[BUG] cython.final extension type with fused type method

[lorentzenchr]

Describe the bug

When using cython.final decorator for a cdef class that has a method with an argument of fused type, the generated C code lacks some prefix __pyx_fuse_0 and __pyx_fuse_1 in a few places.
The code compiles without error. But when loading/importing one gets an error:

ImportError: dlopen(XXX.cpython-311-darwin.so, 0x0002): symbol not found in flat namespace '___pyx_f_54_cython_magic_570ee92356009cb8197cfd5d5ebfbcc01ad73c1d_6AClass_c_sum'

Code to reproduce the behaviour:

import cython
%load_ext cython

# %%
%%cython

import cython
import numpy as np

ctypedef unsigned char uint8_t
ctypedef unsigned short uint16_t
ctypedef fused uint_fused:
    uint8_t
    uint16_t

@cython.final
cdef class AClass:
    def compute_sum(self, a):
        if a.dtype == np.float32:
            return self.c_sum[uint8_t](a)
        else:
            return self.c_sum[uint16_t](a)

    cdef c_sum(self, uint_fused[:] a):
        cdef:
            int i
            uint16_t s = 0
        for i in range(a.shape[0]):
            s += a[i]
        return s

# %%
aclass = AClass()
aclass.compute_sum(np.arange(10).astype(np.uint16))

Note that without the @cython.final, everything works as expected without error.

Expected behaviour

No error, result 45.

OS

macOS

Python version

3.11.7

Cython version

3.0.8

Additional context

No response

[lorentzenchr]

@da-woods You seem active right now and this is a road blocker for one of my projects.

[da-woods]

I'll try to have a look in the next week or so. I should warn: this fused stuff is usually quite heavy going to debug and fix, so I'm really not going to promise anything quick.

[lorentzenchr]

Thanks for having a look.
A striking argument for Cython is allocating numpy arrays in Python and then use memoryviews in Cython, in particular in a prange. Then one quickly wants c++ templates, the next best thing being fused types, last resort tempita.

So the combination of C++ with memoryviews would be super helpful, but the C(++) structure of memoryviews is quite hidden.

TLDR: If Cython works, it‘s very cool. If it doesn’t, try n error can drive you crazy 😜

[lorentzenchr]

@da-woods @matusvalo Thanks for the fix, really appreciated 🥳