Fused functions don't necessarily pass __Pyx_CyFunction_Check

[da-woods]

Reproducible test-case:

cdef extern from *:
    int __Pyx_CyFunction_Check(object)

ctypedef fused IntOrDouble:
    int
    double

def f(IntOrDouble x):
    # fused function
    pass

def run_cyfunction_check():
    """
    >>> run_cyfunction_check()
    <class 'fused_cython_function'>
    1
    """
    print(type(f))
    print(__Pyx_CyFunction_Check(f))  # should be True

Run this as part of the test-suite with a compiled version of Cython (fails) and a non-compiled version of Cython (passes).

---

I believe this issue is as follows

__pyx_FusedFunctionType_type uses as its base __pyx_CyFunctionType_type:
https://github.com/cython/cython/blob/master/Cython/Utility/CythonFunction.c#L1408

__Pyx_CyFunction_Check uses __pyx_CyFunctionType (note lack of _type suffix)

cython/Cython/Utility/CythonFunction.c

Line 55 in cbedb54

#define __Pyx_CyFunction_Check(obj) (__Pyx_TypeCheck(obj, __pyx_CyFunctionType))

__pyx_CyFunctionType is initialized from using __Pyx_FetchCommonType
__pyx_CyFunctionType = __Pyx_FetchCommonType(&__pyx_CyFunctionType_type);
This means that when using a compiled version of Cython, it can end up different to __pyx_CyFunctionType_type, meaning fused functions no longer register as cyfunctions.

Unfortunately I have no idea how this should be fixed....

---

For context (not really part of this bug report) this is causing me problems in __Pyx_Method_ClassMethod because it's deciding that fused functions are unknown types and throwing an exception. This is actually an easily-fixed bug in __Pyx_Method_ClassMethod, which should follow the behaviour of the Python classmethod and accept any object then fail at runtime if it isn't callable.

[scoder]

Excellent analysis, thanks. The check function must use the correct (possibly imported) type pointer.

[da-woods]

I think the check function does use the correct type pointer. It's the creation of __pyx_FusedFunctionType_type that uses the wrong type pointer.

[scoder]

Ok, got it. We can change the base pointer after importing the type pointer and before initialising the fused function type.

[scoder]

Closed in #3386