gh-128813: deprecate cval field of the PyComplexObject struct (#137271)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Doc/c-api/complex.rst

@@ -118,6 +118,16 @@ Complex Numbers as Python Objects
 
    This subtype of :c:type:`PyObject` represents a Python complex number object.
 
+   .. c:member:: Py_complex cval
+
+      The complex number value, using the C :c:type:`Py_complex` representation.
+
+      .. deprecated-removed:: next 3.20
+         Use :c:func:`PyComplex_AsCComplex` and
+         :c:func:`PyComplex_FromCComplex` to convert a
+         Python complex number to/from the C :c:type:`Py_complex`
+         representation.
+
 
 .. c:var:: PyTypeObject PyComplex_Type
 

Doc/deprecations/c-api-pending-removal-in-3.20.rst

@@ -0,0 +1,7 @@
+Pending removal in Python 3.20
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+* The ``cval`` field in :c:type:`PyComplexObject` (:gh:`128813`).
+  Use :c:func:`PyComplex_AsCComplex` and :c:func:`PyComplex_FromCComplex`
+  to convert a Python complex number to/from the C :c:type:`Py_complex`
+  representation.

Doc/deprecations/index.rst

@@ -18,4 +18,6 @@ C API deprecations
 
 .. include:: c-api-pending-removal-in-3.18.rst
 
+.. include:: c-api-pending-removal-in-3.20.rst
+
 .. include:: c-api-pending-removal-in-future.rst

Doc/whatsnew/3.15.rst

@@ -570,6 +570,13 @@ Deprecated C APIs
   signed integer type of the same size is now deprecated.
   (Contributed by Serhiy Storchaka in :gh:`132629`.)
 
+* Deprecate :c:member:`~PyComplexObject.cval` field of the the
+  :c:type:`PyComplexObject` type.
+  Use :c:func:`PyComplex_AsCComplex` and :c:func:`PyComplex_FromCComplex`
+  to convert a Python complex number to/from the C :c:type:`Py_complex`
+  representation.
+  (Contributed by Sergey B Kirpichev in :gh:`128813`.)
+
 * Functions :c:func:`_Py_c_sum`, :c:func:`_Py_c_diff`, :c:func:`_Py_c_neg`,
   :c:func:`_Py_c_prod`, :c:func:`_Py_c_quot`, :c:func:`_Py_c_pow` and
   :c:func:`_Py_c_abs` are :term:`soft deprecated`.