bpo-40170: Convert PyObject_IS_GC() macro to a function

Doc/c-api/gcsupport.rst

@@ -60,6 +60,12 @@ Constructors for container types must conform to two rules:
    followed by the :c:member:`~PyTypeObject.tp_traverse` handler become valid, usually near the
    end of the constructor.
 
+.. c:function:: int PyObject_IS_GC(PyObject *obj)
+
+   Return non-zero if the object have GC head. Return zero otherwise.
+
+   .. versionadded:: 3.9
+
 
 Similarly, the deallocator for the object must conform to a similar pair of
 rules:

Include/cpython/objimpl.h

@@ -121,9 +121,7 @@ PyAPI_FUNC(Py_ssize_t) _PyGC_CollectIfEnabled(void);
 
 
 /* Test if an object has a GC head */
-#define PyObject_IS_GC(o) \
-    (PyType_IS_GC(Py_TYPE(o)) \
-     && (Py_TYPE(o)->tp_is_gc == NULL || Py_TYPE(o)->tp_is_gc(o)))
+PyAPI_FUNC(int) PyObject_IS_GC(PyObject *obj);
 
 /* GC information is stored BEFORE the object structure. */
 typedef struct {

Misc/NEWS.d/next/C API/2020-04-11-06-12-44.bpo-40170.cmM9oK.rst

@@ -0,0 +1,2 @@
+Convert :c:func:`PyObject_IS_GC` macro to a function to hide
+implementation details: the macro test an object have GC head or not.

Modules/gcmodule.c

@@ -2203,6 +2203,16 @@ PyObject_GC_UnTrack(void *op_raw)
     }
 }
 
+/* Test if an object has a GC head */
+int
+PyObject_IS_GC(PyObject *obj)
+{
+    if (PyType_IS_GC(Py_TYPE(obj))) {
+        return (Py_TYPE(obj)->tp_is_gc == NULL || Py_TYPE(obj)->tp_is_gc(obj));
+    }
+    return 0;
+}
+
 static PyObject *
 _PyObject_GC_Alloc(int use_calloc, size_t basicsize)
 {