[Bug c++/109434] New: std::optional weird -Wmaybe-unitialized and behaviour with -O2

["tomas.pecka at cesnet dot cz" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109434

            Bug ID: 109434
           Summary: std::optional weird -Wmaybe-unitialized and behaviour
                    with -O2
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tomas.pecka at cesnet dot cz
  Target Milestone: ---

Created attachment 54816
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54816&action=edit
reproducer

Hello,

I've seen a compile warning on a code (see attachment) that looks bogus. 

  In file included from optional.cpp:2:
  In member function ‘constexpr bool std::_Optional_base_impl<_Tp,
_Dp>::_M_is_engaged() const [with _Tp = int; _Dp = std::_Optional_base<int,
true, true>]’,
      inlined from ‘constexpr bool std::optional<_Tp>::has_value() const [with
_Tp = int]’ at /usr/include/c++/12.2.1/optional:988:35,
      inlined from ‘int main()’ at optional.cpp:21:63:
  /usr/include/c++/12.2.1/optional:471:58: warning: ‘*(unsigned
char*)((char*)&optInt +
offsetof(std::optional<int>,std::optional<int>::<unnamed>.std::_Optional_base<int,
true, true>::_M_payload.std::_Optional_payload<int, true, true,
true>::<unnamed>.std::_Optional_payload_base<int>::_M_engaged))’ may be used
uninitialized [-Wmaybe-uninitialized]
    471 |       { return static_cast<const _Dp*>(this)->_M_payload._M_engaged;
}
        |                                                          ^~~~~~~~~~
  optional.cpp: In function ‘int main()’:
  optional.cpp:15:24: note: ‘*(unsigned char*)((char*)&optInt +
offsetof(std::optional<int>,std::optional<int>::<unnamed>.std::_Optional_base<int,
true,   true>::_M_payload.std::_Optional_payload<int, true, true,
true>::<unnamed>.std::_Optional_payload_base<int>::_M_engaged))’ was declared
here
     15 |     std::optional<int> optInt;
        |                        ^~~~~~

The warning is present only when compiling with -Wall -O2. When I run the
executable, I see unexpected weird output, e.g.

  catch 56
  val=72704.000000

which look like some uninitialized variable is really there. When run under
valgrind, I see reports about unitialized variables.

Code seems to be working correctly when compiled with -O1 or lower and the
executable seem to be behaving expectedly as well.
Compiling and executing under clang++ works as well regardless of the
optimization level. 
I don't have older versions of g++ (< 12) but compiling the attached code on
godbolt with g++ 11 and lower does not trigger any warning.

Interesting part is that declaring optDbl as std::optional<int> makes the code
behave correctly.