[3.9] bpo-44392: Add Py_GenericAlias to C API docs (GH-26724) (GH-26757)

(cherry picked from commit 6773c3ea)

Doc/c-api/concrete.rst

@@ -115,3 +115,4 @@ Other Objects
    coro.rst
    contextvars.rst
    datetime.rst
+   typehints.rst

Doc/c-api/typehints.rst

+.. highlight:: c
+
+.. _typehintobjects:
+
+Objects for Type Hinting
+------------------------
+
+Various built-in types for type hinting are provided.
+Only :ref:`GenericAlias <types-genericalias>` is exposed to C.
+
+.. c:function:: PyObject* Py_GenericAlias(PyObject *origin, PyObject *args)
+
+   Create a :ref:`GenericAlias <types-genericalias>` object.
+   Equivalent to calling the Python class
+   :class:`types.GenericAlias`.  The *origin* and *args* arguments set the
+   ``GenericAlias``\ 's ``__origin__`` and ``__args__`` attributes respectively.
+   *origin* should be a :c:type:`PyTypeObject*`, and *args* can be a
+   :c:type:`PyTupleObject*` or any ``PyObject*``.  If *args* passed is
+   not a tuple, a 1-tuple is automatically constructed and ``__args__`` is set
+   to ``(args,)``.
+   Minimal checking is done for the arguments, so the function will succeed even
+   if *origin* is not a type.
+   The ``GenericAlias``\ 's ``__parameters__`` attribute is constructed lazily
+   from ``__args__``.  On failure, an exception is raised and ``NULL`` is
+   returned.
+
+   Here's an example of how to make an extension type generic::
+
+      ...
+      static PyMethodDef my_obj_methods[] = {
+          // Other methods.
+          ...
+          {"__class_getitem__", (PyCFunction)Py_GenericAlias, METH_O|METH_CLASS, "See PEP 585"}
+          ...
+      }
+
+   .. seealso:: The data model method :meth:`__class_getitem__`.
+
+   .. versionadded:: 3.9
+
+.. c:var:: PyTypeObject Py_GenericAliasType
+
+   The C type of the object returned by :c:func:`Py_GenericAlias`. Equivalent to
+   :class:`types.GenericAlias` in Python.
+
+   .. versionadded:: 3.9

Misc/NEWS.d/next/Documentation/2021-06-16-18-09-49.bpo-44392.6RF1Sc.rst

+Added a new section in the C API documentation for types used in type
+hinting.  Documented ``Py_GenericAlias`` and ``Py_GenericAliasType``.