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When building with `make DEVELOPER=1` we explicitly pass "-std=gnu99" to
the compiler so that we don't start leaning on features exposed by more
recent versions of the C standard. Unfortunately though, glibc 2.43
started to use type-generic expressions. This works alright with GCC,
but when compiling with Clang this leads to errors:
$ make DEVELOPER=1 CC=clang
CC daemon.o
In file included from daemon.c:3:
./git-compat-util.h:344:11: error: '_Generic' is a C11 extension [-Werror,-Wc11-extensions]
344 | return !!strchr(path, '/');
| ^
/usr/include/string.h:265:3: note: expanded from macro 'strchr'
265 | __glibc_const_generic (S, const char *, strchr (S, C))
| ^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:838:3: note: expanded from macro '__glibc_const_generic'
838 | _Generic (0 ? (PTR) : (void *) 1, \
| ^
In theory, the `__glibc_const_generic` macro does have feature gating:
#if !defined __cplusplus \
&& (__GNUC_PREREQ (4, 9) \
|| __glibc_has_extension (c_generic_selections) \
|| (!defined __GNUC__ && defined __STDC_VERSION__ \
&& __STDC_VERSION__ >= 201112L))
# define __HAVE_GENERIC_SELECTION 1
#else
# define __HAVE_GENERIC_SELECTION 0
#endif
But this feature gating isn't effective because `_has_extension()` will
always evaluate to true as C generics _are_ available as a language
extension to GNU C99 when using Clang. This would have been different if
`_has_feature()` was used instead, in which case it would have properly
evaluated to `false`.
GCC has a workaround to squelch this warning from standard system
headers, but because clang fails due to [-Werror,-Wc11-extensions],
as it lacks the corresponding workaround.
For both meson and Makefile, pass -Wno-c11-extensions when we are
building with clang.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Shardul Natu <snatu@google.com>
[jc: replaced Makefile side with Shardul's approach]
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Headers from glibc 2.43 when used with clang does not allow disabling C11 language features, causing build failures.. * ps/clang-w-glibc-2.43-and-_Generic: build: tolerate use of _Generic from glibc 2.43 with Clang
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