Many -Werror=cast-function-type with GCC 8

[fishilico]

The code generated by Cython heavily uses function casts between function types which have incompatible signatures. GCC 8.1.1 (on Arch Linux) does not like this and reports many Werror=cast-function-type errors:

setools/policyrep/libpolicyrep.c:8874:108: error: cast between incompatible function types from ‘PyObject * (*)(PyObject *, PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *, struct _object *)’} to ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *)’} [-Werror=cast-function-type]
 static PyMethodDef __pyx_mdef_7setools_9policyrep_12libpolicyrep_10PolicyEnum_3__format__ = {"__format__", (PyCFunction)__pyx_pw_7setools_9policyrep_12libpolicyrep_10PolicyEnum_3__format__, METH_VARARGS|METH_KEYWORDS, 0};
                                                                                                            ^
setools/policyrep/libpolicyrep.c:9084:100: error: cast between incompatible function types from ‘PyObject * (*)(PyObject *, PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *, struct _object *)’} to ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *)’} [-Werror=cast-function-type]
 static PyMethodDef __pyx_mdef_7setools_9policyrep_12libpolicyrep_10PolicyEnum_5__eq__ = {"__eq__", (PyCFunction)__pyx_pw_7setools_9policyrep_12libpolicyrep_10PolicyEnum_5__eq__, METH_VARARGS|METH_KEYWORDS, 0};
                                                                                                    ^
setools/policyrep/libpolicyrep.c:9500:100: error: cast between incompatible function types from ‘PyObject * (*)(PyObject *, PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *, struct _object *)’} to ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *)’} [-Werror=cast-function-type]
 static PyMethodDef __pyx_mdef_7setools_9policyrep_12libpolicyrep_10PolicyEnum_9lookup = {"lookup", (PyCFunction)__pyx_pw_7setools_9policyrep_12libpolicyrep_10PolicyEnum_9lookup, METH_VARARGS|METH_KEYWORDS, __pyx_doc_7setools_9policyrep_12libpolicyrep_10PolicyEnum_8lookup};
                                                                                                    ^
...

Have you already encountered this issue?

As -Wcast-function-type does not exist in previous GCC releases, adding it to the set of disabled warnings in setup.py will break building with older compilers.
In order to silence the warning, I can think of two possible ways:

• either by adding code in setup.py which tries running $CC -Wcast-function-type on a dummy C file and adds -Wno-cast-function-type to the compiler flags if this succeeds,
• or by adding a pragma which disables the warning flag for GCC versions which support it, in the Cython file (if adding inline C code at the beginning of the file is possible).

What do you think about these options?

[pebenito]

I've only got gcc 7.3 right now, and I added -Wno-cast-function-type locally, and it didn't complain.

[fishilico]

Indeed. It seems that gcc never warns for unknown warning which are disabled, even in old versions like 4.9.2:

$ gcc --version
gcc (Debian 4.9.2-10+deb8u1) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ echo 'int main(){return 0;}' > test.c
$ gcc -Wcast-function-type test.c
gcc: error: unrecognized command line option ‘-Wcast-function-type’
$ gcc -Wno-cast-function-type test.c
$

Thanks.