[Bug sanitizer/107298] New: Failure to compile code with std::optional and ASan/UBSan

[Bug sanitizer/107298] New: Failure to compile code with std::optional and ASan/UBSan

[jzwinck at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107298

            Bug ID: 107298
           Summary: Failure to compile code with std::optional and
                    ASan/UBSan
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jzwinck at gmail dot com
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at gcc dot gnu.org
  Target Milestone: ---

Test case which is expected to compile:

    #include <optional>

    std::optional<bool> f(bool cond)
    {
        if (cond) {
            return std::make_optional(true);
        }
        return {};
    }

    bool g()
    {
        return *f(true);
    }

GCC 10, 11 and 12 all fail to compile this with "-Wall -Wextra -Werror -O2
-fsanitize=undefined":

    error: '*(unsigned char*)((char*)&<unnamed> +
offsetof(std::optional<bool>,std::optional<bool>::<unnamed>.std::_Optional_base<bool,
true, true>::<unnamed>))' may be used uninitialized
[-Werror=maybe-uninitialized]
        return {};

Removing std::make_optional or replacing it with std::optional<bool> makes the
error go away, though these changes should be irrelevant since they're not on
the code path the compiler complains about.  Compiling without a sanitizer also
works fine.

Demo: https://godbolt.org/z/TafE9cxWW

[Bug sanitizer/107298] Failure to compile code with std::optional and ASan/UBSan

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107298

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
It's a known limitation that sanitizer trigger more warnings, please disable
-Werrors.

[Bug sanitizer/107298] Failure to compile code with std::optional and ASan/UBSan

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107298

--- Comment #2 from Martin Liška <marxin at gcc dot gnu.org> ---
I mean warnings that are false-positives.

[Bug sanitizer/107298] Failure to compile code with std::optional and ASan/UBSan

[jzwinck at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107298

--- Comment #3 from John Zwinck <jzwinck at gmail dot com> ---
Martin, this bug is the only false positive I see in a large corporate
codebase.  I think your comment makes it sound like this is a documented thing
where there are tons of false positives and it's not practical to do anything
about them, and I am not sure how to square this with my experience that almost
all warnings from -Wall and -Wextra work just fine with ASan & UBSan.  Is there
some documentation explaining this, or telling Sanitizer users to turn off
warnings-as-errors?

[Bug sanitizer/107298] Failure to compile code with std::optional and ASan/UBSan

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107298

--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
> Is there some documentation explaining this, or telling Sanitizer
> users to turn off warnings-as-errors?

Unfortunately, it is not, but we should include it.

Generally speaking, -Wmaybe-uninitialized is a warning with a pretty high
false-positive rate. And it's known that sanitizers tend to introduce new
warnings which are false positives. Anyway, let me update the documentation.

[Bug sanitizer/107298] Failure to compile code with std::optional and ASan/UBSan

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107298

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Liska <marxin@gcc.gnu.org>:

https://gcc.gnu.org/g:3f9c071324eff2222249b23a7531e447fc17d928

commit r13-3539-g3f9c071324eff2222249b23a7531e447fc17d928
Author: Martin Liska <mliska@suse.cz>
Date:   Wed Oct 26 13:07:57 2022 +0200

    docs: document sanitizers can trigger warnings

            PR sanitizer/107298

    gcc/ChangeLog:

            * doc/invoke.texi: Document sanitizers can trigger warnings.