Expand comments to explain why we ignore errno

Author: hpkfft

Modules/cmathmodule.c

@@ -1004,8 +1004,10 @@ cmath_phase_impl(PyObject *module, Py_complex z)
 {
     double phi;
 
-    phi = atan2(z.imag, z.real); /* should not cause any exception */
-    /* gh-153144: Ignore atan2() errno on purpose. */
+    phi = atan2(z.imag, z.real);
+    /* gh-153144: Ignore atan2() errno on purpose since it can optionally be
+     *            EDOM, which we should ignore, or ERANGE if phi underflows,
+     *            which is silent on Python.  Overflow is not possible. */
     return PyFloat_FromDouble(phi);
 }
 

Modules/mathmodule.c

@@ -1149,11 +1149,14 @@ FUNC1(atan, atan, 0,
       "atan($module, x, /)\n--\n\n"
       "Return the arc tangent (measured in radians) of x.\n\n"
       "The result is between -pi/2 and pi/2.")
-FUNC2NE(atan2, atan2,  // gh-153144: Ignore atan2() errno on purpose.
+/* gh-153144: Ignore atan2 and atan2pi errno on purpose since it can optionally
+ *            be EDOM, which we should ignore, or ERANGE on underflow, which is
+ *            silent on Python.  Overflow is not possible. */
+FUNC2NE(atan2, atan2,
       "atan2($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.")
-FUNC2NE(atan2pi, m_atan2pi,  // gh-153144: Ignore atan2pi() errno on purpose.
+FUNC2NE(atan2pi, m_atan2pi,
       "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.")