Add cmath tests and update NEWS entry

Author: hpkfft

Lib/test/test_cmath.py

@@ -421,6 +421,16 @@ def test_phase(self):
         self.assertEqual(phase(complex(-0.0, 0.0)), pi)
         self.assertEqual(phase(complex(-0.0, -0.0)), -pi)
 
+        # overflow and underflow of imag/real
+        self.assertEqual(phase(complex(1E300, 1E-320)), 0.0)
+        self.assertEqual(phase(complex(1E300, -1E-320)), -0.0)
+        self.assertAlmostEqual(phase(complex(1E-320, 1E300)), pi/2)
+        self.assertAlmostEqual(phase(complex(-1E-320, 1E300)), pi/2)
+        self.assertAlmostEqual(phase(complex(-1E300, 1E-320)), pi)
+        self.assertAlmostEqual(phase(complex(-1E300, -1E-320)), -pi)
+        self.assertAlmostEqual(phase(complex(1E-320, -1E300)), -pi/2)
+        self.assertAlmostEqual(phase(complex(-1E-320, -1E300)), -pi/2)
+
         # infinities
         self.assertAlmostEqual(phase(complex(-INF, -0.0)), -pi)
         self.assertAlmostEqual(phase(complex(-INF, -2.3)), -pi)

Misc/NEWS.d/next/Library/2026-07-20-09-35-32.gh-issue-153144.0a2AoO.rst

@@ -1,7 +1,7 @@
-The return values of ``atan2(0.0, 0.0)``, ``atan2(0.0, -0.0)``, etc. are
-specified in Annex F of the C standard.  The Intel and Solaris math libraries
-also set ``errno`` to indicate a domain error when both arguments are zero.
-Python's :func:`math.atan2`, :func:`math.atan2pi`, and :func:`cmath.phase`
-should return the values specified by Annex F, not raise ``ValueError``.
-This patch ignores ``errno`` for the C ``atan2()`` and ``atan2pi()`` functions.
+Given a complex number ``z`` for which ``z.imag/z.real`` underflows to zero,
+the :func:`cmath.phase` of ``z`` should not raise ``OverflowError``.  Also,
+given arguments of zero, Python's :func:`math.atan2`, :func:`math.atan2pi`,
+and :func:`cmath.phase` should return the values specified by Annex F of the
+C standard, not raise ``ValueError``, regardless of whether the platform's
+math library sets ``errno`` for ``atan2(0.0, 0.0)``, ``atan2(0.0, -0.0)``, etc.
 Contributed by High Performance Kernels LLC.