Build fails on Fedora 38 with clang 16 (-Wgnu-offsetof-extensions)

[airlockgithubci]

The build fails on Fedora 38 with clang 16.

.bazelrc file to use clang:

build --config=clang
build --cxxopt=-std=c++17

build:clang --action_env=BAZEL_COMPILER=clang
build:clang --action_env=CC=clang --action_env=CXX=clang++
build:clang --linkopt=-fuse-ld=lld

Build error:

ERROR: /home/user/upb/BUILD:102:11: Compiling upb/arena.c failed: (Exit 1): clang-16 failed: error executing command (from target //:upb) /usr/bin/clang-16 -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -Wunused-but-set-parameter -Wno-free-nonheap-object -fcolor-diagnostics -fno-omit-frame-pointer -MD -MF ... (remaining 22 arguments skipped)

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
upb/arena.c:177:25: error: defining a type within '__builtin_offsetof' is a Clang extension [-Werror,-Wgnu-offsetof-extensions]
  n = UPB_ALIGN_DOWN(n, UPB_ALIGN_OF(upb_Arena));
                        ^~~~~~~~~~~~~~~~~~~~~~~
./upb/port_def.inc:95:38: note: expanded from macro 'UPB_ALIGN_OF'
#define UPB_ALIGN_OF(type) offsetof (struct { char c; type member; }, member)
                                     ^~~~~~
/usr/lib64/clang/16/include/stddef.h:111:43: note: expanded from macro 'offsetof'
#define offsetof(t, d) __builtin_offsetof(t, d)
                                          ^
./upb/port_def.inc:93:48: note: expanded from macro 'UPB_ALIGN_DOWN'
#define UPB_ALIGN_DOWN(size, align) ((size) / (align) * (align))
                                               ^~~~~
upb/arena.c:177:25: error: defining a type within '__builtin_offsetof' is a Clang extension [-Werror,-Wgnu-offsetof-extensions]
  n = UPB_ALIGN_DOWN(n, UPB_ALIGN_OF(upb_Arena));
                        ^~~~~~~~~~~~~~~~~~~~~~~
./upb/port_def.inc:95:38: note: expanded from macro 'UPB_ALIGN_OF'
#define UPB_ALIGN_OF(type) offsetof (struct { char c; type member; }, member)
                                     ^~~~~~
/usr/lib64/clang/16/include/stddef.h:111:43: note: expanded from macro 'offsetof'
#define offsetof(t, d) __builtin_offsetof(t, d)
                                          ^
./upb/port_def.inc:93:58: note: expanded from macro 'UPB_ALIGN_DOWN'
#define UPB_ALIGN_DOWN(size, align) ((size) / (align) * (align))
                                                         ^~~~~
2 errors generated.

[mkauf]

I have created this issue with the wrong GitHub account, sorry. If possible, please change the reporter to @mkauf .

[esrauchg]

It looks like something else is happening here; if you look at the UPB_ALIGN_OF() macro definition it already has a separate definition for #ifdef clang with a different definition:
https://github.com/protocolbuffers/upb/blob/main/upb/port/def.inc#L120

It seems like the warning you're disabling is based on hitting the non-clang path with a clang compiler, is it possible your configuration is somehow using clang but without clang defined for the preprocessor?

[mkauf]

I think that the bug has been fixed and the issue can be closed :-)

Probably I have tested with the git revision that Envoy 1.26.0 used, and not with the main branch (sorry...). That was git revision protocolbuffers/upb@e4635f2

Commit protocolbuffers/upb@896e74c removed the compiler flags "-pedantic" and "-Werror=pedantic", so the build does not fail anymore.

Commit protocolbuffers/upb@21af783 fixed UPB_ALIGN_OF for clang, so clang now uses a different code path than gcc (thanks @esrauchg)