gh-133895: provide C99 Annex G return values for cmath functions

Now this information is available as the "value" attribute
of the ValueError exception object:

```pycon
>>> import cmath
>>> try:
...     cmath.log(complex(-0.0, 0))
... except ValueError as exc:
...     print(exc.value)
...
(-inf+3.141592653589793j)
```

Also uses the AC magic for return value of the cmath.rect().

Author: skirpichev

Doc/library/cmath.rst

@@ -38,6 +38,12 @@ the function is then applied to the result of the conversion.
       1.4142135623730951j
 
 
+Most functions computes and returns the C99 Annex G recommended result.  If the
+standard recommends raising ``FE_DIVBYZERO`` or ``FE_INVALID`` floating-point
+exceptions --- the :exc:`ValueError` is raised and recommended result available
+as the ``value`` attribute of the exception object.
+
+
 ====================================================  ============================================
 **Conversions to and from polar coordinates**
 --------------------------------------------------------------------------------------------------

Doc/whatsnew/3.15.rst

@@ -89,6 +89,14 @@ New modules
 Improved modules
 ================
 
+cmath
+-----
+
+* Provide C99 Annex G return values for :mod:`cmath`'s functions as the
+  "value" attribute of the ValueError exception object.
+  (Contributed by Sergey B Kirpichev in :gh:`133895`.)
+
+
 difflib
 -------
 

Lib/test/test_cmath.py

@@ -318,13 +318,13 @@ def polar_complex(z):
             if 'divide-by-zero' in flags or 'invalid' in flags:
                 try:
                     actual = function(arg)
-                except ValueError:
-                    continue
+                except ValueError as exc:
+                    actual = exc.value
                 else:
                     self.fail('ValueError not raised in test '
                           '{}: {}(complex({!r}, {!r}))'.format(id, fn, ar, ai))
 
-            if 'overflow' in flags:
+            elif 'overflow' in flags:
                 try:
                     actual = function(arg)
                 except OverflowError:
@@ -333,7 +333,8 @@ def polar_complex(z):
                     self.fail('OverflowError not raised in test '
                           '{}: {}(complex({!r}, {!r}))'.format(id, fn, ar, ai))
 
-            actual = function(arg)
+            else:
+                actual = function(arg)
 
             if 'ignore-real-sign' in flags:
                 actual = complex(abs(actual.real), actual.imag)

Misc/NEWS.d/next/Library/2025-06-01-10-38-00.gh-issue-133895.0X7c-V.rst

@@ -0,0 +1,3 @@
+Provide C99 Annex G return values for :mod:`cmath`'s functions as the
+"value" attribute of the ValueError exception object.  Patch by Sergey B
+Kirpichev.