Ok, I found it.  It was an assert() in debug mode that the MemoryError from
PyObject_IsInstance() was swallowed.  In production mode AttributeError
would have been raised.


Program received signal SIGABRT, Aborted.
0x00007ffff71e0475 in *__GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
64      ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb)
(gdb) bt
#0  0x00007ffff71e0475 in *__GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1  0x00007ffff71e36f0 in *__GI_abort () at abort.c:92
#2  0x00007ffff71d9621 in *__GI___assert_fail (assertion=0x688db8 "!PyErr_Occurred()", file=<optimized out>, line=1211,
    function=0x68a0c0 "PyEval_EvalFrameEx") at assert.c:81
#3  0x000000000058fe3a in PyEval_EvalFrameEx (f=0xb328d8, throwflag=0) at Python/ceval.c:1211
#4  0x00000000005a4192 in PyEval_EvalCodeEx (_co=0x7ffff05a3100, globals=0x7ffff0580d78, locals=0x0, args=0x7ffff059b288, argcount=1,
    kws=0x0, kwcount=0, defs=0x0, defcount=0, kwdefs=0x0, closure=0x0) at Python/ceval.c:3570
#5  0x0000000000650efa in function_call (func=0x7fffef54de00, arg=0x7ffff059b260, kw=0x0) at Objects/funcobject.c:632
#6  0x000000000045e2af in PyObject_Call (func=0x7fffef54de00, arg=0x7ffff059b260, kw=0x0) at Objects/abstract.c:2087
#7  0x000000000045f3ea in PyObject_CallFunctionObjArgs (callable=0x7fffef54de00) at Objects/abstract.c:2379
#8  0x000000000064b101 in property_descr_get (self=0x7fffef53cc58, obj=0x7ffff05a25a0, type=0xae3778) at Objects/descrobject.c:1345
#9  0x00000000004c27cd in _PyObject_GenericGetAttrWithDict (obj=0x7ffff05a25a0, name=0x7ffff7faf9e0, dict=0x0) at Objects/object.c:1181
#10 0x00000000004c2b9c in PyObject_GenericGetAttr (obj=0x7ffff05a25a0, name=0x7ffff7faf9e0) at Objects/object.c:1241
#11 0x00000000004c201e in PyObject_GetAttr (v=0x7ffff05a25a0, name=0x7ffff7faf9e0) at Objects/object.c:1015
#12 0x00000000004c1ba3 in PyObject_GetAttrString (v=0x7ffff05a25a0, name=0x7fffef275987 "numerator") at Objects/object.c:914
#13 0x00007fffef2319ec in numerator_as_decimal (r=0x7ffff05a25a0, context=0x7fffef1e6430)
    at /home/stefan/hg/master3/Modules/_decimal/_decimal.c:2951
#14 0x00007fffef231e4e in convert_op_cmp (vcmp=0x7fffffffc670, wcmp=0x7fffffffc668, v=0x7fffefea9760, w=0x7ffff05a25a0, op=0,
    context=0x7fffef1e6430) at /home/stefan/hg/master3/Modules/_decimal/_decimal.c:3013
#15 0x00007fffef23d10e in dec_richcompare (v=0x7fffefea9760, w=0x7ffff05a25a0, op=0)