Fused Type + Memoryview Parameter Throws unused-function Warning on Master

Credit to @tacaswell for originally finding in https://github.com/pandas-dev/pandas/issues/33224

Using this as a minimum reproducible example:

ctypedef fused number_t:
    int
    float

def add_one(number_t[:] arr):
    cdef Py_ssize_t i

    for i in range(len(arr)):
        arr[i] = arr[i] + 1

Compilation using Cython master will throw the following warning

cytest.c:3476:18: warning: ‘__pyx_pf_6cytest_8__defaults__’ defined but not used [-Wunused-function]
 static PyObject *__pyx_pf_6cytest_8__defaults__(CYTHON_UNUSED PyObject *__pyx_self) {
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

No warning is thrown using 0.29.14

From what I can tell, it seems to be specific using a fused type and a memoryview together in the function signature - changing the parameter type to say just int[:] does not throw a warning

[da-woods]

I’m relatively sure this would be related to https://github.com/cython/cython/pull/3366 (or possibly a combination of that and https://github.com/cython/cython/pull/3398). I don’t know how helpful this actually is as a place to start though.

I’m a bit suspicious of https://github.com/cython/cython/blob/2ca3f569254ddee7d1561d102ccca0c9d3250c91/Cython/Compiler/ExprNodes.py#L9443-L9449 - maybe it means that the thing that would normally use the __defaults__ isn’t set?

[scoder]

I was guessing that there was just some inconsistent condition somewhere. Let’s see if #3491 fixes it.