If a classic class defined a __coerce__() method that just returned its two

arguments in reverse, the interpreter would infinitely recourse trying to get a
coercion that worked.  So put in a recursion check after a coercion is made and
the next call to attempt to use the coerced values.

Fixes bug #992017 and closes crashers/coerce.py .

Lib/test/crashers/coerce.py

-
-# http://python.org/sf/992017
-
-class foo:
-    def __coerce__(self, other):
-        return other, self
-
-if __name__ == '__main__':
-    foo()+1   # segfault: infinite recursion in C

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.5 beta 1?
 Core and builtins
 -----------------
 
+- Bug #992017: A classic class that defined a __coerce__() method that returned
+  its arguments swapped would infinitely recurse and segfault the interpreter.
+
 - Fix the socket tests so they can be run concurrently.
 
 - Removed 5 integers from C frame objects (PyFrameObject).

Objects/classobject.c

@@ -1368,10 +1368,13 @@ half_binop(PyObject *v, PyObject *w, char *opname, binaryfunc thisfunc,
 		 * argument */
 		result = generic_binary_op(v1, w, opname);
 	} else {
+		if (Py_EnterRecursiveCall(" after coercion"))
+		    return NULL;
 		if (swapped)
 			result = (thisfunc)(w, v1);
 		else
 			result = (thisfunc)(v1, w);
+		Py_LeaveRecursiveCall();
 	}
 	Py_DECREF(coerced);
 	return result;