only way to put the closer ahead of a
poller in that queue is a poller that blocks after the close began. Reaching it
would need a seam that chooses which waiter a notify goes to. The standard does
not promise FIFO, so what survives here is the implementation's doing.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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