I think it's already compiled in debug mode (https://github.com/pyenv/pyenv/blob/c057a80c8296a7c694e4ef80ecbac0d0c169df7a/plugins/python-build/bin/python-build#L2050).

`-X dev` won't help (it's a 3.6.6 not a 3.7).

The debug come from _Py_ForgetReference which prints op, op->_ob_prev->_ob_next and op->_ob_next->_ob_prev, crash occurs on the 2nd one, which make sense according to:

    (gdb) p *(((PyObject*)(op)))
    $12 = {_ob_next = 0x7ffff1307e90, _ob_prev = 0x7ffff12df950, ob_refcnt = 0, ob_type = 0x555555ad9f80 <PyBytes_Type>}
    (gdb) p *(((PyObject*)(op->_ob_prev)))
    $13 = {_ob_next = 0x7ffff0e667bf, _ob_prev = 0x7ffff10c58b0, ob_refcnt = 1, ob_type = 0x555555ae89a0 <PyLong_Type>}
    (gdb) p *(((PyObject*)(op->_ob_prev->_ob_next)))
    $14 = {_ob_next = 0x7ffff1307e90fb, _ob_prev = 0x7ffff12df95000, ob_refcnt = 0, ob_type = 0x555555ad9f8000}