GCC 9 compiler warnings on MacOS Catalina

[remilapeyre]

BPO	40516
Nosy	@ronaldoussoren, @ned-deily, @remilapeyre
PRs	bpo-40516: Silence GCC 9 warnings on MacOS Catalina #19925

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-05.13:32:24.115>
labels = ['OS-mac', 'type-bug', '3.9', 'build']
title = 'GCC 9 compiler warnings on MacOS Catalina'
updated_at = <Date 2020-05-06.01:35:47.989>
user = 'https://github.com/remilapeyre'

bugs.python.org fields:

activity = <Date 2020-05-06.01:35:47.989>
actor = 'ned.deily'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Build', 'macOS']
creation = <Date 2020-05-05.13:32:24.115>
creator = 'remi.lapeyre'
dependencies = []
files = []
hgrepos = []
issue_num = 40516
keywords = ['patch']
message_count = 2.0
messages = ['368146', '368221']
nosy_count = 3.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'remi.lapeyre']
pr_nums = ['19925']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40516'
versions = ['Python 3.9']

[remilapeyre]

Building master with GCC 9.3.0 gives some warnings on posix and nis:

./Modules/posixmodule.c: In function 'os_chown_impl':
./Modules/posixmodule.c:3397:39: warning: the comparison will always evaluate as 'false' for the address of 'lchown' will never be NULL [-Waddress]
3397 | if ((!follow_symlinks) && (lchown == NULL)) {
| ^~
./Modules/posixmodule.c: In function 'PyInit_posix':
./Modules/posixmodule.c:14939:16: warning: the comparison will always evaluate as 'false' for the address of 'lchown' will never be NULL [-Waddress]
14939 | if (lchown == NULL) {
| ^~
/Users/remi/src/cpython/Modules/nismodule.c: In function 'nis_cat':
/Users/remi/src/cpython/Modules/nismodule.c:216:18: warning: cast between incompatible function types from 'int ()(int, char *, int, char *, int, struct ypcallback_data *)' to 'int ()(long unsigned int, char *, int, char *, int, void *)' [-Wcast-function-type]
216 | cb.foreach = (foreachfunc)nis_foreach;
| ^
/Users/remi/src/cpython/Modules/nismodule.c: In function 'nis_xdr_ypmaplist':
/Users/remi/src/cpython/Modules/nismodule.c:302:42: warning: cast between incompatible function types from 'int ()(XDR *, nismaplist *)' {aka 'int ()(struct __rpc_xdr *, struct nismaplist *)'} to 'int ()(XDR *, void *, unsigned int)' {aka 'int ()(struct __rpc_xdr *, void *, unsigned int)'} [-Wcast-function-type]
302 | sizeof(nismaplist), (xdrproc_t)nis_xdr_ypmaplist))
| ^
/Users/remi/src/cpython/Modules/nismodule.c: In function 'nis_xdr_ypresp_maplist':
/Users/remi/src/cpython/Modules/nismodule.c:328:42: warning: cast between incompatible function types from 'int ()(XDR *, nismaplist *)' {aka 'int ()(struct __rpc_xdr *, struct nismaplist *)'} to 'int ()(XDR *, void *, unsigned int)' {aka 'int ()(struct __rpc_xdr *, void *, unsigned int)'} [-Wcast-function-type]
328 | sizeof(nismaplist), (xdrproc_t)nis_xdr_ypmaplist))
| ^
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/rpc/rpc.h:76,
from /Users/remi/src/cpython/Modules/nismodule.c:17:
/Users/remi/src/cpython/Modules/nismodule.c: In function 'nisproc_maplist_2':
/Users/remi/src/cpython/Modules/nismodule.c:344:19: warning: cast between incompatible function types from 'int ()(XDR *, char **)' {aka 'int ()(struct __rpc_xdr *, char **)'} to 'int ()(XDR *, void *, unsigned int)' {aka 'int ()(struct __rpc_xdr *, void *, unsigned int)'} [-Wcast-function-type]
344 | (xdrproc_t)nis_xdr_domainname, (caddr_t)argp,
| ^
/Users/remi/src/cpython/Modules/nismodule.c:345:19: warning: cast between incompatible function types from 'int ()(XDR *, nisresp_maplist *)' {aka 'int ()(struct __rpc_xdr *, struct nisresp_maplist *)'} to 'int ()(XDR *, void *, unsigned int)' {aka 'int ()(struct __rpc_xdr *, void *, unsigned int)'} [-Wcast-function-type]
345 | (xdrproc_t)nis_xdr_ypresp_maplist, (caddr_t)&res,
| ^

I'm not sure about the fix, especially for nis but I will propose a PR. There is also some warnings concerning libffi_osx but this looks like a vendored dependency so I left it untouched.

[ned-deily]

See proposed PR 19925 for further discussion.