Why does this async generator/anext snippet result in SystemError: <class 'StopIteration'> returned with exception set?

This is a Python bug, and should be reported.

When the anext-with-default-value implementation hits StopAsyncIteration, it calls _PyGen_SetStopIterationValue to replace the exception with a StopIteration. I'm not quite sure whether this is happening in __next__ or send, but both methods do essentially the same thing in this case. Here's __next__'s handling:

    if (PyErr_ExceptionMatches(PyExc_StopAsyncIteration)) {
        _PyGen_SetStopIterationValue(obj->default_value);
    }

_PyGen_SetStopIterationValue looks like this:

int
_PyGen_SetStopIterationValue(PyObject *value)
{
    PyObject *e;

    if (value == NULL ||
        (!PyTuple_Check(value) && !PyExceptionInstance_Check(value)))
    {
        /* Delay exception instantiation if we can */
        PyErr_SetObject(PyExc_StopIteration, value);
        return 0;
    }
    /* Construct an exception instance manually with
     * PyObject_CallOneArg and pass it to PyErr_SetObject.
     *
     * We do this to handle a situation when "value" is a tuple, in which
     * case PyErr_SetObject would set the value of StopIteration to
     * the first element of the tuple.
     *
     * (See PyErr_SetObject/_PyErr_CreateException code for details.)
     */
    e = PyObject_CallOneArg(PyExc_StopIteration, value);
    if (e == NULL) {
        return -1;
    }
    PyErr_SetObject(PyExc_StopIteration, e);
    Py_DECREF(e);
    return 0;
}

This function needs to perform special handling when the argument is a tuple or an exception object. In your case, the argument is a tuple (2, 3). But the special handling has an issue.

There is already an active exception set, the StopAsyncIteration. It is not safe to call arbitrary Python APIs with an exception set. All the calls in the usual-case handling are okay, but this line in the special-case handling:

e = PyObject_CallOneArg(PyExc_StopIteration, value);

is unsafe. Python detects an inconsistent state, resulting in the error you see.

To fix this issue, either _PyGen_SetStopIterationValue or the code that calls it should use PyErr_Clear to cancel the current exception before the PyObject_CallOneArg(PyExc_StopIteration, value) call happens.