New issue 2819: PyObject_TypeCheck requires argument to have ob_type
https://bitbucket.org/pypy/pypy/issues/2819/pyobject_typecheck-requires-argument-to
Hrvoje Nikšić:
PyPy defines `PyObject_TypeCheck` as:
```
#define PyObject_TypeCheck(ob, tp) \
((ob)->ob_type == (tp) || PyType_IsSubtype((ob)->ob_type, (tp)))
```
whereas CPython defines it as:
```
#define PyObject_TypeCheck(ob, tp) \
(Py_TYPE(ob) == (tp) || PyType_IsSubtype(Py_TYPE(ob), (tp)))
```
The difference is that `Py_TYPE` explicitly casts its argument to `PyObject *`.
This means that CPython's `PyObject_TypeCheck` works for objects without an
`ob_type` member. These can appear in two cases:
* for types that embed an existing Python type so they can inherit from it, and
can't use `PyObject_HEAD` because they inherit from something other than base
`object` - for example, a type that inherits from `list` or `dict` must embed
`PyListObject` or `PyDictObject` and will not have an `obj_type`.
* for C++ wrappers for `PyObject *` that define a cast to `PyObject *`.
The issue can be worked around with an explicit cast to `PyObject *` before the
call to `PyObject_TypeCheck`, but the fix in the PyPy header seems trivial.
(Also, there is no reason _not_ to use `Py_TYPE` instead of explicit `ob_type`
dereference.)
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