The following code causes the interpreter to crash:

import _collections
class BadDeque(_collections.deque):
    def __new__(cls, *args):
        if len(args):
            return 42
        return _collections.deque.__new__(cls)

BadDeque() * 42

(The interpreter would crash also if we replaced 'BadDeque() * 42' with
'BadDeque() + _collections.deque([42])'.)

This is because deque_copy() (in Modules/_collectionsmodule.c) returns whatever
BadDeque() returned, without verifying it is a deque.
deque_repeat() assumes that deque_copy() returned a deque, and passes it to
deque_inplace_repeat(), which assumes it is a deque, and crashes.

(Similarly, deque_concat() assumes that deque_copy() returned a deque, which
is the reason for the other crash.)

ISTM it is a very unlikely corner case, so that adding a test (as well as
a NEWS.d item) for it is unnecessary.
What do you think?

All other sequence objects return an instance of the base class rather than a subclass. list, tuple, str, bytes, bytearray, array. Only deque tries to create an instance of a subclass.

I meant that if make deque.copy(), deque.__add__() and deque.__mul__() returning an exact deque for subclasses, this would make the deque class more consistent with other collections and solve this issue. This could even make them (almost) atomic.

New changeset 24bd50b by Benjamin Peterson (Oren Milman) in branch 'master':
closes bpo-31608: Fix a crash in methods of a subclass of _collections.deque with a bad __new__(). (GH-3788)
24bd50b

New changeset 536e45a by Miss Islington (bot) in branch '3.7':
closes bpo-31608: Fix a crash in methods of a subclass of _collections.deque with a bad __new__(). (GH-3788)
536e45a

New changeset ccbbdd0 by Benjamin Peterson in branch '3.6':
[3.6] closes bpo-31608: Fix a crash in methods of a subclass of _collections.deque with a bad __new__(). (GH-9178)
ccbbdd0

New changeset 253279c by Benjamin Peterson in branch '2.7':
[2.7] closes bpo-31608: Fix a crash in methods of a subclass of _collections.deque with a bad __new__(). (GH-9179)
253279c