bpo-20092. Use __index__ in constructors of int, float and complex. (GH-13108)
diff --git a/Doc/c-api/complex.rst b/Doc/c-api/complex.rst
index 675bd01..06dbb25 100644
--- a/Doc/c-api/complex.rst
+++ b/Doc/c-api/complex.rst
@@ -129,4 +129,10 @@
If *op* is not a Python complex number object but has a :meth:`__complex__`
method, this method will first be called to convert *op* to a Python complex
- number object. Upon failure, this method returns ``-1.0`` as a real value.
+ number object. If ``__complex__()`` is not defined then it falls back to
+ :meth:`__float__`. If ``__float__()`` is not defined then it falls back
+ to :meth:`__index__`. Upon failure, this method returns ``-1.0`` as a real
+ value.
+
+ .. versionchanged:: 3.8
+ Use :meth:`__index__` if available.