Implicit signedness conversion in PyFunctionFastCall

[JustAnotherArchivist]

With Cython 0.29 with Python 3.7.1 on Debian Sid, I get the following error (with -Werror) when compiling pyfuse3:

[...]
x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/python3.7m -c src/pyfuse3.c -o build/temp.linux-x86_64-3.7/src/pyfuse3.o -I/usr/include/fuse3 -DFUSE_USE_VERSION=32 -Wall -Wextra -Wconversion -Wsign-compare -DPYFUSE3_VERSION="1.1" -Wno-unused-function -Wno-implicit-fallthrough -Wno-unused-function -Wno-unused-parameter -Werror -Wfatal-errors -Wno-error=unreachable-code -Werror=conversion -Werror=sign-compare
src/pyfuse3.c: In function ‘__pyx_pymod_exec_pyfuse3’:
src/pyfuse3.c:1845:73: error: conversion to ‘long unsigned int’ from ‘Py_ssize_t’ {aka ‘long int’} may change the sign of the result [-Werror=sign-conversion]
      (void)(__pyx_pyframe_localsplus_offset = PyFrame_Type.tp_basicsize - Py_MEMBER_SIZE(PyFrameObject, f_localsplus)))
                                                                         ^
src/pyfuse3.c:47086:3: note: in expansion of macro ‘__Pxy_PyFrame_Initialize_Offsets’
   __Pxy_PyFrame_Initialize_Offsets();
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated due to -Wfatal-errors.
cc1: all warnings being treated as errors
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

According to python-llfuse/python-llfuse#21, this also happens with Cython 0.28 on macOS.