bpo-37250: define tp_print for backwards compatibility

Author: jdemeyer

Include/cpython/object.h

@@ -174,6 +174,10 @@ typedef struct {
  * backwards-compatibility */
 typedef Py_ssize_t printfunc;
 
+/* Define tp_print as alias of tp_vectorcall to allow for code accessing
+ * this field (hopefully not actually doing anything with it). */
+#define tp_print tp_vectorcall
+
 typedef struct _typeobject {
     PyObject_VAR_HEAD
     const char *tp_name; /* For printing, in format "<module>.<name>" */

Misc/NEWS.d/next/C API/2019-06-12-11-45-36.bpo-37221.RhP1E7.rst

@@ -0,0 +1,2 @@
+``tp_print`` is defined as alias of ``tp_vectorcall`` to restore support for
+old code (in particular generated by Cython) setting ``tp_print = 0``.

Modules/_testcapimodule.c

@@ -6008,6 +6008,10 @@ PyInit__testcapi(void)
     Py_INCREF(&MyList_Type);
     PyModule_AddObject(m, "MyList", (PyObject *)&MyList_Type);
 
+    /* Old Cython code sets tp_print to 0, we check that
+     * this doesn't break anything. */
+    MyList_Type.tp_print = 0;
+
     if (PyType_Ready(&MethodDescriptorBase_Type) < 0)
         return NULL;
     Py_INCREF(&MethodDescriptorBase_Type);