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In 2.6 a deprecation warning was added if `object.__new__` was called
with arguments. Per se this is fine, but the detection seems to be faulty.
The following code shows the problem:
>>> class A(object):
... def __new__(self):
... raise TypeError('i do not exist')
... 
>>> class B(A):
... __new__ = object.__new__
... def __init__(self, x):
... self.x = x
... 
>>> B(1)
__main__:1: DeprecationWarning: object.__new__() takes no parameters
<__main__.B object at 0x88dd0>
In the `B` case `__new__` is not overridden (in the sense that it
differs from object.__new__) but `__init__` is. Which is the default
behaviour. Nonetheless a warning is raised.
I used the pattern with the "__new__ switch" to achieve a
cStringIO.StringIO behavior that supports typechecks: IterIO() returns
either a IterI or IterO object, both instances of IterIO so that
typechecks can be performed.
Real-world use case here:
http://dev.pocoo.org/projects/werkzeug/browser/werkzeug/contrib/iterio.py 

The problem seems to be caused by tp_new being slot_tp_new which then
invokes whatever __new__ in the class dict is.
I'm not so sure what would be the solution to this. One could of course
check if tp_new is either object_new or slot_tp_new and in the latter
case check if the class dict's __new__ item is object_new...

I think the real problem here is `update_one_slot` and not `object_new`. It
is impossible to set "__new__" to a PyCFunction inside Python code, which
may be a feature, but is in fact very irritating.
For example the following snippet:
>>> class Dict(dict): __new__ = object.__new__
...
>>> Dict.__new__ is object.__new__
True
>>> Dict()
{}
I would rather expect this behaviour (or at least that Dict.__new__ is not
object.__new__):
>>> Dict.__new__ is object.__new__
True
>>> Dict()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: object.__new__(Dict) is not safe, use dict.__new__()
The attached patch leads to that behaviour, which also fixes the argument
calling autodetection of `object.__new__`.

I'm sorry, I don't have any opinion on this.

The problem is that `type_setattro()` sets the new "__new__" attribute
in the type's dict (through `PyObject_GenericSetAttr()`), but the
corresponding slot will never be updated if the new "__new__" is a
PyCFunction.
The affected code in `update_one_slot()` was added by Guido van Rossum
in r28090, so maybe he would like to comment on that.

See also issue #1694663.

I think it needs tests.

Here are updated patches with tests for 3.x and 2.7.

New changeset a37cc3d926ec by Serhiy Storchaka in branch '2.7':
Issue #5322: Fixed setting __new__ to a PyCFunction inside Python code.
https://hg.python.org/cpython/rev/a37cc3d926ec 

Will commit to 3.5-3.7 after releasing 3.6.0.

New changeset 1f31bf3f76f5 by Serhiy Storchaka in branch '3.5':
Issue #5322: Fixed setting __new__ to a PyCFunction inside Python code.
https://hg.python.org/cpython/rev/1f31bf3f76f5
New changeset 747de8acb7e4 by Serhiy Storchaka in branch '3.6':
Issue #5322: Fixed setting __new__ to a PyCFunction inside Python code.
https://hg.python.org/cpython/rev/747de8acb7e4
New changeset 9605c558ab58 by Serhiy Storchaka in branch 'default':
Issue #5322: Fixed setting __new__ to a PyCFunction inside Python code.
https://hg.python.org/cpython/rev/9605c558ab58 

test_file started to crash after the change "Issue #5322: Fixed setting __new__ to a PyCFunction inside Python code." :-/ (so all buildbots became red.)
Can someone fix it or revert it? (Sorry, I don't have the bandwith right to investigate the crash.)

The test is fixed if change order of base classes of UnsupportedOperation. This is rather a workaround, we should find more general fix.

New changeset 4a610bc8577b by Serhiy Storchaka in branch '3.5':
Change order of io.UnsupportedOperation base classes.
https://hg.python.org/cpython/rev/4a610bc8577b 

This change breaks backward compatibility in Python 2.7. This is the example that also broke in #25731. In that case the change was reverted. See https://bugs.python.org/issue25731#msg262922
$ cat foo.pxd 
cdef class B:
 cdef object b
$ cat foo.pyx 
cdef class A:
 pass
cdef class B:
 def __init__(self, b):
 self.b = b
$ cat bar.py
from foo import A, B
class C(A, B):
 def __init__(self):
 B.__init__(self, 1)
C()
$ cython foo.pyx && gcc -I/usr/include/python2.7 -Wall -shared -fPIC -o foo.so foo.c
$ python -c 'import bar'
Traceback (most recent call last):
 File "<string>", line 1, in <module>
 File "bar.py", line 7, in <module>
 C()
TypeError: foo.A.__new__(C) is not safe, use foo.B.__new__()

Here is more minimal breaking example. This clearly shows that this patch breaks backwards compatibility.
```
$ cat obj.pyx
cdef class OBJ(object):
 pass
$ ipython
Python 2.7.13rc1 (default, Dec 11 2016, 14:21:24) 
Type "copyright", "credits" or "license" for more information.
IPython 5.1.0 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]: import pyximport; pyximport.install()
Out[1]: (None, <pyximport.pyximport.PyxImporter at 0x7f8e8c585910>)
In [2]: import obj
In [3]: class X(obj.OBJ, dict):
 ...: pass
 ...: 
In [4]: X()
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-4-a7d4f7b89654> in <module>()
----> 1 X()
TypeError: obj.OBJ.__new__(X) is not safe, use dict.__new__()
```

Does changing the order of base classes help or there is an unavoidable conflict?

https://trac.sagemath.org/ticket/22037 reports about another regression.

@serhiy.storchaka: yes, changing the order of the base classes fixes the issue with __new__. Also manually assigning __new__ works, like
class C(A, B):
 __new__ = B.__new__
What is broken by this patch is only the auto-detection of which __new__ (really, which tp_new) should be called.
@doko: not "another regression", it's exactly the one that we are talking about.

Thank you Jeroen. It looks to me that all problems can be resolved by reordering base classes and making Cython not generating trivial __new__. But that is possible only in new Python version. In maintained versions we should keep the old behavior for backward compatibility even if it contradicts normal rules for method resolution and the behavior of Python classes. We should find other solution for making explicit __new__ assigning working.

Wouldn't it be possible to fix assignment of __new__ without breaking backwards compatibility (and then apply the same patch for all Python versions)? I have a feeling that breaking the auto-detection of tp_new is a new bug introduced by this patch and not a fundamental feature needed to fix assignment of __new__.

New changeset 5315db3171b0 by Benjamin Peterson in branch '2.7':
revert a37cc3d926ec (#5322)
https://hg.python.org/cpython/rev/5315db3171b0 

Since this change seems to break the backward compatibility, is it safe to apply it to Python 3.5.x and Python 3.6.x? The bug was reported in 2009, 7 years ago. Can the fix wait for Python 3.7?
test_file contains code which worked well before the change and started to crash after the change. If it occurs for an application, I expect users to be unhappy of getting such "behaviour change" in a minor release, no?
--
Is it possible to prevent the crash of test_file without modifying its code (without the change 4a610bc8577b "Change order of io.UnsupportedOperation base classes")? Sorry, I didnd't follow this issue.

New changeset f89ef18f9824 by Serhiy Storchaka in branch '2.7':
Issue #5322: Restored tests for __new__.
https://hg.python.org/cpython/rev/f89ef18f9824
New changeset 06e4b9f2e4b0 by Serhiy Storchaka in branch '3.5':
Revert changeset 1f31bf3f76f5 (issue5322) except tests.
https://hg.python.org/cpython/rev/06e4b9f2e4b0 

BTW, at least for #25731, I think the right approach in the MI case is to synthesize a __new__ on the subclass that calls the solid base __new__.

Yes, it was what the patch did by setting tp_new to slot_tp_new. The problem is that the same code is used for inherited __new__ and assigned in class body. It is hard to distinguish between these cases.
In any case I think that Cython shouldn't generate trivial __new__. This will help to change the order of __new__ resolution at least in 3.7.

It worries me that nothing in the Python docs nor in any PEP describes how tp_new is inherited. In my opinion, ​this patch makes a significant change which should be subject to a PEP. However, neither the old nor new behaviour is described anywhere. This also makes it harder to argue which behaviour is correct.

Can we close this now?
>>> class A(object):
... def __new__(self):
... raise TypeError('i do not exist')
... 
>>> class B(A):
... __new__ = object.__new__
... def __init__(self, x):
... self.x = x
... 
>>> B(1)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: object.__new__() takes exactly one argument (the type to instantiate)
>>>

The bug is still there, just that it's now not just a warning but an error. The auto detection is incorrect here. It should allow the instantiation of the object with arguments.

Right, I see now.

Python has the same behavior since Python 2.6. While it annoys a few persons, the majority doesn't care. I suggest to close the issue.
It's easy to workaround limitation that object.__new__() which doesn't accept arguments.