gh-153144: Avoid checking errno for atan2

Lib/test/test_cmath.py

@@ -406,8 +406,6 @@ def polar_with_errno_set(z):
                 _testcapi.set_errno(0)
         self.check_polar(polar_with_errno_set)
 
-    @unittest.skipIf(sys.platform.startswith("sunos"),
-                     "skipping, see gh-138573")
     def test_phase(self):
         self.assertAlmostEqual(phase(0), 0.)
         self.assertAlmostEqual(phase(1.), 0.)

Lib/test/test_math.py

@@ -287,8 +287,6 @@ def testAtanh(self):
         self.assertRaises(ValueError, math.atanh, NINF)
         self.assertTrue(math.isnan(math.atanh(NAN)))
 
-    @unittest.skipIf(sys.platform.startswith("sunos"),
-                     "skipping, see gh-138573")
     def testAtan2(self):
         self.assertRaises(TypeError, math.atan2)
         self.ftest('atan2(-1, 0)', math.atan2(-1, 0), -math.pi/2)

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

@@ -0,0 +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,
+for legacy compatibility, 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.  Contributed by High Performance Kernels LLC.

Modules/cmathmodule.c

@@ -1004,12 +1004,9 @@ cmath_phase_impl(PyObject *module, Py_complex z)
 {
     double phi;
 
-    errno = 0;
     phi = atan2(z.imag, z.real); /* should not cause any exception */
-    if (errno != 0)
-        return math_error();
-    else
-        return PyFloat_FromDouble(phi);
+    /* gh-153144: Ignore atan2() errno on purpose. */
+    return PyFloat_FromDouble(phi);
 }
 
 /*[clinic input]

Modules/mathmodule.c

@@ -1017,7 +1017,7 @@ math_1a(PyObject *arg, double (*func) (double), const char *err_msg)
    The last rule is used to catch overflow on platforms which follow
    C89 but for which HUGE_VAL is not an infinity.
 
-   For most two-argument functions (copysign, fmod, hypot, atan2)
+   For most two-argument functions (copysign, fmod, hypot)
    these rules are enough to ensure that Python's functions behave as
    specified in 'Annex F' of the C99 standard, with the 'invalid' and
    'divide-by-zero' floating-point exceptions mapping to Python's
@@ -1060,6 +1060,28 @@ math_2(PyObject *const *args, Py_ssize_t nargs,
         return PyFloat_FromDouble(r);
 }
 
+/* variant of math_2, to be used when the function being wrapped is known NOT
+   to need error checking (i.e., no overflow, invalid, or divide-by-zero).  */
+
+static PyObject *
+math_2ne(PyObject *const *args, Py_ssize_t nargs,
+         double (*func) (double, double), const char *funcname)
+{
+    double x, y, r;
+    if (!_PyArg_CheckPositional(funcname, nargs, 2, 2))
+        return NULL;
+    x = PyFloat_AsDouble(args[0]);
+    if (x == -1.0 && PyErr_Occurred()) {
+        return NULL;
+    }
+    y = PyFloat_AsDouble(args[1]);
+    if (y == -1.0 && PyErr_Occurred()) {
+        return NULL;
+    }
+    r = (*func)(x, y);  // Ignore errno on purpose.
+    return PyFloat_FromDouble(r);
+}
+
 #define FUNC1(funcname, func, can_overflow, docstring)                  \
     static PyObject * math_##funcname(PyObject *self, PyObject *args) { \
         return math_1(args, func, can_overflow, NULL);                  \
@@ -1090,6 +1112,12 @@ math_2(PyObject *const *args, Py_ssize_t nargs,
     }\
     PyDoc_STRVAR(math_##funcname##_doc, docstring);
 
+#define FUNC2NE(funcname, func, docstring) \
+    static PyObject * math_##funcname(PyObject *self, PyObject *const *args, Py_ssize_t nargs) { \
+        return math_2ne(args, nargs, func, #funcname); \
+    }\
+    PyDoc_STRVAR(math_##funcname##_doc, docstring);
+
 FUNC1D(acos, acos, 0,
       "acos($module, x, /)\n--\n\n"
       "Return the arc cosine (measured in radians) of x.\n\n"
@@ -1121,11 +1149,11 @@ FUNC1(atan, atan, 0,
       "atan($module, x, /)\n--\n\n"
       "Return the arc tangent (measured in radians) of x.\n\n"
       "The result is between -pi/2 and pi/2.")
-FUNC2(atan2, atan2,
+FUNC2NE(atan2, atan2,  // gh-153144: Ignore atan2() errno on purpose.
       "atan2($module, y, x, /)\n--\n\n"
       "Return the arc tangent (measured in radians) of y/x.\n\n"
       "Unlike atan(y/x), the signs of both x and y are considered.")
-FUNC2(atan2pi, m_atan2pi,
+FUNC2NE(atan2pi, m_atan2pi,  // gh-153144: Ignore atan2pi() errno on purpose.
       "atan2pi($module, y, x, /)\n--\n\n"
       "Return the arc tangent (measured in half-turns) of y/x.\n\n"
       "Unlike atanpi(y/x), the signs of both x and y are considered.")