[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