amend! build: tolerate use of _Generic from glibc 2.43 with Clang

build: tolerate use of _Generic from glibc 2.43 with Clang

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>
Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

config.mak.dev

@@ -21,17 +21,14 @@ endif
 endif
 
 ifneq ($(uname_S),FreeBSD)
-ifneq ($(filter gcc6,$(COMPILER_FEATURES)),)
+ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang7,$(COMPILER_FEATURES))),)
 ifndef USE_MIMALLOC
 DEVELOPER_CFLAGS += -std=gnu99
 endif
 endif
 else
 # FreeBSD cannot limit to C99 because its system headers unconditionally
 # rely on C11 features.
-#
-# Clang cannot limit to C99 when using glibc 2.43 because its system headers
-# depend on the _Generic C11 feature. This works with GCC though.
 endif
 
 DEVELOPER_CFLAGS += -Wdeclaration-after-statement
@@ -103,6 +100,13 @@ endif
 endif
 endif
 
+# glibc 2.43 headers unconditionally use _Generic even when we ask the
+# compiler to stick to -std=gnu99 and unlike GCC, clang lacks a
+# workaround to squelch warnings from system headers.
+ifneq ($(filter clang1,$(COMPILER_FEATURES)),)     # if we are using clang
+DEVELOPER_CFLAGS += -Wno-c11-extensions
+endif
+
 # https://bugzilla.redhat.com/show_bug.cgi?id=2075786
 ifneq ($(filter gcc12,$(COMPILER_FEATURES)),)
 DEVELOPER_CFLAGS += -Wno-error=stringop-overread