As Jonathan Wakely pointed out[1], my change in commit
f9ddb696a2 should have been rounding to
the target clock duration type rather than the input clock duration type
in __atomic_futex_unsigned::_M_load_when_equal_until just as (e.g.)
condition_variable does.
As well as fixing this, let's create a rather contrived test that fails
with the previous code, but unfortunately only when run on a machine
with an uptime of over 208.5 days, and even then not always.
[1] https://gcc.gnu.org/pipermail/libstdc++/2020-September/051004.html
libstdc++-v3/ChangeLog:
PR libstdc++/91486
* include/bits/atomic_futex.h:
(__atomic_futex_unsigned::_M_load_when_equal_until): Use target
clock duration type when rounding.
* testsuite/30_threads/async/async.cc (test_pr91486_wait_for):
Rename from test_pr91486.
(float_steady_clock): New class for test.
(test_pr91486_wait_until): New test.
f33a43f9f7