[BUG] Cannot put fused type pointer in return type for cdef'd function with latest cython

[david-cortes]

Describe the bug

This sort of code was working fine with earlier versions of cython (I think also with 3.0.0):

ctypedef fused int_t:
    int
    int64_t
    size_t

...
    
cdef int_t* get_ptr_int(np.ndarray[int_t, ndim=1] a):
    if a.shape[0]:
        return &a[0]
    else:
        return NULL

Example in usage:
https://github.com/david-cortes/readsparse/blob/7869e7a4e7097f8f82a8eb25da82c2db6e10738a/readsparse/cpp_interface.pxi#L220

But with Cython 3.0.5, it now throws an error when compiling:

cdef int_t* get_ptr_int(np.ndarray[int_t, ndim=1] a):
                       ^
------------------------------------------------------------

readsparse/cpp_interface.pxi:257:23: Type is not specialized

Instead, one now has to change the type to void* and cast the result in the lines where it is called:

cdef void* get_ptr_int(np.ndarray[int_t, ndim=1] a):
    if a.shape[0]:
        return &a[0]
    else:
        return NULL

...

cdef int_t *ptr = <int_t*>get_ptr_int(x)

Example in usage:
https://github.com/david-cortes/readsparse/blob/c7e04ed66ac1aa91e7eb72231e114f26e8506aa5/readsparse/cpp_interface.pxi#L1101

Code to reproduce the behaviour:

from libc.stdint cimport int64_t
import numpy as np
cimport numpy as np

ctypedef fused int_t:
    int
    int64_t
    size_t
    
cdef int_t* get_ptr_int(np.ndarray[int_t, ndim=1] a):
    if a.shape[0]:
        return &a[0]
    else:
        return NULL

Expected behaviour

Should compile and work as if it were a C++ template

OS

Linux

Python version

3.11

Cython version

3.0.5

Additional context

No response

[da-woods]

Looks to bisect to b480ae6

[da-woods]

This is fixed in #5710

[da-woods]

I'll re-open this since I really meant "This will be fixed by a pending PR", not "this has been fixed already"