[Bug c++/111525] New: Inconsistent Wsign-compare in c++ with ubsan

[zhroma at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111525

            Bug ID: 111525
           Summary: Inconsistent Wsign-compare in c++ with ubsan
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhroma at gcc dot gnu.org
  Target Milestone: ---

$ cat test.cc 
extern long global;

int foo (unsigned x) {
  return x == ((global ? 64 : 32) - 1) / 8;
}
$ gcc -x c -c -Werror=sign-compare test.cc
$ gcc -x c -c -Werror=sign-compare test.cc -fsanitize=undefined
$ gcc -x c++ -c -Werror=sign-compare test.cc
$ gcc -x c++ -c -Werror=sign-compare test.cc -fsanitize=undefined
test.cc: In function ‘int foo(unsigned int)’:
test.cc:4:12: error: comparison of integer expressions of different signedness:
‘unsigned int’ and ‘int’ [-Werror=sign-compare]
    4 |   return x == ((global ? 64 : 32) - 1) / 8;
      |          ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: some warnings being treated as errors


Warning appears only for C++ with ubsan enabled.  It seems that both sanitizer
and warning are implemented in front-end part, and C++ somehow issues warning
after instrumentation.

This actually breaks ubsan-bootstrap with -Werror enabled, from what I know all
versions since gcc-8 are affected.

configure --enable-languages=c,c++ --disable-multilib
--with-build-config=bootstrap-ubsan --enable-werror=yes && make
BOOT_CFLAGS="-Wno-error=array-bounds -Wno-error=format-overflow"

-Wno-error=array-bounds and -Wno-error=format-overflow are necessary as these
warnings have known issues under ubsan.