I'm not sure about the unit tests: not sure if they are portable on any IEEE 754 implementation, and if the test would work on non-IEEE 754 implementation. Maybe the test should be decorated ith @requires_IEEE_754?

Maybe the test should be decorated ith @requires_IEEE_754?

Yes, I think so. There's nothing to guarantee that the existing test would work on a non IEEE 754 system, should Python ever encounter one.

Please could we also have tests covering all the various corner cases: signed zeros, infinities, nans, subnormals, etc.? There's a lot that can go wrong, and I don't think it's safe to trust that the platform's libm does the right thing. (We've tried that before.)

Oh, and once we have more comprehensive tests, it might be worth trying out the new "test on the buildbots" functionality with this PR, to ferret out any platforms that have dodgy implementations of nextafter.

Please could we also have tests covering all the various corner cases: signed zeros, infinities, nans, subnormals, etc.?

I added more tests. Which tests do you want to signed zeros? The sign doesn't seem to matter:

>>> math.nextafter(0.0, 1.0)
5e-324
>>> math.nextafter(-0.0, 1.0)
5e-324
>>> math.nextafter(-0.0, -1.0)
-5e-324
>>> math.nextafter(+0.0, -1.0)
-5e-324

For subnormals, I wrote the following tests, is it what you expect?

        # around 0.0
        self.assertEqual(math.nextafter(0.0, INF),
                         sys.float_info.min * sys.float_info.epsilon)
        self.assertEqual(math.nextafter(0.0, -INF),
                         (-sys.float_info.min) * sys.float_info.epsilon)

There's a lot that can go wrong, and I don't think it's safe to trust that the platform's libm does the right thing. (We've tried that before.)

Do you that if nextafter() behaves differently on a platform, Python should patch the function?

🤖 New build scheduled with the buildbot fleet by @vstinner for commit c12293b 🤖

If you want to schedule another build, you need to add the "🔨 test-with-buildbots" label again.

Oh, and once we have more comprehensive tests, it might be worth trying out the new "test on the buildbots" functionality with this PR, to ferret out any platforms that have dodgy implementations of nextafter.

Let me try this new toy :-)

I don't understand this result, but nextafter() gives me the expected result, whereas 1.0-epsilon gives me a surprising smaller value.

>>> math.nextafter(1.0, 2.0) == 1.0 + sys.float_info.epsilon
True
>>> math.nextafter(1.0, 0.0) == 1.0 - sys.float_info.epsilon
False
>>> math.nextafter(1.0, 0.0), 1.0 - sys.float_info.epsilon
(0.9999999999999999, 0.9999999999999998)
>>> math.nextafter(1.0, 0.0).hex(), (1.0 - sys.float_info.epsilon).hex()
('0x1.fffffffffffffp-1', '0x1.ffffffffffffep-1')